[time-nuts] HP 11729C versus 11848A

[Adrian]

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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