LMX2485,LMX2485E
LMX2485/LMX2485E 50 MHz - 3.0 GHz High Performance Delta-Sigma Low Power
Dual PLLatinum Frequency Synthesizers with 800 MHz Integer PLL
Literature Number: SNAS236E
February 27, 2008
LMX2485/LMX2485E
50 MHz - 3.0 GHz High Performance Delta-Sigma Low
Power Dual PLLatinum™ Frequency Synthesizers with 800
MHz Integer PLL
General Description
The LMX2485 is a low power, high performance delta-sigma
fractional-N PLL with an auxiliary integer-N PLL. The device
is fabricated using National Semiconductor’s advanced pro-
cess.
With delta-sigma architecture, fractional spurs at lower offset
frequencies are pushed to higher frequencies outside the loop
bandwidth. The ability to push close in spur and phase noise
energy to higher frequencies is a direct function of the mod-
ulator order. Unlike analog compensation, the digital feed-
back technique used in the LMX2485 is highly resistant to
changes in temperature and variations in wafer processing.
The LMX2485 delta-sigma modulator is programmable up to
fourth order, which allows the designer to select the optimum
modulator order to fit the phase noise, spur, and lock time
requirements of the system.
Serial data for programming the LMX2485 is transferred via
a three line high speed (20 MHz) MICROWIRE interface. The
LMX2485 offers fine frequency resolution, low spurs, fast pro-
gramming speed, and a single word write to change the
frequency. This makes it ideal for direct digital modulation
applications, where the N counter is directly modulated with
information. The LMX2485 is available in a 24 lead
4.0 X 4.0 X 0.8 mm LLP package.
Applications
■Cellular phones and base stations
CDMA, WCDMA, GSM/GPRS, TDMA, EDGE, PDC
■Direct digital modulation applications
■Satellite and cable TV tuners
■WLAN Standards
Features
Quadruple Modulus Prescalers for Lower Divide Ratios
■RF PLL: 8/9/12/13 or 16/17/20/21
■IF PLL: 8/9 or 16/17
Advanced Delta Sigma Fractional Compensation
■12 bit or 22 bit selectable fractional modulus
■Up to 4th order programmable delta-sigma modulator
Features for Improved Lock Times and Programming
■Fastlock / Cycle slip reduction
■Integrated time-out counter
■Single word write to change frequencies with Fastlock
Wide Operating Range
■LMX2485 RF PLL: 500 MHz to 3.0 GHz
■LMX2485E RF PLL: 50 MHz to 3.0 GHz
Useful Features
■Digital lock detect output
■Hardware and software power-down control
■On-chip crystal reference frequency doubler.
■RF phase comparison frequency up to 50 MHz
■2.5 to 3.6 volt operation with ICC = 5.0 mA at 3.0 V
Functional Block Diagram
20087701
PLLatinum™ is a trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation 200877 www.national.com
LMX2485/LMX2485E High Performance Delta-Sigma Low Power Dual PLLatinum Frequency
Synthesizer
Connection Diagram
Top View
24-Pin LLP (SQ)
20087722
Pin Descriptions
Pin # Pin Name I/O Pin Description
0 GND - Ground Substrate. This is on the bottom of the package and must be grounded.
1 CPoutRF O RF PLL charge pump output.
2 GND - RF PLL analog ground.
3 VddRF1 - RF PLL analog power supply.
4 FinRF I RF PLL high frequency input pin.
5 FinRF* I RF PLL complementary high frequency input pin. Shunt to ground with a 100 pF capacitor.
6 LE I MICROWIRE Load Enable. High impedance CMOS input. Data stored in the shift registers is
loaded into the internal latches when LE goes HIGH
7 DATA I MICROWIRE Data. High impedance binary serial data input.
8 CLK I MICROWIRE Clock. High impedance CMOS Clock input. Data for the various counters is
clocked into the 24 bit shift register on the rising edge
9 VddRF2 - Power supply for RF PLL digital circuitry.
10 CE I Chip Enable control pin. Must be pulled high for normal operation.
11 VddRF5 I Power supply for RF PLL circuitry.
12 Ftest/LD O Test frequency output / Lock Detect.
13 FinIF I IF PLL high frequency input pin.
14 VddIF1 - IF PLL analog power supply.
15 GND - IF PLL digital ground.
16 CPoutIF O IF PLL charge pump output
17 VddIF2 - IF PLL power supply.
18 OSCout O Buffered output of the OSCin signal.
19 ENOSC I Oscillator enable. When this is set to high, the OSCout pin is enabled regardless of the state
of other pins or register bits.
20 OSCin I Input for TCXO signal.
21 NC I This pin must be left open.
22 VddRF3 - Power supply for RF PLL digital circuitry.
23 FLoutRF O RF PLL Fastlock Output. Also functions as Programmable TRI-STATE CMOS output.
24 VddRF4 - RF PLL analog power supply.
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LMX2485/LMX2485E
Absolute Maximum Ratings (Notes 1, 2)
Parameter Symbol Value Units
Min Typ Max
Power Supply Voltage VCC -0.3 4.25 V
Voltage on any pin with GND = 0V Vi-0.3 VCC+0.3 V
Storage Temperature Range Ts-65 +150 °C
Lead Temperature (Solder 4 sec.) TL +260 °C
Recommended Operating Conditions
Parameter Symbol Value Units
Min Typ Max
Power Supply Voltage (Note 1) VCC 2.5 3.0 3.6 V
Operating Temperature TA-40 25 +85 °C
Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur. "Recommended Operating Conditions" indicate conditions
for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the
Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. The voltage at all the power supply pins of VddRF1, VddRF2,
VddRF3, VddRF4, VddRF5, VddIF1 and VddIF2 must be the same. VCC will be used to refer to the voltage at these pins and ICC will be used to refer to the sum
of all currents through all these power pins.
Note 2: This Device is a high performance RF integrated circuit with an ESD rating < 2 kV and is ESD sensitive. Handling and assembly of this device should
only be done at ESD-free workstations.
Electrical Characteristics (VCC = 3.0V; -40°C ≤ TA ≤ +85°C unless otherwise specified)
Symbol Parameter Conditions Value Units
Min Typ Max
Icc PARAMETERS
ICCRF Power Supply Current,
RF Synthesizer
IF PLL OFF
RF PLL ON
Charge Pump TRI-STATE
3.3 mA
ICCIF Power Supply Current, IF
Synthesizer
IF PLL ON
RF PLL OFF
Charge Pump TRI-STATE
1.7 mA
ICCTOTAL Power Supply Current,
Entire Synthesizer
IF PLL ON
RF PLL ON
Charge Pump TRI-STATE
5.0 mA
ICCPD Power Down Current CE = ENOSC = 0V
CLK, DATA, LE = 0V 1 10 µA
RF SYNTHESIZER PARAMETERS
fFinRF
Operating
Frequency
(Note 3)
LMX2485 RF_P = 8 500 2000
MHz
RF_P = 16 500 3000
LMX2485
E
RF_P = 8 50 2000
RF_P = 16 50 3000
pFinRF Input Sensitivity 500 - 3000 MHz -15 0 dBm
50 - 500 MHz (LMX2485E only) -8 8
fCOMP
Phase Detector
Frequency
(Note 4)
50 MHz
ICPoutRFSRCE
RF Charge Pump Source
Current
(Note 5)
RF_CPG = 0
VCPoutRF = VCC/2 95 µA
RF_CPG = 1
VCPoutRF = VCC/2 190 µA
... ... µA
RF_CPG = 15
VCPoutRF = VCC/2 1520 µA
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LMX2485/LMX2485E
Symbol Parameter Conditions Value Units
Min Typ Max
ICPoutRFSINK
RF Charge Pump Sink
Current
(Note 5)
RF_CPG = 0
VCPoutRF = VCC/2 -95 µA
RF_CPG = 1
VCPoutRF = VCC/2 -190 µA
... ... µA
RF_CPG = 15
VCPoutRF = VCC/2 -1520 µA
ICPoutRFTRI
RF Charge Pump TRI-
STATE Current
Magnitude
0.5 ≤ VCPoutRF ≤ VCC -0.5 2 10 nA
| ICPoutRF%MIS | Magnitude of RF CP Sink
vs. CP Source Mismatch
VCPoutRF = VCC/2
TA = 25°C
RF_CPG > 2 3 10 %
RF_CPG ≤ 2 3 13 %
| ICPoutRF%V | Magnitude of RF CP
Current vs. CP Voltage
0.5 ≤ VCPoutRF ≤ VCC -0.5
TA = 25°C 2 8 %
| ICPoutRF%T | Magnitude of RF CP
Current vs. Temperature VCPoutRF = VCC/2 4 %
IF SYNTHESIZER PARAMETERS
fFinIF Operating Frequency 75 800 MHz
pFinIF IF Input Sensitivity -10 5 dBm
fCOMP
Phase Detector
Frequency 10 MHz
ICPoutIFSRCE IF Charge Pump Source
Current VCPoutIF = VCC/2 3.5 mA
ICPoutIFSINK IF Charge Pump Sink
Current VCPoutIF = VCC/2 -3.5 mA
ICPoutIFTRI
IF Charge Pump TRI-
STATE Current
Magnitude
0.5 ≤ VCPoutIF ≤ VCC RF -0.5 2 10 nA
| ICPoutIF%MIS | Magnitude of IF CP Sink
vs. CP Source Mismatch
VCPoutIF = VCC/2
TA = 25°C 1 8 %
| ICPoutIF%V | Magnitude of IF CP
Current vs. CP Voltage
0.5 ≤ VCPoutIF ≤ VCC -0.5
TA = 25°C 4 10 %
| ICPoutIF%TEMP Magnitude of IF CP
Current vs. Temperature VCPoutIF = VCC/2 4 %
OSCILLATOR PARAMETERS
fOSCin
Oscillator Operating
Frequency
OSC2X = 0 5 110 MHz
OSC2X = 1 5 20 MHz
vOSCin
Oscillator Input
Sensitivity 0.5 VCC VP-P
IOSCin Oscillator Input Current -100 100 µA
SPURS
Spurs in band (Note 6) -55 dBc
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LMX2485/LMX2485E
Symbol Parameter Conditions Value Units
Min Typ Max
PHASE NOISE
LF1HzRF
RF Synthesizer
Normalized Phase Noise
Contribution (Note 7)
RF_CPG = 0 -202
dBc/Hz
RF_CPG = 1 -202
RF_CPG = 3 -206
RF_CPG = 7 -208
RF_CPG = 15 -210
LF1HzIF
IF Synthesizer
Normalized Phase Noise
Contribution
-209 dBc/Hz
DIGITAL INTERFACE (DATA, CLK, LE, ENOSC, CE, Ftest/LD, FLoutRF)
VIH High-Level Input Voltage 1.6 VCC V
VIL Low-Level Input Voltage 0.4 V
IIH High-Level Input Current VIH = VCC -1.0 1.0 µA
IIL Low-Level Input Current VIL = 0 V -1.0 1.0 µA
VOH
High-Level Output
Voltage IOH = -500 µA VCC-0.4 V
VOL
Low-Level Output
Voltage IOL = 500 µA 0.4 V
MICROWIRE INTERFACE TIMING
tCS
Data to Clock Set Up
Time See MICROWIRE Input Timing 25 ns
tCH Data to Clock Hold Time See MICROWIRE Input Timing 8 ns
tCWH Clock Pulse Width High See MICROWIRE Input Timing 25 ns
tCWL Clock Pulse Width Low See MICROWIRE Input Timing 25 ns
tES
Clock to Load Enable Set
Up Time See MICROWIRE Input Timing 25 ns
tEW Load Enable Pulse Width See MICROWIRE Input Timing 25 ns
Note 3: A slew rate of at least 100 V/uS is recommended for frequencies below 500 MHz for optimal performance.
Note 4: For Phase Detector Frequencies above 20 MHz, Cycle Slip Reduction (CSR) may be required. Legal divide ratios are also required.
Note 5: Refer to table in Section 2.4.2 RF_CPG -- RF PLL Charge Pump Gain for complete listing of charge pump currents.
Note 6: In order to measure the in-band spur, the fractional word is chosen such that when reduced to lowest terms, the fractional numerator is one. The spur
offset frequency is chosen to be the comparison frequency divided by the reduced fractional denominator. The loop bandwidth must be sufficiently wide to negate
the impact of the loop filter. Measurement conditions are: Spur Offset Frequency = 10 kHz, Loop Bandwidth = 100 kHz, Fraction = 1/2000, Comparison Frequency
= 20 MHz, RF_CPG = 7, DITH = 0, and a 4th Order Modulator (FM = 0). These are relatively consistent over tuning range.
Note 7: Normalized Phase Noise Contribution is defined as: LN(f) = L(f) – 20log(N) – 10log(fCOMP) where L(f) is defined as the single side band phase noise
measured at an offset frequency, f, in a 1 Hz Bandwidth. The offset frequency, f, must be chosen sufficiently smaller than the PLL loop bandwidth, yet large
enough to avoid substantial phase noise contribution from the reference source. Measurement conditions are: Offset Frequency = 11 kHz, Loop Bandwidth = 100
kHz for RF_CPG = 7, Fraction = 1/2000, Comparison Frequency = 20 MHz, FM = 0, DITH = 0.
MICROWIRE INPUT TIMING DIAGRAM
20087775
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LMX2485/LMX2485E
IF PLL Fin Sensitivity
TA = 25°C, IF_P = 16
20087747
IF PLL Fin Sensitivity
VCC = 3.0 V, IF_P = 16
20087748
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LMX2485/LMX2485E
OSCin Sensitivity
TA = 25°C, OSC_2X = 0
20087749
OSCin Sensitivity
VCC = 3.0 V, OSC_2X = 0
20087756
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LMX2485/LMX2485E
OSCin Sensitivity
TA = 25°C, OSC_2X = 1
20087773
OSCin Sensitivity
VCC = 3.0 V, OSC_2X = 1
20087774
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LMX2485/LMX2485E
Typical Performance Characteristic : FinRF Input Impedance (Note 8)
20087768
FinRF Input Impedance
Frequency (MHz) Real (Ohms) Imaginary (Ohms)
50 670 -276
100 531 -247
200 452 -209
300 408 -212
400 373 -222
500 337 -231
600 302 -237
700 270 -239
800 241 -236
900 215 -231
1000 192 -221
1100 172 -218
1200 154 -209
1300 139 -200
1400 127 -192
1500 114 -184
1600 104 -175
1700 96 -168
1800 88 -160
1900 80 -153
2000 74 -147
2200 64 -134
2400 56 -123
2600 50 -113
2800 45 -103
3000 39 -94
3200 37 -86
3400 33 -78
3600 30 -72
3800 28 -69
4000 26 -66
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LMX2485/LMX2485E
Typical Performance Characteristic : FinIF Input Impedance (Note 8)
20087754
FinIF Input Impedance
Frequency (MHz) Real (Ohms) Imaginary (Ohms)
50 583 -286
75 530 -256
100 499 -241
200 426 -209
300 384 -209
400 347 -219
500 310 -224
600 276 -228
700 244 -228
800 216 -223
900 192 -218
1000 173 -208
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LMX2485/LMX2485E
Typical Performance Characteristic : OSCin Input Impedance (Note 8)
20087755
Frequency
(MHz)
Powered Up Powered Down
Real Imaginary Magnitude Real Imaginary Magnitude
5 1730 -3779 4157 392 -8137 8146
10 846 -2236 2391 155 -4487 4490
20 466 -1196 1284 107 -2215 2217
30 351 -863 932 166 -1495 -1504
40 316 -672 742 182 -1144 1158
50 278 -566 631 155 -912 925
60 261 -481 547 153 -758 774
70 252 -425 494 154 -652 669
80 239 -388 456 147 -576 595
90 234 -358 428 145 -518 538
100 230 -337 407 140 -471 492
110 225 -321 392 138 -436 458
120 219 -309 379 133 -402 123
130 214 -295 364 133 -374 397
140 208 -285 353 132 -349 373
150 207 -279 348 133 -329 355
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LMX2485/LMX2485E
RF PLL Charge Pump Current
VCC = 3.0 Volts
20087767
IF PLL Charge Pump Current
VCC = 3.0 Volts
20087765
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LMX2485/LMX2485E
Charge Pump Leakage
RF PLL
VCC = 3.0 Volts
20087764
Charge Pump Leakage
IF PLL
VCC = 3.0 Volts
20087763
Note 8: Typical performance characteristics do not imply any sort of guarantee. Guaranteed specifications are in the electrical characteristics section.
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LMX2485/LMX2485E
Bench Test Setups
20087769
Charge Pump Current Measurement Procedure
The above block diagram shows the test procedure for testing
the RF and IF charge pumps. These tests include absolute
current level, mismatch, and leakage measurement. In order
to measure the charge pump currents, a signal is applied to
the high frequency input pins. The reason for this is to guar-
antee that the phase detector gets enough transitions in order
to be able to change states. If no signal is applied, it is possible
that the charge pump current reading will be low due to the
fact that the duty cycle is not 100%. The OSCin Pin is tied to
the supply. The charge pump currents can be measured by
simply programming the phase detector to the necessary po-
larity. For instance, in order to measure the RF charge pump,
a 10 MHz signal is applied to the FinRF pin. The source cur-
rent can be measured by setting the RF PLL phase detector
to a positive polarity, and the sink current can be measured
by setting the phase detector to a negative polarity. The IF
PLL currents can be measured in a similar way. Note that the
magnitude of the RF PLL charge pump current is controlled
by the RF_CPG bit. Once the charge pump currents are
known, the mismatch can be calculated as well. In order to
measure leakage, the charge pump is set to a TRI-STATE
mode by enabling the RF_CPT and IF_CPT bits. The table
below shows a summary of the various charge pump tests.
Current Test RF_CPG RF_CPP RF_CPT IF_CPP IF_CPT
RF Source 0 to 15 0 0 X X
RF Sink 0 to 15 1 0 X X
RF TRI-STATE X X 1 X X
IF Source X X X 0 0
IF Sink X X X 1 0
IF TRI-STATE X X X X 1
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LMX2485/LMX2485E
Charge Pump Current Specification Definitions
20087750
I1 = Charge Pump Sink Current at VCPout = Vcc - ΔV
I2 = Charge Pump Sink Current at VCPout = Vcc/2
I3 = Charge Pump Sink Current at VCPout = ΔV
I4 = Charge Pump Source Current at VCPout = Vcc - ΔV
I5 = Charge Pump Source Current at VCPout = Vcc/2
I6 = Charge Pump Source Current at VCPout = ΔV
ΔV = Voltage offset from the positive and negative supply
rails. Defined to be 0.5 volts for this part.
vCPout refers to either VCPoutRF or VCPoutIF
ICPout refers to either ICPoutRF or ICPoutIF
Charge Pump Output Current Magnitude Variation vs.
Charge Pump Output Voltage
20087751
Charge Pump Sink Current vs. Charge Pump Output
Source Current Mismatch
20087752
Charge Pump Output Current Magnitude Variation vs.
Temperature
20087753
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LMX2485/LMX2485E
20087770
Frequency Input Pin DC Blocking Capacitor Corresponding
Counter
Default Counter Value MUX Value
OSCin 1000 pF RF_R / 2 50 14
FinRF 100 pF// 1000 pF RF_N / 2 502 + 2097150 /
4194301
15
FinIF 100 pF IF_N / 2 534 13
OSCin 1000 pF IF_R / 2 50 12
Sensitivity Measurement Procedure
Sensitivity is defined as the power level limits beyond which
the output of the counter being tested is off by 1 Hz or more
of its expected value. It is typically measured over frequency,
voltage, and temperature. In order to test sensitivity, the MUX
[3:0] word is programmed to the appropriate value. The
counter value is then programmed to a fixed value and a fre-
quency counter is set to monitor the frequency of this pin. The
expected frequency at the Ftest/LD pin should be the signal
generator frequency divided by twice the corresponding
counter value. The factor of two comes in because the
LMX2485 has a flip-flop which divides this frequency by two
to make the duty cycle 50% in order to make it easier to read
with the frequency counter. The frequency counter input
impedance should be set to high impedance. In order to per-
form the measurement, the temperature, frequency, and volt-
age is set to a fixed value and the power level of the signal is
varied. Note that the power level at the part is assumed to be
4 dB less than the signal generator power level. This accounts
for 1 dB for cable losses and 3 dB for the pad. The power level
range where the frequency is correct at the Ftest/LD pin to
within 1 Hz accuracy is recorded for the sensitivity limits. The
temperature, frequency, and voltage can be varied in order to
produce a family of sensitivity curves. Since this is an open-
loop test, the charge pump is set to TRI-STATE and the
unused side of the PLL (RF or IF) is powered down when not
being tested. For this part, there are actually four frequency
input pins, although there is only one frequency test pin (Ftest/
LD). The conditions specific to each pin are shown in above
table.
Note that for the RF N counter, a fourth order fractional mod-
ulator is used in 22-bit mode with a fraction of 2097150 /
4194301 is used. The reason for this long fraction is to test
the RF N counter and supporting fractional circuitry as com-
pletely as possible.
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LMX2485/LMX2485E
20087771
Input Impedance Measurement Procedure
The above block diagram shows the test setup used for mea-
suring the input impedance for the LMX2485. The DC block-
ing capacitor used between the input SMA connector and the
pin being measured must be changed to a zero Ohm resistor.
This procedure applies to the FinRF, FinIF, and OSCin pins.
The basic test procedure is to calibrate the network analyzer,
ensure that the part is powered up, and then measure the
input impedance. The network analyzer can be calibrated by
using either calibration standards or by soldering resistors di-
rectly to the evaluation board. An open can be implemented
by putting no resistor, a short can be implemented by solder-
ing a zero ohm resistor as close as possible to the pin being
measured, and a short can be implemented by soldering two
100 ohm resistors in parallel as close as possible to the pin
being measured. Calibration is done with the PLL removed
from the PCB. This requires the use of a clamp down fixture
that may not always be generally available. If no clamp down
fixture is available, then this procedure can be done by cali-
brating up to the point where the DC blocking capacitor usu-
ally is, and then implementing port extensions with the
network analyzer. Zero ohm resistor is added back for the
actual measurement. Once the setup is calibrated, it is nec-
essary to ensure that the PLL is powered up. This can be done
by toggling the power down bits (RF_PD and IF_PD) and ob-
serving that the current consumption indeed increases when
the bit is disabled. Sometimes it may be necessary to apply
a signal to the OSCin pin in order to program the part. If this
is necessary, disconnect the signal once it is established that
the part is powered up. It is useful to know the input
impedance of the PLL for the purposes of debugging RF
problems and designing matching networks. Another use of
knowing this parameter is make the trace width on the PCB
such that the input impedance of this trace matches the real
part of the input impedance of the PLL frequency of operation.
In general, it is good practice to keep trace lengths short and
make designs that are relatively resistant to variations in the
input impedance of the PLL.
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LMX2485/LMX2485E
Functional Description (Note 9)
1.0 GENERAL
The LMX2485 consists of integrated N counters, R counters,
and charge pumps. The TCXO, VCO and loop filter are sup-
plied external to the chip. The various blocks are described
below.
1.1 TCXO, OSCILLATOR BUFFER, AND R COUNTER
The oscillator buffer must be driven single-ended by a signal
source, such as a TCXO. The OSCout pin is included to pro-
vide a buffered output of this input signal and is active when
the OSC_OUT bit is set to one. The ENOSC pin can be also
pulled high to ensure that the OSCout pin is active, regardless
of the status of the registers in the LMX2485.
The R counter divides this TXCO frequency down to the com-
parison frequency.
1.2 PHASE DETECTOR
The maximum phase detector operating frequency for the IF
PLL is straightforward, but it is a little more involved for the
RF PLL since it is fractional. The maximum phase detector
frequency for the LMX2485 RF PLL is 50 MHz. However, this
is not possible in all circumstances due to illegal divide ratios
of the N counter. The crystal reference frequency also limits
the phase detector frequency, although the doubler helps with
this limitation. There are trade-offs in choosing the phase de-
tector frequency. If this frequency is run higher, then phase
noise will be lower, but lock time may be increased due to
cycle slipping and the capacitors in the loop filter may become
rather large.
1.3 CHARGE PUMP
For the majority of the time, the charge pump output is high
impedance, and the only current through this pin is the Tri-
State leakage. However, it does put out fast correction pulses
that have a width that is proportional to the phase error pre-
sented at the phase detector.
The charge pump converts the phase error presented at the
phase detector into a correction current. The magnitude of
this current is theoretically constant, but the duty cycle is pro-
portional to the phase error. For the IF PLL, this current is not
programmable, but for the RF PLL it is programmable in 16
steps. Also, the RF PLL allows for a higher charge pump cur-
rent to be used when the PLL is locking in order to reduce the
lock time.
1.4 LOOP FILTER
The loop filter design can be rather involved. In addition to the
regular constraints and design parameters, delta-sigma PLLs
have the additional constraint that the order of the loop filter
should be one greater than the order of the delta sigma mod-
ulator. This rule of thumb comes from the requirement that the
loop filter must roll off the delta sigma noise at 20 dB/decade
faster than it rises. However, since the noise can not have
infinite power, it must eventually roll off. If the loop bandwidth
is narrow, this requirement may not be necessary. For the
purposes of discussion in this datasheet, the pole of the loop
filter at 0 Hz is not counted. So a second order filter has 3
components, a 3rd order loop filter has 5 components, and
the 4th order loop filter has 7 components. Although a 5th
order loop filter is theoretically necessary for use with a 4th
order modulator, typically a 4th order filter is used in this case.
The loop filter design, especially for higher orders can be
rather involved, but there are many simulation tools and ref-
erences available, such as the one given at the end of the
functional description block.
1.5 N COUNTERS AND HIGH FREQUENCY INPUT PINS
The N counter divides the VCO frequency down to the com-
parison frequency. Because prescalers are used, there are
limitations on how small the N value can be. The N counters
are discussed in greater depth in the programming section.
Since the input pins to these counters ( FinRF and FinIF ) are
high frequency, layout considerations are important.
High Frequency Input Pins, FinRF and FinIF
It is generally recommended that the VCO output go through
a resistive pad and then through a DC blocking capacitor be-
fore it gets to these high frequency input pins. If the trace
length is sufficiently short ( < 1/10th of a wavelength ), then
the pad may not be necessary, but a series resistor of about
39 ohms is still recommended to isolate the PLL from the
VCO. The DC blocking capacitor should be chosen at least to
be 27 pF, depending on frequency. It may turn out that the
frequency is above the self-resonant frequency of the capac-
itor, but since the input impedance of the PLL tends to be
capacitive, it actually is a benefit to exceed the tune frequen-
cy. The pad and the DC blocking capacitor should be placed
as close to the PLL as possible
Complementary High Frequency Pin, FinRF*
These inputs may be used to drive the PLL differentially, but
it is very common to drive the PLL in a single ended fashion.
A shunt capacitor should be placed at the FinRF* pin. The
value of this capacitor should be chosen such that the
impedance, including the ESR of the capacitor, is as close to
an AC short as possible at the operating frequency of the PLL.
100 pF is a typical value, depending on frequency.
1.6 POWER PINS, POWER DOWN, AND POWER UP
MODES
It is recommended that all of the power pins be filtered with a
series 18 ohm resistor and then placing two capacitors shunt
to ground, thus creating a low pass filter. Although it makes
sense to use large capacitor values in theory, the ESR
( Equivalent Series Resistance ) is greater for larger capaci-
tors. For optimal filtering minimize the sum of the ESR and
theoretical impedance of the capacitor. It is therefore recom-
mended to provide two capacitors of very different sizes for
the best filtering. 1 µF and 100 pF are typical values. The
small capacitor should be placed as close as possible to the
pin.
The power down state of the LMX2485 is controlled by many
factors. The one factor that overrides all other factors is the
CE pin. If this pin is low, the part will be powered down. As-
serting a high logic level on this pin is necessary to power up
the chip, however, there are other bits in the programming
registers that can override this and put the PLL back in a
power down state. Provided that the voltage on the CE pin is
high, programming the RF_PD and IF_PD bits to zero guar-
antees that the part will be powered up. Programming either
one of these bits to one will power down the appropriate sec-
tion of the synthesizer, provided that the ATPU bit does not
override this.
www.national.com 20
LMX2485/LMX2485E
CE Pin RF_PD ATPU
Bit Enabled +
Write to RF
N Counter
PLL State
Low X X Powered Down
(Asynchronous)
High X Yes Powered Up
High 0 No Powered Up
High 1 No Powered Down
( Asynchronous )
1.7 DIGITAL LOCK DETECT OPERATION
The RF PLL digital lock detect circuitry compares the differ-
ence between the phase of the inputs of the phase detector
to a RC generated delay of ε. To indicate a locked state (Lock
= HIGH) the phase error must be less than the ε RC delay for
5 consecutive reference cycles. Once in lock (Lock = HIGH),
the RC delay is changed to approximately δ. To indicate an
out of lock state (Lock = LOW), the phase error must become
greater δ. The values of ε and δ are dependent on which PLL
is used and are shown in the table below:
PLL ε δ
RF 10 ns 20 ns
IF 15 ns 30 ns
When the PLL is in the power down mode and the Ftest/LD
pin is programmed for the lock detect function, it is forced
LOW. The accuracy of this circuit degrades at higher com-
parison frequencies. To compensate for this, the DIV4 word
should be set to one if the comparison frequency exceeds 20
MHz. The function of this word is to divide the comparison
frequency presented to the lock detect circuit by 4. Note that
if the MUX[3:0] word is set such as to view lock detect for both
PLLs, an unlocked (LOW) condition is shown whenever either
one of the PLLs is determined to be out of lock.
20087704
21 www.national.com
LMX2485/LMX2485E
1.8 CYCLE SLIP REDUCTION AND FASTLOCK
The LMX2485 offers both cycle slip reduction (CSR) and
Fastlock with timeout counter support. This means that it re-
quires no additional programming overhead to use them. It is
generally recommended that the charge pump current in the
steady state be 8X or less in order to use cycle slip reduction,
and 4X or less in steady state in order to use Fastlock. The
next step is to decide between using Fastlock or CSR. This
determination can be made based on the ratio of the com-
parison frequency ( fCOMP ) to loop bandwidth ( BW ).
Comparison
Frequency
( fCOMP )
Fastlock
Cycle Slip
Reduction
( CSR )
fCOMP ≤ 1.25 MHz Noticeable better
than CSR
Likely to provide a
benefit, provided
that
fCOMP > 100 X BW
1.25 MHz < fCOMP ≤
2 MHz
Marginally better
than CSR
fCOMP > 2 MHz Same or worse
than CSR
Cycle Slip Reduction (CSR)
Cycle slip reduction works by reducing the comparison fre-
quency during frequency acquisition while keeping the same
loop bandwidth, thereby reducing the ratio of the comparison
frequency to the loop bandwidth. In cases where the ratio of
the comparison frequency exceeds about 100 times the loop
bandwidth, cycle slipping can occur and significantly degrade
lock times. The greater this ratio, the greater the benefit of
CSR. This is typically the case of high comparison frequen-
cies. In circumstances where there is not a problem with cycle
slipping, CSR provides no benefit. There is a glitch when CSR
is disengaged, but since CSR should be disengaged long be-
fore the PLL is actually in lock, this glitch is not an issue. A
good rule of thumb for CSR disengagement is to do this at the
peak time of the transient response. Because this time is typ-
ically much sooner than Fastlock should be disengaged, it
does not make sense to use CSR and Fastlock in combina-
tion.
Fastlock
Fastlock works by increasing the loop bandwidth only during
frequency acquisition. In circumstances where the compari-
son frequency is less than or equal to 2 MHz, Fastlock may
provide a benefit beyond what CSR can offer. Since Fastlock
also reduces the ratio of the comparison frequency to the loop
bandwidth, it may provide a significant benefit in cases where
the comparison frequency is above 2 MHz. However, CSR
can usually provide an equal or larger benefit in these cases,
and can be implemented without using an additional resistor.
The reason for this restriction on frequency is that Fastlock
has a glitch when it is disengaged. As the time of engagement
for Fastlock decreases and becomes on the order of the fast
lock time, this glitch grows and limits the benefits of Fastlock.
This effect becomes worse at higher comparison frequencies.
There is always the option of reducing the comparison fre-
quency at the expense of phase noise in order to satisfy this
constraint on comparison frequency. Despite this glitch, there
is still a net improvement in lock time using Fastlock in these
circumstances. When using Fastlock, it is also recommended
that the steady state charge pump state be 4X or less. Also,
Fastlock was originally intended only for second order filters,
so when implementing it with higher order filters, the third and
fourth poles can not be too close in, or it will not be possible
to keep the loop filter well optimized when the higher charge
pump current and Fastlock resistor are engaged.
1.8.1 Using Cycle Slip Reduction (CSR) to Avoid Cycle
Slipping
Once it is decided that CSR is to be used, the cycle slip re-
duction factor needs to be chosen. The available factors are
1/2, 1/4, and 1/16. In order to preserve the same loop char-
acteristics, it is recommended that the following constraint be
satisfied:
(Fastlock Charge Pump Current) / (Steady State Charge
Pump Current) = CSR
In order to satisfy this constraint, the maximum charge pump
current in steady state is 8X for a CSR of 1/2, 4X for a CSR
of 1/4, and 1X for a CSR of 1/16. Because the PLL phase
noise is better for higher charge pump currents, it makes
sense to choose CSR only as large as necessary to prevent
cycle slipping. Choosing it larger than this will not improve lock
time, and will result in worse phase noise.
Consider an example where the desired loop bandwidth in
steady state is 100 kHz and the comparison frequency is 20
MHz. This yields a ratio of 200. Cycle slipping may be present,
but would not be too severe if it was there. If a CSR factor of
1/2 is used, this would reduce the ratio to 100 during frequen-
cy acquisition, which is probably sufficient. A charge pump
current of 8X could be used in steady state, and a factor of
16X could be used during frequency acquisition. This yields
a ratio of 1/2, which is equal to the CSR factor and this satis-
fies the above constraint. In this circumstance, it could also
be decided to just use 16X charge pump current all the time,
since it would probably have better phase noise, and the
degradation in lock time would not be too severe.
1.8.2 Using Fastlock to Improve Lock Times
20087740
Once it is decided that Fastlock is to be used, the loop band-
width multiplier, K, is needed in order to determine the theo-
retical impact of Fastlock on the loop bandwidth and the
resistor value, R2p, that is switched in parallel during Fast-
lock. This ratio is calculated as follows:
K = ( Fastlock Charge Pump Current ) / ( Steady State
Charge Pump Current )
KLoop
Bandwidth R2p Value Lock Time
1 1.00 X Open 100 %
2 1.41 X R2/0.41 71 %
3 1.73 X R2/0.73 58%
4 2.00 X R2 50%
8 2.83 X R2/1.83 35%
9 3.00 X R2/2 33%
16 4.00 X R2/3 25%
The above table shows how to calculate the Fastlock resistor
and theoretical lock time improvement, once the ratio , K, is
known. This all assumes a second order filter (not counting
the pole at 0 Hz). However, it is generally recommended that
the loop filter order be one greater than the order of the delta
sigma modulator, which means that a second order filter is
www.national.com 22
LMX2485/LMX2485E
never recommended. In this case, the value for R2p is typi-
cally about 80% of what it would be for a second order filter.
Because the Fastlock disengagement glitch gets larger and it
is harder to keep the loop filter optimized as the K value be-
comes larger, designing for the largest possible value for K
usually, but not always yields the best improvement in lock
time. To get a more accurate estimate requires more simula-
tion tools, or trial and error.
1.8.3 Capacitor Dielectric Considerations for Lock Time
The LMX2485 has a high fractional modulus and high charge
pump gain for the lowest possible phase noise. One consid-
eration is that the reduced N value and higher charge pump
may cause the capacitors in the loop filter to become larger
in value. For larger capacitor values, it is common to have a
trade-off between capacitor dielectric quality and physical
size. Using film capacitors or NPO/COG capacitors yields the
best possible lock times, where as using X7R or Z5R capac-
itors can increase lock time by 0 – 500%. However, it is a
general tendency that designs that use a higher compare fre-
quency tend to be less sensitive to the effects of capacitor
dielectrics. Although the use of lesser quality dielectric ca-
pacitors may be unavoidable in many circumstances, allow-
ing a larger footprint for the loop filter capacitors, using a lower
charge pump current, and reducing the fractional modulus are
all ways to reduce capacitor values. Capacitor dielectrics
have very little impact on phase noise and spurs.
1.9 FRACTIONAL SPUR AND PHASE NOISE CONTROLS
Control of the fractional spurs is more of an art than an exact
science. The first differentiation that needs to be made is be-
tween primary fractional and sub-fractional spurs. The prima-
ry fractional spurs are those that occur at increments of the
channel spacing only. The sub-fractional spurs are those that
occur at a smaller resolution than the channel spacing, usu-
ally one-half or one-fourth. There are trade-offs between frac-
tional spurs, sub-fractional spurs, and phase noise. The rules
of thumb presented in this section are just that. There will be
exceptions. The bits that impact the fractional spurs are FM
and DITH, and these bits should be set in this order.
The first step to do is choose FM, for the delta sigma modu-
lator order. It is recommended to start with FM = 3 for a third
order modulator and use strong dithering. In general, there is
a trade-off between primary and sub-fractional spurs. Choos-
ing the highest order modulator (FM = 0 for 4th order) typically
provides the best primary fractional spurs, but the worst sub-
fractional spurs. Choosing the lowest modulator order (FM =
2 for 2nd order), typically gives the worst primary fractional
spurs, but the best sub-fractional spurs. Choosing FM = 3, for
a 3rd order modulator is a compromise.
The second step is to choose DITH, for dithering. Dithering
has a very small impact on primary fractional spurs, but a
much larger impact on sub-fractional spurs. The only problem
is that it can add a few dB of phase noise, or even more if the
loop bandwidth is very wide. Disabling dithering (DITH = 0),
provides the best phase noise, but the sub-fractional spurs
are worst (except when the fractional numerator is 0, and in
this case, they are the best). Choosing strong dithering (DITH
= 2) significantly reduces sub-fractional spurs, if not eliminat-
ing them completely, but adds the most phase noise. Weak
dithering (DITH = 1) is a compromise.
The third step is to tinker with the fractional word. Although
1/10 and 400/4000 are mathematically the same, expressing
fractions with much larger fractional numerators often im-
prove the fractional spurs. Increasing the fractional denomi-
nator only improves spurs to a point. A good practical limit
could be to keep the fractional denominator as large as pos-
sible, but not to exceed 4095, so it is not necessary to use the
extended fractional numerator or denominator.
This steps can be done in different orders and it might take a
few iterations to find the optimum performance. Special con-
siderations should be taken for lower frequencies that are
below about 100 MHz. In addition squaring up the wave, it is
often helpful to use lowest terms fractions instead of highest
terms fractions. Also, dithering may turn out to not be so use-
ful. All the things are to introduce a methodical way of thinking
about optimizing spurs, not an exact method. There will be
exceptions to all these rules.
Note 9: For more information concerning delta-sigma PLLs, loop filter design, cycle slip reduction, Fastlock, and many other topics, visit wireless.national.com.
Here there is the EasyPLL simulation tool and an online reference called "PLL Performance, Simulation, and Design", by Dean Banerjee.
23 www.national.com
LMX2485/LMX2485E
Programming Description
2.0 GENERAL PROGRAMMING INFORMATION
The 24-bit data registers are loaded through a MICROWIRE Interface. These data registers are used to program the R counter,
the N counter, and the internal mode control latches. The data format of a typical 24-bit data register is shown below. The control
bits CTL [3:0] decode the register address. On the rising edge of LE, data stored in the shift register is loaded into one of the
appropriate latches (selected by address bits). Data is shifted in MSB first. Note that it is best to program the N counter last, since
doing so initializes the digital lock detector and Fastlock circuitry. Note that initialize means it resets the counters, but it does NOT
program values into these registers. The exception is when 22-bit is not being used. In this case, it is not necessary to program
the R7 register.
MSB LSB
DATA [21:0] CTL [3:0]
23 4 3 2 1 0
2.0.1 Register Location Truth Table
The control bits CTL [3:0] decode the internal register address. The table below shows how the control bits are mapped to the
target control register.
C3 C2 C1 C0 DATA Location
x x x 0 R0
0 0 1 1 R1
0 1 0 1 R2
0 1 1 1 R3
1 0 0 1 R4
1 0 1 1 R5
1 1 0 1 R6
1 1 1 1 R7
2.0.2 Control Register Content Map
Because the LMX2485 registers are complicated, they are
organized into two groups, basic and advanced. The first four
registers are basic registers that contain critical information
necessary for the PLL to achieve lock. The last 5 registers are
for features that optimize spur, phase noise, and lock time
performance. The next page shows these registers.
Quick Start Register Map
Although it is highly recommended that the user eventually take advantage of all the modes of the LMX2485, the quick start register
map is shown in order for the user to get the part up and running quickly using only those bits critical for basic functionality. The following
default conditions for this programming state are a third order delta-sigma modulator in 12-bit mode with no dithering and no Fastlock.
RE
GIS
TE
R
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[19:0] ( Except for the RF_N Register, which is [22:0] ) C3 C2 C1 C0
R0 RF_N[10:0] RF_FN[11:0] 0
R1 RF
_P
D
RF
_P
RF_R[5:0] RF_FD[11:0] 0 0 1 1
R2 IF_
PD
IF_N[18:0] 0 1 0 1
R3 0001 RF_CPG[3:0] IF_R[11:0] 0 1 1 1
R4 0 0 1 0 0 0 0 0 1 1 0 0 0 1 1 1 0 0 0 0 1 0 0 1
www.national.com 24
LMX2485/LMX2485E
Complete Register Map
The complete register map shows all the functionality of all registers, including the last five.
RE
GIS
TE
R
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[19:0] ( Except for the RF_N Register, which is [22:0] ) C3 C2 C1 C0
R0 RF_N[10:0] RF_FN[11:0] 0
R1 RF
_P
D
RF
_P
RF_R[5:0] RF_FD[11:0] 0 0 1 1
R2 IF_
PD
IF_N[18:0] 0 1 0 1
R3 ACCESS[3:0] RF_CPG[3:0] IF_R[11:0] 0 1 1 1
R4 AT
PU
0 1 0 0 0 DITH
[1:0]
FM
[1:0]
0 OS
C
_2X
OS
C
_O
UT
IF_
CP
P
RF
_
CP
P
IF_
P
MUX
[3:0]
1 0 0 1
R5 RF_FD[21:12] RF_FN[21:12] 1 0 1 1
R6 CSR[1:0] RF_CPF[3:0] RF_TOC[13:0] 1 1 0 1
R70000000000DIV
4
0 1 0 0 1 IF_
RS
T
RF
_R
ST
IF_
CP
T
RF
_C
PT
1 1 1 1
2.1 R0 REGISTER
Note that this register has only one control bit, so the N counter value to be changed with a single write statement to the PLL.
RE
GIS
TE
R
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[22:0] C0
R0 RF_N[10:0] RF_FN[11:0] 0
2.1.1 RF_FN[11:0] -- Fractional Numerator for RF PLL
Refer to section 2.6.1 for a more detailed description of this control word.
2.1.2 RF_N[10:0] -- RF N Counter Value
The RF N counter contains an 8/9/12/13 and a 16/17/20/21 prescaler. The N counter value can be calculated as follows:
N = RF_P·RF_C + 4·RF_B + RF_A
RF_C ≥Max{RF_A, RF_B} , for N-2FM-1 ... N+2FM is a necessary condition. This rule is slightly modified in the case where the
RF_B counter has an unused bit, where this extra bit is used by the delta-sigma modulator for the purposes of modulation. Consult
the tables below for valid operating ranges for each prescaler.
25 www.national.com
LMX2485/LMX2485E
Operation with the 8/9/12/13 Prescaler (RF_P=0)
RF_N RF_N [10:0]
RF_C [6:0] RF_B [1:0] RF_A [1:0]
<25 N values less than 25 are prohibited.
25-26 Possible only with a second order delta-sigma engine.
27-30 Possible only with a second or third order delta-sigma engine.
3100000110111
..........0...
102311111110111
>1023 N values above 1023 are prohibited.
Operation with the 16/17/20/21 Prescaler (RF_P=1)
RF_N RF_N [10:0]
RF_C [6:0] RF_B [1:0] RF_A [1:0]
<49 N values less than 49 are prohibited.
49-50 Possible only with a second order delta-sigma engine.
51-54 Possible with a second or third order delta-sigma engine.
5500000110111
..............
203911111110111
2040-20
43
Possible with a second or third order delta-sigma engine.
2044-20
45
Possible only with a second order delta-sigma engine.
>2045 N values greater than 2045 are prohibited.
www.national.com 26
LMX2485/LMX2485E
2.2 R1 REGISTER
REGISTER 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[19:0] C3 C2 C1 C0
R1 RF_PD RF_P RF_R[5:0] RF_FD[11:0] 0 0 1 1
2.2.1 RF_FD[11:0] -- RF PLL Fractional Denominator
The function of these bits are described in section 2.6.2.
2.2.2 RF_R [5:0] -- RF R Divider Value
The RF R Counter value is determined by this control word. Note that this counter does allow values down to one.
R Value RF_R[5:0]
1000001
... . . . . . .
63 1 1 1 1 1 1
2.2.3 RF_P -- RF Prescaler bit
The prescaler used is determined by this bit.
RF_P Prescaler Maximum Frequency
0 8/9/12/13 2000 MHz
1 16/17/20/21 3000 MHz
2.2.4 RF_PD -- RF Power Down Control Bit
When this bit is set to 0, the RF PLL operates normally. When it is set to one, the RF PLL is powered down and the RF Charge
pump is set to a TRI-STATE mode. The CE pin and ATPU bit also control power down functions, and will override the RF_PD bit.
The order of precedence is as follows. First, if the CE pin is LOW, then the PLL will be powered down. Provided this is not the
case, the PLL will be powered up if the ATPU bit says to do so, regardless of the state of the RF_PD bit. After the CE pin and the
ATPU bit are considered, then the RF_PD bit then takes control of the power down function for the RF PLL.
27 www.national.com
LMX2485/LMX2485E
2.3 R2 REGISTER
REGISTER 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[19:0] C3 C2 C1 C0
R2 IF_PD IF_N[18:0] 0 1 0 1
2.3.1 IF_N[18:0] -- IF N Divider Value
IF_N Counter Programming with the 8/9 Prescaler (IF_P=0)
N
Valu
e
IF_N[18:0]
IF_B IF_A
≤23 N values less than or equal to 23 are prohibited because IF_B ≥ 3 is required.
24-5
5
Legal divide ratios in this range are:
24-27, 32-36, 40-45, 48-54
560000000000001110000
570000000000001110001
......................
2621
43
1111111111111110111
Operation with the 16/17 Prescaler (IF_P=1)
N
Valu
e
IF_B IF_A
≤47 N values less than or equal to 47 are prohibited because IF_B ≥ 3 is required.
48-2
39
Legal divide ratios in this range are:
48-51, 64-68, 80-85, 96-102, 112-119, 128-136, 144-153, 160-170, 176-187, 192-204, 208-221, 224-238
2400000000000011110000
2410000000000011110001
......................
5242
87
1111111111111111111
2.3.4 IF_PD -- IF Power Down Bit
When this bit is set to 0, the IF PLL operates normally. When it is set to 1, the IF PLL powers down and the output of the IF PLL
charge pump is set to a TRI-STATE mode. If the ATPU bit is set and register R0 is written to, the IF_PD will be reset to 0 and the
IF PLL will be powered up. If the CE pin is held low, the IF PLL will be powered down, overriding the IF_PD bit.
www.national.com 28
LMX2485/LMX2485E
2.4 R3 REGISTER
REGISTER 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[19:0] C3 C2 C1 C0
R3 ACCESS[3:0] RF_CPG[3:0] IF_R[11:0] 0 1 1 1
2.4.1 IF_R[11:0] -- IF R Divider Value
For the IF R divider, the R value is determined by the IF_R[11:0] bits in the R3 register. The minimum value for IF_R is 3.
R Value IF_R[11:0]
3000000000011
...............
4095111111111111
2.4.2 RF_CPG -- RF PLL Charge Pump Gain
This is used to control the magnitude of the RF PLL charge pump in steady state operation.
RF_CPG Charge Pump State Typical RF Charge Pump Current at 3
Volts (µA)
0 1X 95
1 2X 190
2 3X 285
3 4X 380
4 5X 475
5 6X 570
6 7X 665
7 8X 760
8 9X 855
9 10X 950
10 11X 1045
11 12X 1140
12 13X 1235
13 14X 1330
14 15X 1425
15 16X 1520
2.4.3 ACCESS -- Register Access word
It is mandatory that the first 5 registers R0-R4 be programmed. The programming of registers R5-R7 is optional. The ACCESS
[3:0] bits determine which additional registers need to be programmed. Any one of these registers can be individually programmed.
According to the table below, when the state of a register is in default mode, all the bits in that register are forced to a default state
and it is not necessary to program this register. When the register is programmable, it needs to be programmed through the
MICROWIRE. Using this register access technique, the programming required is reduced up to 37%.
ACCESS Bit Register Location Register Controlled
ACCESS[0] R3[20] Must be set to 1
ACCESS[1] R3[21] R5
ACCESS[2] R3[22] R6
ACCESS[3] R3[23] R7
The default conditions the registers is shown below:
29 www.national.com
LMX2485/LMX2485E
Re
gis
ter
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Data[19:0] C3 C2 C1 C0
R4 R4 Must be programmed manually.
R5 000000000000000000001011
R6 000000000000000000001101
R7 000000000000100100001111
This corresponds to the following bit settings.
Register Bit Location Bit Name Bit Description Bit Value Bit State
R4
R4[23] ATPU Autopowerup 0 Disabled
R4[17:16] DITH Dithering 2 Strong
R4[15:14] FM Modulation Order 3 3rd Order
R4[12] OSC_2X Oscillator Doubler 0 Disabled
R4[11] OSC_OUT OSCout Pin Enable 0 Disabled
R4[10] IF_CPP IF Charge Pump
Polarity 1 Positive
R4[9] RF_CPP RF Charge Pump
Polarity 1 Positive
R4[8] IF_P IF PLL Prescaler 1 16/17
R4[7:4] MUX Ftest/LD Output 0 Disabled
R5
R5[23:14] RF_FD[21:12] Extended Fractional
Denominator 0 Disabled
R5[13:4] RF_FN[21:12] Extended Fractional
Numerator 0 Disabled
R6
R6[23:22] CSR Cycle Slip
Reduction 0 Disabled
R6[21:18] RF_CPF Fastlock Charge
Pump Current 0 Disabled
R6[17:4] RF_TOC RF Timeout Counter 0 Disabled
R7
R7[13] DIV4 Lock Detect
Adjustment 0Disabled (Fcomp ≤
20 MHz)
R7[7] IF_RST IF PLL Counter
Reset 0 Disabled
R7[6] RF_RST RF PLL Counter
Reset 0 Disabled
R7[5] IF_CPT IF PLL Tri-State 0 Disabled
R7[4] RF_CPT RF PLL Tri-State 0 Disabled
www.national.com 30
LMX2485/LMX2485E
2.5 R4 REGISTER
This register controls the conditions for the RF PLL in Fastlock.
REGISTER 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[19:0] C3 C2 C1 C0
R4 ATPU 0 1 0 0 0 DITH
[1:0]
FM
[1:0] 0OSC_
2X
OSC_
OUT
IF_
CPP
RF_
CPP IF_P MUX
[3:0] 1 0 0 1
2.5.1 MUX[3:0] Frequency Out & Lock Detect MUX
These bits determine the output state of the Ftest/LD pin.
MUX[3:0] Output Type Output
Description
0 0 0 0 High Impedance Disabled
0 0 0 1 Push-Pull General purpose
output, Logical
“High” State
0 0 1 0 Push-Pull General purpose
output, Logical
“Low” State
0 0 1 1 Push-Pull RF & IF Digital Lock
Detect
0 1 0 0 Push-Pull RF Digital Lock
Detect
0 1 0 1 Push-Pull IF Digital Lock
Detect
0 1 1 0 Open Drain RF & IF Analog Lock
Detect
0 1 1 1 Open Drain RF Analog Lock
Detect
1 0 0 0 Open Drain IF Analog Lock
Detect
1 0 0 1 Push-Pull RF & IF Analog Lock
Detect
1 0 1 0 Push-Pull RF Analog Lock
Detect
1 0 1 1 Push-Pull IF Analog Lock
Detect
1 1 0 0 Push-Pull IF R Divider divided
by 2
1 1 0 1 Push-Pull IF N Divider divided
by 2
1 1 1 0 Push-Pull RF R Divider divided
by 2
1 1 1 1 Push-Pull RF N Divider divided
by 2
2.5.2 IF_P -- IF Prescaler
When this bit is set to 0, the 8/9 prescaler is used. Otherwise the 16/17 prescaler is used.
IF_P IF Prescaler Maximum Frequency
0 8/9 800 MHz
1 16/17 800 MHz
31 www.national.com
LMX2485/LMX2485E
2.5.3 RF_CPP -- RF PLL Charge Pump Polarity
RF_CPP RF Charge Pump Polarity
0 Negative
1 Positive (Default)
2.5.4 IF_CPP -- IF PLL Charge Pump Polarity
For a positive phase detector polarity, which is normally the case, set this bit to 1. Otherwise set this bit for a negative phase
detector polarity.
IF_CPP IF Charge Pump Polarity
0 Negative
1 Positive
2.5.5 OSC_OUT Oscillator Output Buffer Enable
OSC_OUT OSCout Pin
0 Disabled (High Impedance)
1 Buffered output of OSCin pin
2.5.6 OSC2X -- Oscillator Doubler Enable
When this bit is set to 0, the oscillator doubler is disabled and the TCXO frequency presented to the IF R and RF R counters is
equal to that of the input frequency of the OSCin pin. When this bit is set to 1, the TCXO frequency presented to the RF R counter
is doubled. Phase noise added by the doubler is negligible.
OSC2X Frequency Presented to RF R Counter Frequency Presented to IF R Counter
0 fOSCin fOSCin
1 2 x fOSCin
2.5.7 FM[1:0] -- Fractional Mode
Determines the order of the delta-sigma modulator. Higher order delta-sigma modulators reduce the spur levels closer to the carrier
by pushing this noise to higher frequency offsets from the carrier. In general, the order of the loop filter should be at least one
greater than the order of the delta-sigma modulator in order to allow for sufficient roll-off.
FM Function
0 Fractional PLL mode with a 4th order delta-sigma modulator
1 Disable the delta-sigma modulator. Recommended for test use
only.
2 Fractional PLL mode with a 2nd order delta-sigma modulator
3 Fractional PLL mode with a 3rd order delta-sigma modulator
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LMX2485/LMX2485E
2.5.8 DITH[1:0] -- Dithering Control
Dithering is a technique used to spread out the spur energy. Enabling dithering can reduce the main fractional spurs, but can also
give rise to a family of smaller spurs. Whether dithering helps or hurts is application specific. Enabling the dithering may also
increase the phase noise. In most cases where the fractional numerator is zero, dithering usually degrades performance.
Dithering tends to be most beneficial in applications where there is insufficient filtering of the spurs. This often occurs when the
loop bandwidth is very wide or a higher order delta-sigma modulator is used. Dithering tends not to impact the main fractional spurs
much, but has a much larger impact on the sub-fractional spurs. If it is decided that dithering will be used, best results will be
obtained when the fractional denominator is at least 1000.
DITH Dithering Mode Used
0 Disabled
1 Weak Dithering
2 Strong Dithering
3 Reserved
2.5.9 ATPU -- PLL Automatic Power Up
When this bit is set to 1, both the RF and IF PLL power up when the R0 register is written to. When the R0 register is written to,
the PD_RF and PD_IF bits are changed to 0 in the PLL registers. The exception to this case is when the CE pin is low. In this case,
the ATPU function is disabled.
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LMX2485/LMX2485E
2.6 R5 REGISTER
REGISTER 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[19:0] C3 C2 C1 C0
R5 RF_FD[21:12] RF_FN[21:12] 1 0 1 1
2.6.1 Fractional Numerator Determination { RF_FN[21:12], RF_FN[11:0], ACCESS[1] }
In the case that the ACCESS[1] bit is 0, then the part operates in 12-bit fractional mode, and the RF_FN2[21:12] bits become do
not care bits. When the ACCESS[1] bit is set to 1, the part operates in 22-bit mode and the fractional numerator is expanded from
12 to 22-bits.
Fra
ctio
nal
RF_FN[21:12]
RF_FN[11:0]
Nu
mer
ator
( These bits only apply in 22- bit mode)
0
In 12- bit mode, these are do not care.
In 22- bit mode, for N <4096,
these bits should be all set to 0.
000000000000
1 000000000001
... ............
409
5
111111111111
409
6
0000000001000000000000
.........................
419
430
3
1111111111111111111111
2.6.2 Fractional Denominator Determination { RF_FD[21:12], RF_FD[11:0], ACCESS[1]}
In the case that the ACCESS[1] bit is 0, then the part is operates in the 12-bit fractional mode, and the RF_FD[21:12] bits become
do not care bits. When the ACCESS[1] is set to 1, the part operates in 22-bit mode and the fractional denominator is expanded
from 12 to 22-bits.
Fra
ctio
nal
RF_FD[21:12] RF_FD[11:0]
Den
omi
nat
or
( These bits only apply in 22- bit mode)
0 In 12- bit mode, these are do not care.
In 22- bit mode, for N <4096,
these bits should be all set to 0.
000000000000
1 000000000001
... ............
409
5
111111111111
409
6
0000000001000000000000
.........................
419
430
3
1111111111111111111111
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LMX2485/LMX2485E
2.7 R6 REGISTER
REGISTER 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA[19:0] C3 C2 C1 C0
R6 CSR[1:0] RF_CPF[3:0] RF_TOC[13:0] 1 1 0 1
2.7.1 RF_TOC -- RF Time Out Counter and Control for FLoutRF Pin
The RF_TOC[13:0] word controls the operation of the RF Fastlock circuitry as well as the function of the FLoutRF output pin. When
this word is set to a value between 0 and 3, the RF Fastlock circuitry is disabled and the FLoutRF pin operates as a general purpose
CMOS TRI-STATE I/O. When RF_TOC is set to a value between 4 and 16383, the RF Fastlock mode is enabled and the FLoutRF
pin is utilized as the RF Fastlock output pin. The value programmed into the RF_TOC[13:0] word represents two times the number
of phase detector comparison cycles the RF synthesizer will spend in the Fastlock state.
RF_TOC Fastlock Mode Fastlock Period [CP events] FLoutRF Pin Functionality
0 Disabled N/A High Impedance
1 Manual N/A Logic “0” State.
Forces all Fastlock conditions
2 Disabled N/A Logic “0” State
3 Disabled N/A Logic “1” State
4 Enabled 4X2 = 8 Fastlock
5 Enabled 5X2 = 10 Fastlock
…Enabled …Fastlock
16383 Enabled 16383X2 = 32766 Fastlock
2.7.2 RF_CPF -- RF PLL Fastlock Charge Pump Current
Specify the charge pump current for the Fastlock operation mode for the RF PLL. Note that the Fastlock charge pump current,
steady state current, and CSR control are all interrelated.
RF_CPF RF Charge Pump State Typical RF Charge Pump Current at 3
Volts (µA)
0 1X 95
1 2X 190
2 3X 285
3 4X 380
4 5X 475
5 6X 570
6 7X 665
7 8X 760
8 9X 855
9 10X 950
10 11X 1045
11 12X 1140
12 13X 1235
13 14X 1330
14 15X 1425
15 16X 1520
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LMX2485/LMX2485E
2.7.3 CSR[1:0] -- RF Cycle Slip Reduction
CSR controls the operation of the Cycle Slip Reduction Circuit. This circuit can be used to reduce the occurrence of phase detector
cycle slips. Note that the Fastlock charge pump current, steady state current, and CSR control are all interrelated. Refer to section
1.8 for information on how to use this.
CSR CSR State Sample Rate Reduction Factor
0 Disabled 1
1 Enabled 1/2
2 Enabled 1/4
3 Enabled 1/16
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LMX2485/LMX2485E
2.8 R7 REGISTER
RE
GI
ST
ER
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Data[19:0] C3 C2 C1 C0
R70000000000DIV
4
0 1 0 0 1 IF_
RS
T
RF
_R
ST
IF_
CP
T
RF
_C
PT
1111
2.8.1 DIV4 -- RF Digital Lock Detect Divide By 4
Because the digital lock detect function is based on a phase error, it becomes more difficult to detect a locked condition for larger
comparison frequencies. When this bit is enabled, it subdivides the RF PLL comparison frequency (it does not apply to the IF
comparison frequency) presented to the digital lock detect circuitry by 4. This enables this circuitry to work at higher comparison
frequencies. It is recommended that this bit be enabled whenever the comparison frequency exceeds 20 MHz and RF digital lock
detect is being used.
2.8.2 IF_RST -- IF PLL Counter Reset
When this bit is enabled, the IF PLL N and R counters are reset, and the charge pump is put in a Tri-State condition. This feature
should be disabled for normal operation. Note that a counter reset is applied whenever the chip is powered up via software or CE
pin.
IF_RST IF PLL N and R Counters IF PLL Charge Pump
0 (Default) Normal Operation Normal Operation
1 Counter Reset Tri-State
2.8.3 RF_RST -- RF PLL Counter Reset
When this bit is enabled, the RF PLL N and R counters are reset and the charge pump is put in a Tri-State condition. This feature
should be disabled for normal operation. This feature should be disabled for normal operation. Note that a counter reset is applied
whenever the chip is powered up via software or CE pin.
RF_RST RF PLL N and R Counters RF PLL Charge Pump
0 (Default) Normal Operation Normal Operation
1 Counter Reset Tri-State
2.8.4 RF_TRI -- RF Charge Pump Tri-State
When this bit is enabled, the RF PLL charge pump is put in a Tri-State condition, but the counters are not reset. This feature is
typically disabled for normal operation.
RF_TRI RF PLL N and R Counters RF PLL Charge Pump
0 (Default) Normal Operation Normal Operation
1 Normal Operation Tri-State
2.8.5 IF_TRI -- IF Charge Pump Tri-State
When this bit is enabled, the IF PLL charge pump is put in a Tri-State condition, but the counters are not reset. This feature is
typically disabled for normal operation.
IF_TRI IF PLL N and R Counters IF PLL Charge Pump
0 (Default) Normal Operation Normal Operation
1 Normal Operation Tri-State
37 www.national.com
LMX2485/LMX2485E
www.national.com 38
LMX2485/LMX2485E
Physical Dimensions inches (millimeters) unless otherwise noted
Plastic Quad LLP (SQ), Bottom View
Order Number LMX2485SQ or LMX2485ESQ for 1000 Unit Reel
Order Number LMX2485SQX or LMX2485ESQX for 4500 Unit Reel
NS Package Number SQA24A
39 www.national.com
LMX2485/LMX2485E
Notes
LMX2485/LMX2485E High Performance Delta-Sigma Low Power Dual PLLatinum Frequency
Synthesizer
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