On Sun, 12 Jan 2020 14:22:35 -0800, Jeff Liebermann
wrote:

In other words, it's picking up garbage from something in your shop
(or cave). I suggest you start by moving or removing all the
switching power supplies, wall warts, light dimmers, desk lamps, or
gizmos within a few feet that might have a switching power supply
inside. Next, cover the network analyzer with some aluminum foil to
see if the glitches are being delivered via RF.

Yeah, I did try all that and it didn't help, Jeff. Also there's no
glitches or noise when I pull the RF signal away, so at least it's not
something coming from the spectrum analyser.

My desktop wi-fi
access point does that sometimes. If that also fails, drag your
network analyzer and scope to another part of your shop (or cave),
that's away from all the gizmos, and see if the problem persists. In
other words, determine if the garbage is internally generated,
externally radiated, or conducted via the power lines, clip leads,
cables, etc.

I *suspect* noise is somehow getting onto the DC VCO ramp voltage and
manifesting as spikes in the RF output of the VNA accordingly.
Fortunately, there is a jack in the back of the VNA for the
application of an external VCO control voltage, so I plan to avail
myself of that with a big old linear bench supply and see if the
problem goes away.

If you really want to see the noise, glitch, trash, garbage, or
whatever signal, set you scope so that the horizontal trigger is off
and the horizontal sweep is free running. Then, slooooooowly vary the
horizontal sweep until you see fairly stable glitches. If the noise
is periodic, you should be able to see something.

Thanks, Jeff. Duly noted!

--

No deal? No problem! :-D