LMX9830
SNOSAU0C –MAY 2008–REVISED JUNE 2015
www.ti.com
9.3.10.3.2 Post-Detection Filter and Equalizer
The output signals of the FM discriminator first go through a post-detection filter and then through an equalizer.
Both the post-detection filter and equalizer are tuned to the proper frequency by the autotuning circuitry. The
post-detection filter is a low-pass filter intended to suppress all remaining spurious signals, such as the second
harmonic (4 MHz) from the FM detector and noise generated after the limiter.
The post-detection filter also helps for attenuating the first adjacent channel signal. The equalizer improves the
eye-opening for 101010 patterns. The post-detection filter is a third order Butterworth filter.
9.3.10.4 Autotuning Circuitry
The autotuning circuitry is used for tuning the bandpass filter, the detector, the post-detection filter, the equalizer,
and the transmit filters for process and temperature variations. The circuit also includes an offset compensation
for the FM detector.
9.3.10.5 Synthesizer
The synthesizer consists of a phase-frequency detector, a charge pump, a low-pass loop filter, a programmable
frequency divider, a voltage-controlled oscillator (VCO), a delta-sigma modulator, and a lookup table.
The frequency divider consists of a divide-by-2 circuit (divides the 5 GHz signal from the VCO down to 2.5 GHz),
a divide-by-8-or-9 divider, and a digital modulus control. The delta-sigma modulator controls the division ratio and
also generates an input channel value to the lookup table.
9.3.10.5.1 Phase-Frequency Detector
The phase-frequency detector is a 5-state phase-detector. It responds only to transitions, hence phase-error is
independent of input waveform duty cycle or amplitude variations. Loop lockup occurs when all the negative
transitions on the inputs, F_REF and F_MOD, coincide. Both outputs (that is, Up and Down) then remain high.
This is equal to the zero error mode. The phase-frequency detector input frequency range operates at 12 MHz.
9.3.10.6 Transmitter Circuitry
The transmitter consists of ROM tables, two Digital to Analog (DA) converters, two low-pass filters, IQ mixers,
and a power amplifier (PA).
The ROM tables generate a digital IQ signal based on the transmit data. The output of the ROM tables is
inserted into IQ-DA converters and filtered through two low-pass filters. The two signal components are mixed up
to 2.5 GHz by the TX mixers and added together before being inserted into the transmit PA.
9.3.10.6.1 IQ-DA Converters and TX Mixers
The ROM output signals drive an I- and a Q-DA converter. Two Butterworth low-pass filters filter the DA output
signals. The 6-MHz clock for the DA converters and the logic circuitry around the ROM tables are derived from
the autotuner.
The TX mixers mix the balanced I- and Q-signals up to 2.4-2.5 GHz. The output signals of the I- and Q-mixers
are summed.
9.3.10.7 Crystal Requirements
The LMX9830 contains a crystal driver circuit. This circuit operates with an external crystal and capacitors to
form an oscillator. shows the recommended crystal circuit. Table 13 specifies system clock requirements.
The RF local oscillator and internal digital clocks for the LMX9830 is derived from the reference clock at the
CLK+ input. This reference may either come from an external clock or a dedicated crystal oscillator. The crystal
oscillator connections require an Xtal and two grounded capacitors.
It is also important to consider board and design dependant capacitance in tuning crystal circuit. Equations that
follow allow a close approximation of crystal tuning capacitance required, but actual values on board will vary
with capacitive properties of the board. As a result, some fine tuning of crystal circuit that must be done that
cannot be calculated; tuning must be done by testing different values of load capacitance.
22 Submit Documentation Feedback Copyright © 2008–2015, Texas Instruments Incorporated
Product Folder Links: LMX9830