Frequency Synthesis With Propeller - Performance?
Drone
Posts: 433
Question...
So each Propeller cog-counter can generate up to 125MHz as a frequency synthesizer (i.e., see the "Frequency Synthesis" Object in OBEX). Obviously, the stability of the output will depend on the stability of the crystal driving the Prop. But my question is what is the jitter and/or phase-noise performance of the Propeller as a synthesizer? Has anyone measured this before? If-so, how?
Regards, David
So each Propeller cog-counter can generate up to 125MHz as a frequency synthesizer (i.e., see the "Frequency Synthesis" Object in OBEX). Obviously, the stability of the output will depend on the stability of the crystal driving the Prop. But my question is what is the jitter and/or phase-noise performance of the Propeller as a synthesizer? Has anyone measured this before? If-so, how?
Regards, David
Comments
Tracy Allen had investigated some spectra.. As he has not so many postings it should be not too difficult to find it.
I remember two threads:
(1) High precision frequency generator (1 Hz @ 100 MHz or a similar nonsense..); could be last year
(2) True waveform generation, with intersting contributions from Phil and me, not so long ago
This one is interesting
http://forums.parallax.com/showthread.php?p=583301
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Aka: CosmicBob
To output a square wave of a certain frequency - say 20 MHz - requires you to toggle the line each 25 ns. Of course that's just a laugh:
And doing 13.3 MHz takes just a NOP inbetween.
Well but what's with the values inbetween? Maybe you need 15MHz for some particular reason...
(a) There is the PLL and the timers which will allow you to generate in a very specific (and small!) frequency range some more values. But there is always this fundamental limit of discrete time..... sooner or later.
(b) Don't look so overcritical. What's in a frequency? What's Montague, what's Ca.. Oh sorry wrong track...
When you consider - say - a milisecond, then for 1 MHz 1000 "Highs" (and consequently 1000 "Lows") have to be transmitted. This is the reason an expensive frequency counter tells you :" 1.000.000 Hz". So why not cheat - just a little bit! One could omit each tenth "peak", so we have 9 Highs and Lows just as above and then an addition gap of 1µs. Who cares?
The stupid frequency counter will say "900.000 Hz".
People with musical ears however will not like it.
That is the background of David's question: How to generate such a detailed signal in the first place (are the counters sufficient?) and will it be "compliant" enough for more things but to trick a stupid frequency counter....
Post Edited (deSilva) : 3/6/2008 12:46:41 AM GMT
I have a good spectrum analyzer. I may have a look at the counter output physically just to see how bad it is.
Thanks for the feedback Gentlemen...
David
Between those values exist options, i.e. you have 230 counter settings to choose from between 40 and 20 MHz. The SIGNAL however will look most of the time just as an 40 MHz signal!
And than - surprise, surprise! - there is a "glitch". And than it goes on with 40 MHz...
You can indeed select: 39,999,999 Hz. The "glitch" distinguishing this from a 40 MHz signal will occur around each second....
Just to make this clear: We are obviously not talking of 40 MHz, but of the whatever clock that is derived from the specific crystal at the given temperature!
It should be perfect if done in quadrature, that is, having sine and cosine phases for each of the two tones,
Otherwise, you will probably end up with a mix of the sum and difference frequencies which will not be
a perfect square wave.
Thus if you can make a pure "tone" within 2.5 Mhz of 27 Mhz, then you should be able to synthesize all the CB frequencies
if you wanted to for a project, for example.