Hi everyone and thanks for the suggestions!
I rummaged around in my junk box and found a high quality ovenized 10 MHz oscillator that I pulled out of an organ donor HP signal generator last year. I mounted that bad boy in a box and powered it up with two 9-volt batteries (The oven is off so its slightly off frequency). The plot below shows what it looks like on the 8566B. Thats the cleanest signal Ive ever seen from anything in my shop. The 8566B says the SSB phase noise at a 5 kHz offset is -106.7 dBc/Hz.
I then looked at it on the SDR-14 which is powered from a wall wart and it had lots of 120 Hz sidebands. I whipped up a power cord so I could run the SDR-14 from a lab bench supply, and guess what... Sidebands gone! Its a loaner so I wont use it after this weekend.
Next I went to my SDR-IQ half way expecting to have to dig into it to fix ripple since its powered from the PCs USB port, but it was clean. I had to stick a 21.5 dB pad in the line to keep from overloading the SDR-IQ so the levels are relative.
Now, were starting to cook with Gas! My plan is to take the battery-powered oscillator to work Monday and see if I can check phase noise using something expensive and designed for the job. Then Ill have a standard I can bring back here for comparison. Once we iron out this last ambiguity we should all have confidence when we get to the Rev-B receiver testing.
73, and thanks for the help!
Juan
-----Original Message----- From: Bill Ress [mailto:[email protected]] Sent: Saturday, July 07, 2007 6:14 PM To: [email protected] Cc: [email protected] Subject: Re: Help Needed
Hi Juan,
What your likely seeing when looking at your low noise 10 MHz signal is
the phase noise floors of the three devices your using for measurement.
For instance, the 8566B has a typical phase noise floor at 100 to 320 Hz
of around -95 dBc/Hz and it's spec'ed at -80 dBc/Hz (yours might be
lower) which is essentially the noise of the phase locked YIG first LO
in the 8566B and you won't be able to resolve any thing lower than that,
at those frequency offsets. The -85dBc spur at 120 Hz could very well be
the 120 Hz spur on the 8566B's LO's. I can't recall what the power line
spurious spec is for the 8566B but it should be in your manual.
I can't speak for the SDR's, not having played with them, but I would
guess that their phase noise will be much better than the 8566B. Hence
you would read a different value than obtained on the 8566B. So what now
- right?
Well, from the data you've shown earlier for the 10.7 MHz output of the
URx, its phase noise is greater than that of the 8566B's phase noise
floor (not to be confused with its "sensitivity floor"). Hence the
reading you get on the 8566B is the phase noise of the combined LO's in
the URx and the phase noise of your 435 MHz signal generator (and let's
assume its at least 10dB lower than the phase of the URx LO's and hence
you can ignore it) . Now if you take that data on the 8566B and then run
the URx 10.7 output into the SDR's, you should get the same phase noise
readings (or very close to it).
In other words - if the expected phase noise of the device under test is
greater (by at least 10dB) than the phase noise floor of the measuring
device, the measurement is going to be accurate enough for our application.
Clear as mud - huh? Phase noise measurements are not straight forward in
most cases unless you have a "phase noise analyzer" ala HP or RDL.
Regards...Bill - N6GHz
Juan Rivera wrote:
Hello All,
I'm trying to arrive at a reliable method of measuring phase noise for
the testing I'm doing on the 70 cm Eagle Receiver.
I recently purchased an SDR-IQ software defined radio. It makes a
great spectrum analyzer below 30 MHz, with a very low noise floor and
great resolution. I took it to work to show to my coworkers and the
lab manager liked it so much he purchased the big brother of this
unit, the SDR-14, which I borrowed and brought back to my shop. I also
sent my Agilent 8566B spectrum analyzer out for calibration so now I
have three pieces of test equipment that I can use to measure the
phase noise of the 70 cm Receiver. My problem is that I cant get any
two of them to agree...
Ive fed the same 10 MHz 0 dBm signal into each unit, recorded all of
their settings and created the attached PDF file. As you will see,
they all differ. If we look at the 120 Hz power supply sidebands you
get the following:
SDR-IQ: -102 dBm
SDR-14: -93 dBm
8566B: -84 dBm
The 8566B automatically calculates RMS noise levels normalized to a 1
Hz noise power bandwidth by correcting for the analyzers log
amplifier and detector response and compensating for the resolution
bandwidth setting. Its capable of accurately measuring noise levels
down to 10 dB above the spectrum analyzers noise level (-131.6 dBm)
and reading out in steps of ±0.1 dB. At a 150 Hz offset it measures
-104.0 dBc/Hz. Looking at the two SDR units its hard to reconcile
this measurement with either of them, or to reconcile any of the power
supply sidebands.
Both SDR units measure power levels in a very linear way. Ive
confirmed their accuracy at the levels Im interested in around -90 dBm.
Can anyone shed any light on this? No combination of FFT block size,
span, RF or IF gain, or filter bandwidth can make any two of these
devices agree on the noise floor or the amplitude of the power supply
sidbands.
73,
Juan WA6HTP
/A man with a watch will always know what time it is - a man with two
watched can never be sure./
