using PFD only

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 googled a lot and found a pair of ideas.

Here is my scheme: -----xx4046------- ----DSP----- TTL -->|SIGin PC2out|--> LPF--> ADC--> | | | PFD | | |phase-koeff| |->|COMPin | | | | | | | | | sine | | ------------------ ------------- | | ----------Comparator

Sorry,

my scheme was disturbed. I post a simpler one here.

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

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.

To John: Thank you for the answer. I'm sorry John, I can't write the answer directly. I'm using web-based version and the button "Replay to the Author" doesn't work. Yes, 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? They are suitable for lower frequencies (is 20kHz too much?).

Regards, kirgizz

If you used one of the Analog Devices direct digital frequency synthesis (DDS) chips

as your digitally controlled oscillator - the AD9833 is pretty cheap and if you can't live with the phase noise from a 25MHz synthesis clock, the 400MHz AD9859 isn't all that expensive - you'd get a good bit less phase noise than you'd get out of your DSP chip.

You might want to use a good analogue phase-detector and digitise its output to close the phase/frequency control loop - a product detector based on a good analog multiplier should give you the least noisy control signal.

The control loop does represent a trap for young players - you have to think of the loop as a phase control loop, which makes your frequency-controlled oscillator a phase integrator.

Floyd M. Gardner's "Phaselock Techniques" ISBN: 0-471-43063-3 covers the subject well, as do many, more modern, sources.

------------- Bill Sloman, Nijmegen