[time-nuts] HP 11729C versus 11848A

Christophe Huygens christophe.huygens at cs.kuleuven.ac.be
Fri May 7 14:17:26 UTC 2010

I don t have my notes here, but are these numbers
not for the 11729C system including the 8662A
rather than for the unit standalone? So as long
as you are measuring > 20-30 MHz the difference
848/729 should be much smaller than indicated,
or be attributed to the ref?


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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Christophe Huygens Dept. Computer Science E-mail:
Christophe.Huygens at cs.kuleuven.be Celestijnenlaan 200 A. bus 2402 B-3001
Leuven, Belgium Tel: +32 16 32 70 88, Fax: +32 16 32 79 96

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