[time-nuts] HP 11729C versus 11848A

John Miles jmiles at pop.net
Fri May 7 23:41:21 UTC 2010

> John,
> I'd say you nailed it.
> After some more testing, I can confirm that the limiter amp and the LPF
> are the culprit.
> I opened the box and plugged directly into the mixer LO port.
> And, for the LPF, as a quick 'n dirty solution, I connected the <1 MHz
> front panel output with the LNA input.
> Now, at 10 dBm each into the mixer ports, I'm getting a noise floor of
> <-145 dBc/Hz at 100 Hz and about -170 dBc/Hz at 10 kHz and above.

That's pretty aggressive for 10811s.  The floor on those is usually
around -165 dBc/Hz.  How's your calibration process -- are you accounting
for the 600 ohm output Z of the <1 MHz output port?  It'll lose a few dB if
you try to drive 50 ohms with it, and/or the filter response won't be

-- john, KE5FX

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