Stanford Research Systems SR810 Manual page 31

SR810 Basics
but the instantaneous phase shift has a few
millidegrees of noise. This shows up at the output
as noise in phase or quadrature measurements.
Phase noise can also cause noise to appear at the
X and Y outputs. This is because a reference
oscillator with a lot of phase noise is the same as
a reference whose frequency spectrum is spread
out. That is, the reference is not a single
frequency, but a distribution of frequencies about
the true reference frequency. These spurious
frequencies are attenuated quite a bit but still
cause problems. The
frequencies result in signals close to the reference
being detected. Noise at nearby frequencies now
appears near DC and affects the lock-in output.
spurious reference
3-6
Phase noise in the SR810 is very low and
generally causes no problems. In applications
requiring no phase jitter, the internal reference
mode should be used. Since there is no PLL, the
internal oscillator and the reference sine waves
are directly linked and there is no jitter in the
measured phase. (Actually, the phase jitter is the
phase noise of a crystal oscillator and is very, very
small).
Harmonic Detection
It is possible to compute the two PSD reference
sine waves at a multiple of the internal oscillator
frequency. In this case, the lock-in detects signals
at Nxf which are synchronous with the reference.
ref
The SINE OUT frequency is not affected. The
SR810 can detect at any harmonic up to N=19999
as long as Nxf does not exceed 102 kHz.
ref