Just the roughly 16 dB of insertion loss caused by the 562 ohm resistor
at the input don't make that filter such a great solution ;-)
So, I brewed something better together...
The 10 kHz beat note is now near 0 dBm when the R input signal is
decreased by 40 dB, which makes a lot more sense than before...
And, I have a noise floor of some -165 to -169 dBc/Hz at +7.5 dBm input,
and I'm seeing the 10811A's some 5 dB above that.
Adrian
John Miles schrieb:
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
correct.
-- john, KE5FX
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