A few years back I built a phase locked loop circuit very similar to that of Tom Polk which is linked here: http://www.tompolk.com/radios/frequencysynthesizer/freqsynth.html My circuit topology was the same but some of the component values were different and this is not meant to be critical of Tom's design since his may or may not have the problem noted below.
When on the air with this PLL I noticed a background noise which sounded a bit like a wind whistle in a car and was especially apparent when listening to the carrier on a receiver set to CW mode. Typically, such noise is caused by phase jitter (phase noise) and this was confirmed by viewing the lock signal on pin one of the 74HCT4046 with a scope. There was over 1 microsecond of peak to peak jitter and on the spectrum analyser there appeared a strong subharmonic of the carrier at about 580 kHz.
I did a bit of homework and found this site: http://www.sm0vpo.com/ which is linked from Tom Polk's site. On this site follow projects> synthesizer circuits> improved CMOS synthesizer to find a schematic of a similar circuit. The author shows a schematic with a different loop filter connected to pins 9 and 13 of the 4046 which he claims narrows the skirt on the spectrum for the synthesizer. Since the phase jitter I saw would result in a wider skirt I thought it worth while to try this loop filter so it was installed in my PLL and the noise is now gone. The jitter is now less than 40 nanoseconds peak to peak and is inaudible on the transmitter signal and there is no longer a subharmonic in the spectrum.
If you are using or going to build this circuit it seems beneficial to use the loop filter shown on Harry's page.
Dude, that's the issue I'm having! I found that PLL unsuitable to even bother to amplify. It was on a posting started by Horseman of RADIODIRTYSPRING, where he'd built Tom Polk's PLL and also a full transmitter around it.
I was explaining my issues, and a guy came in from Europe, and says he uses a better loop filter, but hadn't gotten back to explain what he did. I'm glad you've confirmed that there's a better loop filter and that it does work. I was thinking of doing low pass filtering with L and C and all kinds of stuff. Here's Horseman's article, you'll see I had a similar problem..
http://www.part15.us/forum/part15-forums/transmitter-talk/my-homemade-pll-transmitter
Nate,
I recalled that thread but thought I would put this in a new thread to make it easier to find. I had tried various values of R and C but that didn't help. For a while I thought maybe there was something wrong with the timing of the TC and PE connection on the 40103 chip but this is a synchronous load and it looked good on the scope. Note also that I am using the 74HCT series ICs rather than the usual CMOS chips which, according to the data sheet for the 4046, would be operating beyond the maximum frequency spec. The HCT series of chips handle this quite nicely though they require a regulated 5V supply for which I placed a 7805 regulator on board. No heatsink was needed since this PLL draws 14 mA and the regulator runs cool with the 8VDC raw input..
The PLL board is part of this transmitter which I had published a while back: http://www.part15.us/forum/part15-forums/transmitter-talk/new-tube-transmitter-air The noise was annoying enough but it was the sub harmonic at about 580 kHz in the transmitted signal that worried me. It is now gone and the spectrum out of the transmitter antenna shows only some harmonics, the largest of which is -40dBc which is just fine to put on the air.
Neil
Brooce