El 09/12/2011 17:51, ed breya escribió:
When looking at harmonic distortion of the 10 MHz, also consider the
distortion of the SA itself. I rebuilt my filter made from LAN
components that I described a while back, and achieved about 85 dB
rejection from 30 MHz to 70 MHz, which should have made harmonics of
even a crappy sine wave all but disappear. When I put actual signals
through it, I was shocked to see that it hardly made a difference - it
seems that the SA mixer provided way more distortion than what I was
looking for, making it impossible to tell if the filter worked.
Good point. I've checked it and the 8566B is spec for 2nd harmonic at
<-70dBc (100Hz-50MHz, 50-700MHz) and <-80dBc (50-700MHz), but at <-40dBm
mixer input. In my measurement, mixer input ins around -14dBm, and the
second harmonic is a around -68dBc, so not bad. I can compare also with
a Tek 495 (I first did a quick check of the Rb output with it, because I
have it at a nearer distance in the lab...) but as far as I remember,
results were more or less the same.
I gave up trying to measure the filter response to actual signals at
that point, until I review the 8566B specs. I assume that performance
limit was one of the reasons why HP offered a low-band preselector.
I think that the low-band preselector (I don't remember the model) was
more oriented to increase the dynamic range for EMC testing, rather than
due to a crappy harmonic response of the analyzer. It also includes a
preamplifier.
Regards,
Javier
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