On Thu, Jul 14, 2011 at 4:00 PM, RussellMc <apptechnz@gmail.com> wrote:

> > I am trying to make a PLL like scheme using PIC 16F887 based on stored
> sine
> > table values. I have stored 256 step values of sine angles. Its working
> well
> > with the counter increasing in each TMR0 interrupt at certain intervals=
..
> > However the problem now is: how to synchronize with some arbitrary inpu=
t
> > waveform or arbitrary frequency (changed to square wave using
> comparator).
>
>
> Assume:
>
> i  50 Hz mains.
>
> ii Input waveform is reduced to two opposite sense zero crossings per cyc=
le
>
> iii User knows how to determine period of input waveform using
> available zero crossing information.
>
> Then:
>
> Determine period of input waveform.
>
> 1. You can gross-synchronise by just starting your sine wave on its
> rising edge at the negative to positive zero crossing. I think you
> already know that.
>
> 2. If you have a synthesised  sinewave already being output and you
> want to bring it into synchronisation with an external waveform of the
> same frequency then you MUST slip frequency either + or - in order to
> change phase. eg if you are say zero crossing 30 degrees too late you
> need to progressively advance  each of your zero crossings by a
> suitably small amount until you catch up. eg you could decide that an
> OK rate was 1 degree per cycle. By effectively speeding up to 360/359
> x 50 Hz =3D 19,944 uS/cycle instead of 20,000 uS you will catch up in 30
> cycles +` 0.6 seconds. How fast is acceptable is up to you.
>
> 3. Synchronising with phase offset is just a matter [tm] of
> determining where the desired phase point is wrt the input zero
> crossings and steering for that as above.
>
> 4.  To maintain running synchronisation you can monitor input zero
> crossings wrt output zero crossings and make decisions about changing
> output sampling rate as above. Simplest is just add or subtract an
> extra delta count to compensate for observed error. Nicer is to
> introduce correction which takes several cycles to correct. You could
> end up at something like full PID and also introduce input frequency
> filtering in a particularly obnoxious environment but this is unlikely
> to be needed.
>
>
>          Russell McMahon
>          Applied technology
>  --
> http://www.piclist.com PIC/SX FAQ & list archive
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>


*If you have a synthesised  sinewave already being output and you
want to bring it into synchronisation with an external waveform of the
same frequency then.....*
**
How can I generate an output sinewave of the same frequency as that of the
input sinewave? The frequency of the input sinewave although remains steady
over certain duration it may vary by +-10%.

Regards,
Deep.
--=20
 "*Don't worry about people stealing your ideas. If your ideas are any good=
,
you'll have to ram them down people's throats*."----- Howard Aiken
--=20
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