LMX2541
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SNOSB31J –JULY 2009–REVISED DECEMBER 2014
9.3.15.2 Figure 37, LMX2541SQ2690 System Phase Noise Plot Description
For this plot, a third order modulator with dithering disabled was used with a fractional denominator of 500000.
The charge pump gain was 32X and the loop filter components were C1 = 2.2 nF, C2 = 22 nF, R2 = 470 Ω. The
internal loop filter components were C3_LF = 20 pF, C4_LF = 100 pF, R3_LF = 1 kΩ, R4_LF = 200 Ω. The VCO
frequency is 2720.1 MHz. The OSCin signal was a 500 MHz differential LVPECL output of the LMK04033.
9.3.15.3 Phase Noise of PLL
Disregarding the impact of reference oscillator noise, loop filter resistor thermal noise, and loop filter shaping, the
phase noise of the PLL can be decomposed into three components: flicker noise, flat noise, and fractional noise.
These noise sources add in an RMS sense to produce the total PLL noise. In other words:
LPLL(f) = 10·log(10(LPLL_flat(f) / 10 ) + 10(LPLL_flicker (f) / 10 )+ 10(LPLL_fractional(f) / 10 ) (5)
Table 14. Potential Influencing Factors
POTENTIAL INFLUENCING FACTORS
SYMBOL f fVCO fPD KPD FRAC
LPLL_flat(f) No Yes Yes Yes No
LPLL_flicker(f) Yes Yes No Yes No
LPLL_fractional(f) Yes No Yes No Yes
The preceding table shows which factors of offset frequency (f), VCO frequency (fVCO), phase detector frequency
(fPD), charge pump gain (KPD), and the fractional settings (FRAC) can potentially influence each phase noise
component. The fractional settings include the fraction, modulator order, and dithering.
For the flat noise and flicker noise, it is possible to normalize each of these noise sources into a single index. By
normalizing these noise sources to an index, it makes it possible to calculate the flicker and flat noise for an
arbitrary condition. These indices are reported in the electrical characteristics section and in the typical
performance curves.
Table 15. Noise Component
NOISE COMPONENT INDEX RELATIONSHIP
LPLL_flat(f) =
LNPLL_flat
LPLL_flat(f) LNPLL_flat(1 Hz)
(1 Hz) + 20·log(N) + 10·log(fPD)
LPLL_flicker(f) =
LNPLL_flicker
LPLL_flicker(f) LNPLL_flicker(10 kHz)
(10 kHz) - 10·log(10 kHz / f) + 20·log( fVCO / 1 GHz )
The flat noise is dependent on the PLL N divider value (N) and the phase detector frequency (fPD) and the 1 Hz
Normalized phase noise ( LNPLL_flat(1 Hz) ). The 1 Hz normalized phase noise can also depend on the charge
pump gain as well. In order to make an accurate measurement of just the flat noise component, the offset
frequency must be chosen sufficiently smaller then the loop bandwidth of the PLL, and yet large enough to avoid
a substantial noise contribution from the reference and PLL flicker noise. This becomes easier to measure for
lower phase detector frequencies.
The flicker noise, also known as 1/f noise, can be normalized to 1 GHz carrier frequency and 10 kHz offset,
LNPLL_flicker(10 kHz). Flicker noise can dominate at low offsets from the carrier and has a 10 dB/decade slope and
improves with higher charge pump currents and at higher offset frequencies . To accurately measure the flicker
noise it is important to use a high phase detector frequency and a clean crystal to make it such that this
measurement is on the 10 dB/decade slope close to the carrier. LPLL_flicker(f) can be masked by the reference
oscillator performance if a low power or noisy source is used.
An alternative way to interpret the flicker noise is the 1/f noise corner, fcorner. This would be the offset frequency
where the flat noise and flicker noise are equal. This corner frequency changes as a function of the phase
detector frequency and can be related to the flat and flicker noise indices as shown below.
fcorner = 10( (LNPLL_flicker(10 kHz) - LNPLL_flat(1 Hz) - 140) / 10 ) × fPD (6)
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