Datasheet

LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
www.ti.com SNOSB31I –JULY 2009–REVISED FEBRUARY 2013
Noise Component Index Relationship
L
PLL_flat
(f) =
LN
PLL_flat
L
PLL_flat
(f) LN
PLL_flat(1 Hz)
(1 Hz)
+ 20·log(N) + 10·log(f
PD
)
L
PLL_flicker
(f) =
LN
PLL_flicker
L
PLL_flicker
(f) LN
PLL_flicker
(10 kHz)
(10 kHz)
- 10·log(10 kHz / f) + 20·log( f
VCO
/ 1 GHz )
The flat noise is dependent on the PLL N divider value (N) and the phase detector frequency (f
PD
) and the 1 Hz
Normalized phase noise ( LN
PLL_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,
LN
PLL_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. L
PLL_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, f
corner
. 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.
f
corner
= 10
( (LN
PLL_flicker
(10 kHz) - LN
PLL_flat
(1 Hz) - 140) / 10 )
× f
PD
Based on the values for LNPLL_flicker(10 kHz) and LNPLL_flat(1Hz) as reported in the electrical specifications,
the corner frequency can be calculated. For example, one of the plots in the typical performance characteristics
shows the phase noise with a 100 MHz phase detector frequency and 32X charge pump gain. In this case, this
corner frequency works out to be 0.000123 × 100 MHz = 12.3 kHz.
K
PD
LN
PLL_flicker(10 kHz)
LN
PLL_flat(1 Hz)
f
corner
1X -116.0 dBc/Hz -220.8 dBc/Hz 0.000302 × f
PD
32X -124.5 dBc/Hz -225.4 dBc/Hz 0.000123 × f
PD
For integer mode or a first order modulator, there is no fractional noise (disregarding fractional spurs). For higher
order modulators, the fractional engine may or may not add significant phase noise depending on the fraction
and choice of dithering.
Impact of Modulator Order, Dithering, and Larger Equivalent Fractions on Spurs and Phase
Noise
To achieve a fractional N value, an integer N divider is modulated between different values. This gives rise to
three main degrees of freedom with the LMX2541 delta sigma engine: the modulator order, dithering, and the
way that the fractional portion is expressed. The first degree of freedom, the modulator order, can be selected as
zero (integer mode), one, two, three, or four. One simple technique to better understand the impact of the delta
sigma fractional engine on noise and spurs is to tune the VCO to an integer channel and observe the impact of
changing the modulator order from integer mode to a higher order. A higher fractional modulator order in theory
yields lower primary fractional spurs. However, this can also give rise to sub-fractional spurs in some
applications. The second degree of freedom is dithering. Dithering seeks to improve the sub-fractional spurs by
randomizing the sequence of N divider values. In theory, a perfectly randomized sequence would eliminate all
sub-fractional spurs, but add phase noise by spreading the energy that would otherwise be contained in the
spurs. The third degree of freedom is the way that the fraction is expressed. For example, 1/10 can be expressed
as a larger equivalent fraction of 100000/1000000. Using larger equivalent fractions tends to increase
randomization similar to dithering. In general, the very low phase noise of the LMX2541 exposes the modulator
noise when dithering and large fractions are used, so use these with caution. The avid reader is highly
encouraged to read application note 1879 for more details on fractional spurs. The following table summarizes
the relationships between spur types, phase noise, modulator order, dithering and fractional expression.
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E