[time-nuts] Ebay FE-5680A Rb: Are they that good?

[ed breya]

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.

I believe the reason for the discrepancy is that when I designed the 
filter I used a tracking generator, so when the SA passed through the 
harmonics, there was no large fundamental present at the mixer, and 
the true response was shown. When the real signal is present - even a 
very clean sine wave, with or without extra filtering - the SA shows 
fairly large harmonics. The harmonic levels displayed are also 
affected by the mixer input level as expected, but unfortunately, 
cranking down the level makes it run out of dynamic range way before 
it runs out of distortion.

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.

Ed