Thinking out loud and reading a bit on huff puff stabilizing circuits. I am just wrapping my head around simple PLL circuits like that of the old 74HCT4046 PLL IC.
Wouldn't it be possible to take a 10kHz square signal to get the 10kHz steps required for US MW broadcast stations (figure 4060 IC dividing a 10.240MHz crystal by 1024 to get 10KHz) and then inject the 74HCT4046 PLL with the original VFO circuits frequency and compare it against the 10kHz channel spacing and correct the output somehow? Maybe varicap in line with the tunable capacitor?
I think it would be interesting to retrofit an AM transmitter based on the VFO used that is analog tunable and force it into 10kHz steps when tuning the VFO dial.
Has anyone here tried such a thing or have further information?
I am debating if I want to try such an experiment or just rip out the VFO board altogether and replace it with a proper PLL circuit which I also have on my drawing board of projects.
The VFO I use in my AM TX (ChugAM) transmitter is quite stable, but I would love to convert it over to PLL eventually. I am thinking if the above "mod" isn't possible I will just replace the VFO with a simple 74HCT4046 PLL, CD4060 oscillator/divider to get the 10kHz reference, and a 4 bit binary down counter to get 15 channels from 1710kHz to 1560kHz broadcast range. Of course I could use a 8 bit down counter to get far more range if I wanted but I stick to the top of the AM dial anyways so KISS principal comes into play.
I will update this with any progress if my time permits to mess around on the workbench
Actually any interesting information about PLLs for MW is welcome here. There is so little information out there on circuits and designs using very basic old IC chips to get a stable PLL oscillator, or even converting VFO designs to PLL designs, aka retrofitting old rigs easily. I guess huff & puff circuits are a step between, they keep the VFO but force it to step frequency rather than be continuously variable even with the LC circuit.
Looks like you need a couple of PLL Circuits! Here we go:
This synthesiser - using an old CB favourite PLL chip - gives a nice sinewave. I've used this to drive valve ("tube" - US) rigs. The purity is exceptional, and it's beautifully stable. You can put the rock into an "oven" if you're really paranoid about frequency accuracy.
The second synthesiser generates a squarewave output - great for driving big FETs into Class E. It uses the VCO in the 4046 as the carrier oscillator. You can use 10.240MHz crystal (easier to get) for 10kHz channels if you take the 4060 output from pin 15 instead of pin 13. I always buffered the output from the 4046 oscillator with all the gates of a 4049 wired in parallel - this will give a "stiff" enough drive to feed big FETs.
As you can see, the components are cheap and readily available.
Kage - don't bother with Huff & Puff circuits - they're a real pain in transmitters (OK for receivers, though).
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