Steve Kavanagh said:
Receiver local oscillator phase noise sidebands at offsets equal to the
receiver's IF frequency can mix in the receiver's mixer with the LO
carrier to produce noise in the IF which reduces the sensitivity of the
receiver. This can be particularly critical for receivers using low IF
or zero-IF (direct conversion architectures). Can anyone direct me to
a good discussion of this particular effect ? Books, papers or web
links would all be of interest, as are both theoretical treatments and
practical experience.
There are several mechanisms by which LO sideband noise can degrade the
receiver performance. In this case, it sounds like you're describing an
effect known as receiver self-quieting.
Remember that the difference between the wanted RF signal and the Local
Oscillator signal is the IF frequency (if it's not, then your receiver
isn't tuned in properly!). That means that noise on one of the LO's
sidebands at plus-or-minus the IF frequency will be at the same
frequency as the wanted RF signal. If that noise component leaks from
the LO port of the mixer to the RF port of the mixer, then that noise is
co-channel with (i.e. on top of) the signal you wish to receive. This
won't be a problem if the signal you wish to receive is much stronger
than the leaked noise, but if it's not, then the receiver's performance
will be degraded by the fact that the LO's sideband noise has been
plonked squarely on top of the signal you wish to receive. The noise at
one-IF-away-from-the-LO will mix with the "proper" LO to produce noise
at the IF, thereby degrading degrading the signal-to-noise ratio, and
degrading the sensitivity of the receiver.
The reason why it's potentially problematic at low-IF or zero-IF
is that the sideband noise of the LO is much higher close-in to the LO
frequency that it is far away. If your IF is several MHz, then the
sideband noise several MHz away from the intended LO frequency is
usually very small, so the noise leakage at the frequency of interest to
the RF port is small. If, however, you've opted for an IF of a few 100
kHz or even 0 Hz (zero-IF), then the noise on the LO one-IF-frequency
away from the LO frequency is much higher, and hence the leaked noise
is much higher, hence higher noise co-channel with the RF signal you're
trying to detect.