Breadboarded 74HCT7046 PLL very unstable

Philip Pemberton · 2010-12-27T01:27:13+00:00

Hi guys,I'm trying to recover the bit clock from a 6.9375MHz NRZ bit stream. Tomake things easier, this stream has several encoding rules: - Thanks to the use of parity bits (each 7-bit data byte has an oddparity bit in the LSB), the longest possible sequence with no transitionsis 14 bits long (the Clock Cracker: 1111 1110 0111 1111, repeated untilthe end of the packet). - All packets begin with a 2-byte clock run-in (CRI) and framingsequence. This sequence (including sync) is: 10101010 10101010 11100100.By my reckoning this means: - There must be at least one transition every 14*(1/6.9375e6) = 2.02 us. - Transitions can follow one after the other, as seen in the CRI - Thus the time between two transitions can range from 144ns to 2.02us.What I'm trying to do is put together a PLL-based clock recovery circuit,using a 74HCT7046. I need it to start capturing at the CRI, and ideallylock within 16 bits or less (or at least lock well enough to shift in andmatch the frame sync byte).I've breadboarded a test circuit using a Texas Instruments CD74HCT7046A,1% metal film resistors and ceramic disc capacitors (the round brownones).The component values I'm using are: R1 = 33k R2 = 10k C1 = 100pFAnd for the filter: Series resistor between PC2OUT and VCOIN = 18k Capacitor from VCOIN to ground = 120pF t1 = 2.16us 1/t1 = 462962us... but despite all my calculations (a dozen or so sheets of A4, loads ofPost-it notes, and a good few pages of my lab notebook!) saying "this willwork", I can't get the little swine to lock! Somehow the frequency alwaysends up lower than it should, and the phase detector just won't push thefrequency any higher.Given a static input voltage (VCO_in), the VCO frequency jitters quitebadly. If I turn off the trigger delay, the signal looks reasonably good.Add in a 1us delay (that is, the scope starts displaying 1us after thetrigger) and you can see the square-wave VCO output jumping left and righton the screen.I do have a backup plan: a 13.875MHz...

Dec 27, 2010 82 Replies

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Michael A. Terrell 15 years ago

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Sounds like it will work OK for you.

What are you looking for, all the way down here?

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Michael A. Terrell 15 years ago

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Too bad this isn't smaller, the price is right:

You can't fix stupid. You can't even put a band-aid on it, because it's Teflon coated.

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baron 15 years ago

Michael A. Terrell Inscribed thus:

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That picture is very similar to the "Donut Drills" that I mentioned earlier. Pity I'm on the wrong side of the pond. :-(

Best Regards: Baron.

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