Specifications
20
Phase noise
Even though we may not be able to see the actual frequency jitter of a
spectrum analyzer LO system, there is still a manifestation of the LO
frequency or phase instability that can be observed. This is known as phase
noise (sometimes called sideband noise). No oscillator is perfectly stable.
All are frequency or phase modulated by random noise to some extent. As
previously noted, any instability in the LO is transferred to any mixing
products resulting from the LO and input signals. So the LO phase-noise
modulation sidebands appear around any spectral component on the display
that is far enough above the broadband noise floor of the system (Figure 2-11).
The amplitude difference between a displayed spectral component and the
phase noise is a function of the stability of the LO. The more stable the LO,
the farther down the phase noise. The amplitude difference is also a function
of the resolution bandwidth. If we reduce the resolution bandwidth by a
factor of ten, the level of the displayed phase noise decreases by 10 dB
5
.
The shape of the phase noise spectrum is a function of analyzer design, in
particular, the sophistication of the phase lock loops employed to stabilized
the LO. In some analyzers, the phase noise is a relatively flat pedestal out to
the bandwidth of the stabilizing loop. In others, the phase noise may fall away
as a function of frequency offset from the signal. Phase noise is specified in
terms of dBc (dB relative to a carrier) and normalized to a 1 Hz noise power
bandwidth. It is sometimes specified at specific frequency offsets. At other
times, a curve is given to show the phase noise characteristics over a range
of offsets.
Generally, we can see the inherent phase noise of a spectrum analyzer only
in the narrower resolution filters, when it obscures the lower skirts of these
filters. The use of the digital filters previously described does not change
this effect. For wider filters, the phase noise is hidden under the filter skirt,
just as in the case of two unequal sinusoids discussed earlier.
Figure 2-11. Phase noise is displayed only when a signal is displayed far
enough above the system noise floor
5. The effect is the same for the broadband noise floor
(or any broadband noise signal). See Chapter 5,
“Sensitivity and Noise.”