I have used an FM tuner pretty successfully to look at modulation and phase noise in oscillators. For a 10 MHz oscillator you will be looking at the 10th harmonic so modulation and phase noise is multiplied and much easier to see. You do need a square wave output to get a lot of harmonics. Sinewave outputs will be pretty low at the 10th harmonic if the oscillator is working well. This does work and I have tried it both on 10 MHz and 5 MHz oscillators with some success. It's not a replacement for a real phase noise analyzer but its way cheaper and adequate to spot real problems.
The math to transform the output of a tuner into quantifiable phase noise was more than I had patience for. The low frequency limit is in the 5-10 Hz range. The AFC of a good tuner will eliminate most everything below that frequency. More details here http://www.diyaudio.com/forums/blogs/1audio/983-fm-tuner-jitter-analysis.html >Subject: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) >Message-ID: <ccc8bc9e-c7af-4965-88c5-d3d21b41d...@n1k.org> >Content-Type: text/plain; charset=windows-1252 >Hi >Yes indeed, as you go below 1 Hz (or 1 radian/sec) all the things that “help” >you roll off wise now hurt you. If you are worried about sidebands inside 1 >Hz, you need to change a sign here and there. The >only thing that saves you >is that the noise floor is now coming up pretty fast. >If you modulate a crystal oscillator, the loaded frequency of the crystal is >changed to accomplish the modulation. When your FM swings 100 Hz high, your >crystal is tuned 100 Hz high. When your >modulation swings 100 Hz low, your >crystal is tuned 100 Hz low. The Q has no impact in this case. No I did not >believe it worked that way until I did it …. Since then I’ve built a *lot* of >VCXO’s with >modulation bandwidths >> than their crystal Q bandwidths. The >biggest problem comes from crystal spurs rather than crystal Q. >Bob >On Sep 6, 2014, at 6:09 AM, Magnus Danielson <mag...@rubidium.dyndns.org> >wrote: > Bob, > > On 09/06/2014 03:00 AM, Bob Camp wrote: >> Hi >> >> Oddly enough (and yes it is odd) you can modulate an oscillator well outside >> the crystal’s bandwidth. The bigger issue is that the EFC does not pull the >> crystal very far on a normal OCXO. The FM modulation index drops to very >> small numbers pretty fast as you go up in modulation frequency. >> >> You typically only worry about modulation sidebands that are above the phase >> noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM >> modulator (for small modulation index) they get pretty low pretty fast. >> >> If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p >> (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index >> of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite >> large. This gets back to the “this all depends on what you are doing”. >> >> If your EFC is 5V, a reasonably quiet signal would have noise below 0.5 mV. >> That’s already 80 db down. A very quiet supply should be in the < 5 nV / >> sqrt(Hz) range. That would put the noise down 180 db. >> >> It’s unlikely that your OCXO has a phase noise spec of -180 dbc / Hz >> at 10 Hz. We may already be done … >> >> To bring all the numbers together: >> >> At 1 Hz the modulation will do a sideband X db down at your desired >> frequency. >> >> You will drop 20 db by the time you get to 10 Hz simply due to the 1/F >> FM->PM. > > Since the oscillator integrate frequency into phase, you have a 1/(2*pi*f) > factor. The typical LaPlace model for an oscillator is Ko/s, where Ko is the > input sensitivity of the oscillator. > A more complete model needs to include the Q of the crystal, naturally, > unless you are "in-band" of that Q where it has less drastic properties. > >> Bottom line - it’s not all that hard to get a quiet enough EFC voltage. > > Agreed. > > I've found that thinking about systematic noises of low frequency (i.e. > comparator frequency and overtones) as well as loop dynamics is what one > should think about. Lack of DAC resolution hurts. > > Cheers, > Magnus > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. ******** _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
