[time-nuts] HP 11729C versus 11848A

Fri May 7 20:27:46 UTC 2010

With the 11729B/C it's easy to reroute the mixer output directly to the
front panel. The internal mixer is decent, but it can also be easily be
replaced with a high power unit if that's desired. Starting with a box
which needs no S/W to drive it can be useful.

Pete Rawson

On May 7, 2010, at 1:13 PM, John Miles wrote:

> It's not really the mixer's fault -- there are a couple of other things to
> keep in mind with the 11729.  The most important is that you can't measure
> 10 MHz sources with it unless you install an LPF in front of the LNA,
> something with a cutoff on the order of 1-2 MHz.  At 10 MHz and below, the
> USB output from the phase detector at <= 20 MHz will drive the LNA into
> compression.  The manual warns you against using IFs in that range with
> microwave downconversion, but the same problem applies at baseband, and over
> a wider frequency range than they say it does.  Without the additional
> filter in place, you shouldn't try to measure anything under about 30-40
> MHz.
> 
> You will lose the ability to view PN at offsets greater than the LPF cutoff,
> of course, but the 1-10 MHz decade isn't usually very interesting anyway.
> 
> Second, the ALC-limited amplifier in the 11729 seems to have a PN floor in
> the -155 to -160 dBc/Hz neighborhood, while as you noticed the 11848A brings
> its mixer ports directly to the front panel.  (You still have to use
> outboard isolation amps to measure OCXOs with the 11848A, but they can be
> much quieter than what's in the 11729).
> 
> The specs in the 11729 manual are worse because they are talking about the
> best results achievable with microwave downconversion, I believe, and/or the
> best results achievable with downmixing from the 8662A as Christophe says.
> The baseband performance is much better than specified, if you use a clean
> reference, but it's still not quite what you need to measure 10811s and the
> like.  For that you're better off gaining direct access to the mixer.
> 
> It helps to think of the 11729 as a collection of useful modules that can be
> rearranged as needed for specific low-noise measurements.  Adding the
> pre-LNA filter is job #1, IMO.
> 
> -- john, KE5FX
> 
> 
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
>> Behalf Of Christophe Huygens
>> Sent: Friday, May 07, 2010 7:17 AM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] HP 11729C versus 11848A
>> 
>> 
>> 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?
>> 
>> Xtof.
>> 
>> 
>> 
>> 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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