I notice that when I touch my hand onto the bare PCB copper the tone gets somehow better (half the hum).
The main rule in any VCO design is to keep very low RF voltage across the varicaps. Rule that your oscillator doesn't follow, due to the location of your varicaps.
When use variable capacitors should be fine, but using varicaps with high RF voltage across, and also placed in the feedback path of the oscillator, you get behavior that you mentioned.
Franklin oscillator has only advantage that is relative wideband, otherwise has poor frequency stability and poor phase noise.
Make a simple Colpitts (or Clapp) and you get rid of all these problems.
Touching our hand to an amplifier often changes the sound. It might be worse buzzing, it might be less buzzing. It's hard to be sure what your project needs, whether more shielding of signal-carrying wires. Or an enclosing metal box. Or a different grounding arrangement, or star-ground. Etc.
I suspect that high-impedance components isolate a section of the circuit, making it prone to pick up ambient hum. Example, your schematic has a 1M resistor. This is not necessarily an error, although it may pay to try lower ohm values.
I suspect that high-impedance components isolate a section of the circuit, making it prone to pick up ambient hum. Example, your schematic has a 1M resistor. This is not necessarily an error, although it may pay to try lower ohm values.
The varicap is high impedance, so feeding with 1MOhm is sensitive to external coupled voltages. I would prefer an inductor instead, so that you can have low impedance for the control voltage (DC) without loading the oscillator tank circuit (RF).
Another disadvantage of the 1MOhm is that thermal noise of the resistor will make your oscillator noisy (resistor noise voltage tunes the varicaps).
The varicap is high impedance, so feeding with 1MOhm is sensitive to external coupled voltages. I would prefer an inductor instead, so that you can have low impedance for the control voltage (DC) without loading the oscillator tank circuit (RF).
Another disadvantage of the 1MOhm is that thermal noise of the resistor will make your oscillator noisy (resistor noise voltage tunes the varicaps).
Now my thought is to use this ALC mechanism
Your schematic appears to have two potentiometers at left. Did you install them? Do they tune frequency?
Or could they be for adjusting gain?
Or else you could experiment with a component elsewhere on the board. For instance the amount of bias going to a transistor. Or a resistor value in the LC loop.
Accepting the previous comments about noise and stability, I would expect a degree of frequency pulling by the ALC circuit. I'm not sure a BC549 would make a good shunt across the signal at high frequency either.
A better method would be to follow the oscillator with an emitter follower so it presented a high impedance to the oscillator output and maybe apply the ALC voltage to a subsequent stage. I've never tried it but I don't see any reason why a fixed cap & varicap can't be used to bypass the emitter resistor in a conventional common emitter stage to control negative feedback and hence gain.
Brian.
Oh you mean to replace the bjt with a varicap (placed in shunt to the GND with the 470pF feedback capacitor and control it from the VCC of the ALC loop?
...otherwise has poor frequency stability...
Controlling the oscillator emitters would be disastrous and the ALC loop may even cause a secondary oscillation. What I meant was to add a buffer stage before the 2n2222 and shunt it's emitter resistor to AC with a varicap. I agree the voltage recovered from the rectifiers will probably be too low (~2V at a guess) but you can use that voltage amplified to drive a varicap. The essential problem is that the way the oscillator works is rather like a multivibrator with gain too low for it to reliably run so it relies on the external LC to give it enough gain at the resonant frequency. It makes it very susceptible to changes around the tuned circuit which is why I am worried about 'shorting' it out with the ALC shunt. A buffer stage would help by isolating the tuning from the effects of ALC voltage.
Brian.
Varactors really benefit when the frequency is controlled in a loop, for example a PLL where mechanical tuning is impossible.
Beehive trimmers are good but I would have reservations about longevity. The screw thread is intended for 'once off' adjustment and may not last long in continuous use particularly with the extra 'wobble' of an extended shaft.
Brian.
My HF transceiver still uses motors to tune the PA output!Variable capacitors were once controlled in a loop using motors!
My HF transceiver still uses motors to tune the PA output!
Brian.
Sorry for the delayed reply. We have been hit by storms over the past few days and had no electricity most of the time!
My HF transceiver is an Icom IC-756ProII so it is all 'new' technology but still uses servo motors to tune the PA stage because of the high power (up to 100W) involved.
My antenna is not good I'm afraid. I live in a valley and inside a national park that does not allow masts to be erected. It is an end fed horizontal wire about 25m long with an automatic ATU. Unfortunately only about 70m above sea level and the antenna only about 3m above ground level and surrounded by tall trees. We can try if you like but don't expect good results.
Brian.
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