John,
aside of having added the cal level reading wrongly (oops) which adds
some +6 dB to the (+10 dBm input level) noise floor and some +10 dB to
my 10811 readings, so they look a lot more reasonable, I haven't yet
checked the LPF filter response. The corrected 2x 10811 figures ar now
-150 at 100 Hz, -156 at 1 kHz, -159 at 10 kHz and -161 at 100 kHz... 1 MHz.
Calibration for the 10811's was at +7.5 dBm with one of them substituted
by a signal generator at 10.01 MHz, in order to create a 10 kHz beat
note. The signal generator was connected to the R input, and the level
decreased by 30 dB. The measured 10 kHz amplitude was -5.3 dBm.
-(-5.3 dB) +(-30 dB) +(-6 dB) gives a reference level offset of -30.7 dB
for the RF spectrum analyzer. I'm using a R&S FSEA30 and a HP 3582A.
With the noise marker function and ref level offset, the FSEA marker
displays directly in dBc/Hz (it corrects automatically for the log amp,
bandwidth and detector). On the FFT analyzer, I set the ref level as
described in the manual, and added -30 dB -6 dB = -36 dB to the
readings. Both analyzers measure the same values within +/- 1...2 dB.
Just the FSEA is a bit tricky at 10 and 100 Hz offset.
Finally, I checked the filter response, and it looks ok to me, being
within +/- 0.4 dB up to 750 kHz, and +1 dB at 1 MHz, measured with the
beat note.
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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