[time-nuts] HP 11729C versus 11848A

Richard (Rick) Karlquist richard at karlquist.com
Fri May 7 15:16:28 UTC 2010

What the 10811 production line did was to compare two 10811's
to each other by driving a high level mixer.  Anzac AM-123
amplifiers were used to increase the output level of the 10811's.
You can homebrew the AM-123 if you read the patent and can
get a 2N5109/2N5943 type of transistor.  Amplify the mixer
output with an LT1028/LT1128 type of low voltage noise op amp.

The ll729 is a microwave downconverter that is basically irrelevant
to measuring 10811's.  It didn't need to be very good because
an 8662 was used, which meant that the phase detector was
always working with noisy inputs (compared to the 10811).

Rick Karlquist

Adrian wrote:
> I tried to measure phase noise of a 10811A, but found out that the 
> specified PN is below the noise floor of my 11729C.
> Can anyone tell why the (phase detector method) PN noise floor is so 
> much different between the two units?
> 11729C at 100 Hz -126 sBc/Hz (-133 dBc/Hz typ.)
> 11729C at 1 kHz -135 dBc/Hz (-140 dBc/Hz typ.)
> 11848A at 100 Hz -150 dBc/Hz (-160 dBc/Hz typ.)
> 11848A at 1 kHz  -160 dBc/Hz (-170 dBc/Hz typ.)
> Some 25 dB is quite a difference, isn't it?
> Basically, both units apper to be not that much different, except that 
> the 11729C has an IF amp and power splitter between the input and the PD 
> L port, while on the 11848A the L input is fed directly into the mixer. 
> There are some differences in the LNA circuits, but that shouldn't be 
> responsible for the huge noise floor difference.
> 11929C requires 0 dBm (-5...+10 dBm) 'L' (MW Test Signal) input level, 
> that is amplified by the IF amp to >+10 dBm at the mixer input. Btw. the 
> IF amp saturates at input levels grater than -50 dBm. For the 'R' input 
> (5-1280 MHz), the manual specifies -1...+1 dBm.
> For the 11848A, the L input is +15...+23 dBm, and 0...+23 dBm at the R 
> input. Below +15 dBm L and R, the system degrades considerably. Reducing 
> 'L' to +7 dBm adds 10 dB to the noise floor. Reducing 'R' below +15 dBm 
> adds directly to the noise floor. So, reducing it to 0 dBm would add 15 
> dB to the noise floor.
> So, it looks like the 11729C phase detector is more like a +10 dBm 
> mixer, while the 11848A has a +17...+23 dBm mixer.
> Replacing the 11729C PD with a ultra high level mixer should get the 
> noise floor close to 11848A specs. It would just require to feed L and R 
> directly into the mixer rather than using the instrument inputs.
> Any thoughts / experiences referring to this?
> Adrian
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