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DSP, PLL and synchronization

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kirgizz

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dsp pll

Hello,

I want to synchronize an internally generated (in DSP) sine wave with an external TTL signal.
Doing it I want to generate a sine with the same frequency according to the TTL signal that is fed to my board.

I used to measure the period of the external TTL signal (Timer) and generate the sine digitally in my DSP.
While doing it I encountered a problem: a big phase noise (not precise measuring?).
That's why I'm thinking of using PLL (xx4046 series).


I googled a lot and found a pair of ideas.

Here is my simple scheme:

TTL-> SIGin(PLL) -> PC2 (PLL)-> Loop Filter -> ADC -> Phase and Sine (DSP) -> DAC -> Comparator -> COMPin (PLL)


So I don't use VCO in my design (rather DSP as NCO). And the reference requency (SIGin) is 10Hz...20kHz.
The frequency synchronization must be very exact (+- 0.001 Hz). Loop filter used is a passive RC-LPF.

But I'm in doubt whether this solution is good for my design.
Firstly, I've read about some problems related to PFD such as "dead band".
Secondly, using the PC2out I see a strange signal (nothing like the datasheet shows me).
It's 200 mVpp and is not biased at the high impendance level but at oscilates from -100 mV...+100mV.
If I use the loop filter on the output I can't see anything, noise only at 5-10 mV.
And thirdly, a question: is PFD (PC2) suitably for an operation in the mentioned frequency range?


Of course, there is a possibility to implement PLL digitally as software PLL.
I read a pair of articles about SPLL. SPLL are pretty complex to implement, are they?
And they are suitable for lower frequencies (is 20kHz too much?).




Another solutions for this task are welcome.

Thank you for your attention.


Regards,
kirgizz
 

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