What Would It Take to Build a Tube PLL Circuit?
Wanting to follow in Neil's footsteps, I have a dream of building a medium wave transmitter using 100% tube technology. Step 1 is to design a stable oscillator.
As I understand it, basic MW frequency crystals have become difficult to obtain, so another method must be utilized to obtain frequency stability... such as Phase Locked Loop (PLL).
PLL circuits are common in solid-state form, but can they be built with tubes?
It is possible to make a mw frequency with two sw x-tals. You can make two oscillators en couple them. The difference (in frequency) between the two x-tals kan be used for a mw frequency. Use of a LPF is recommanded.
Rob Veld
That's it! Thank you Rob Veld for the wonderful msuggestion! Yes, TWO oscillators, TWO SW crystals, and a mixer circuit with Low Pass Filter (LPF)!
Now we choose the tube types for doing this.
Mixing two signals as suggested would work but would require finding two crystals of the appropriate frequencies whose sum or difference equals the desired AM frequency.
Another approach is to select a crystal frequency which is a sub-multiple of the desired frequency and multiply it or one of a higher multiple and divide it. Unfortunately, finding such crystals is as difficult as finding those cut to the desired frequency.
Try a web search on DDS (direct digital synthesis) modules.which provide generation of almost any frequency at modest cost. Though not vacuum tube based they can, as is also true of a solid state PLL, be integrated into a tube transmitter design.
Implementing a PLL with tubes is possible but the trickiest part would seem to be the phase detector.
Neil
I was skilled enough to do
this kind of transmitter building.
Brooce