[time-nuts] HP 11729C versus 11848A

[Adrian]

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