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.
However, a pair of 10811A's (dual oven versions from a Z3801A), both
running +7.5 dBm into 50 ohms, are still just at or below the system
noise floor :-o
The standard 10811A spec is -140 at 100 Hz and -160 at 10k. A 10811D/E
spec sheet specifies -150 dB at 100 Hz, even down to -153 for some
options, though a lesser performance for the 10811-60158 that should be
inside my double ovens. So, their typical phase noise could be just
those 10 dB better than specified. I wish I had some reference
measurements at hands...
Adrian
John Miles schrieb:
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: [email protected] [mailto:[email protected]]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:
[email protected] 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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