Hi If you are counting on your loop noise to spread your tones out - indeed not a good idea. There are several ways you can “go quiet” in your loop….
Bob On Sep 6, 2014, at 2:10 PM, Magnus Danielson <[email protected]> wrote: > Hi Bob, > > Indeed. The way to keep the MCU PWM doing reasonable stuff is to use a higher > rate, and then update the PWM value in sync with the wrap-around, and then > alter the value (dither or whatever) so that the average has higher > precision. First degree sigma-delta is actually not a bad strategy and fairly > simple to do. > > With FPGA you can do more funky stuff, which is what I do. > > It is even better if you can use a linear DAC of sufficient rate and > resolution. > > If you do hold-over functionality with static steering value, that is you > stop updating the EFC-steering (this is what most folks do), then the > resolution of the full steering can dominate the initial frequency offset. > So, one should think about that too. > > As you go into hold-over, all of a sudden you can run into idle-tones in a > way that normal dynamics would dither out. > > Cheers, > Magnus > > On 09/06/2014 07:01 PM, Bob Camp wrote: >> Hi >> >> One of the easy things to do with PWM is to dither the LSB. That gives you >> one more bit of precision. It still keeps the main tone at the same place. >> >> Your worst case tone happens at 50% duty cycle (perfect square wave). If you >> do your 50/50 as a square wave at Fmax(not Fmin), your fundamental “worst >> tone” is at your highest frequency rather than the lowest. Not easy with MCU >> PWM’s, pretty simple with an FPGA. >> >> By far the best thing to do is to clock your PWM at a nice high frequency >> (like a couple hundred MHz). That way you get lots of bits and your >> fundamental tone is still pretty high. Again, nice for 400 MHz clock FPGA’s, >> not so much for $0.50 MCU’s. >> >> Bob >> >> On Sep 6, 2014, at 12:52 PM, Magnus Danielson <[email protected]> >> wrote: >> >>> Hi Bob, >>> >>> Agreed. I often find that modulations eats your margin out. >>> >>> PWM is interesting in this regard. PWM has the property that the lowest >>> frequency has the highest amplitude and the overtones then decay with 1/f >>> from that. For a given clock rate, as you add a bit of PWM precision, you >>> half the PWM repetition rate and thus move the frequency down... where we >>> are more sensitive to the modulation it causes, and the 1/f slope of the >>> oscillator does not help. >>> >>> I designed a PWM-like signal that has reversed PWM spectrum so that the >>> highest frequency has the strongest amplitude. The 1/f of the oscillator >>> integration makes the modulation flat among the different bits and much >>> easier to handle phase-noise wide. >>> >>> Another approach is sigma-delta style modulation, which noises out the >>> amplitude. Higher-degree sigma-delta needs to avoid idle-tones for optimum >>> result. >>> >>> Thus, paying attention to these details pays of with simplifying the effort >>> to achieve good phase-noise properties. >>> >>> There is more dangers that can occur in PWM-space, but this should be >>> enough of a starting-point. >>> >>> Cheers, >>> Magnus >>> >>> On 09/06/2014 01:39 PM, Bob Camp wrote: >>>> 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 <[email protected]> >>>> 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 -- [email protected] >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
