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Altera_Forum
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16 years ago

PLL megacore probs.build my own??

Hi all, I want to implement a PLL that will lock onto & track an incoming signal of about 30KHz and output a multiple of that locked frequency. I tried using Quartus's PLL megacore function but...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
16 years ago

Does a pure ditital pll still need a low pass filter? I cant seem to acquire lock with the pll I've constructed Im not sure why since all the connections make sense. the XOR output gives basically peaks or 1's when there is a difference between the incoming clock and the output of my hardlimiter (i havent implemented the phase accumulator suggestion yet). The thing with this is that even if the two XOR inputs have the same frequency, if theyre not in phase there will be a train of 1's at the XOR output, which (at the moment) I put through a low pass filter (BW = 34KHz) and the output of that I input into the frequency modulator input of the NCO.

Do I have the wrong configuration perhaps? Another thing I've tried is inputting the LPF output into the PHASE modulator input of the NCO, hoping for a phse shift to allign the two clocks so that the XOR output would eventually die out to zero. This also failed. Im not sure where to go from here.