Regenerative receiver based on my franklin oscillator
时间：04-04
整理：3721RD
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In my page I have designed this franklin oscillator http://qrp.gr/minivfo/
Now I want to investigate it's conversion to a regenerative receiver.
As far as I can understand, I need to limit the feedback or the amps gain in the oscillator section, so that oscillation stops at some point. This will be the edge point of regeneration.
By using a variable voltage to the oscillator this might happen but there will be quite a lot of frequency pull I bet. So I am thinking of the capacitor method.
1. How about replacing the 470pF capacitor with a variable one, do you think that will work without considerable frequency pulling?
Alternatively I could replace one of the 3.3pF with a variable one but I do not think it is a good idea to mess with the resonator-depended components.
2. If I couple the antenna (through an RF preamp for isolation) to the coil, then will demodulated audio present to the top of the 470pF? (collector of the second BJT)
Because there is a big C-B resistor there, the BJT may act as a detector. I may need to change this to 1M or so...
What do you think of these 2 points above?
Replacing the 470p cap with a variable one can quench oscillation, but most of the control will be concentrated in the 0-5 pF range.
I suspect the difficulty in attenuating the Franklin's enormous gain is the reason why it is not traditionally used as a regenerative receiver.
Now I want to investigate it's conversion to a regenerative receiver.
As far as I can understand, I need to limit the feedback or the amps gain in the oscillator section, so that oscillation stops at some point. This will be the edge point of regeneration.
By using a variable voltage to the oscillator this might happen but there will be quite a lot of frequency pull I bet. So I am thinking of the capacitor method.
1. How about replacing the 470pF capacitor with a variable one, do you think that will work without considerable frequency pulling?
Alternatively I could replace one of the 3.3pF with a variable one but I do not think it is a good idea to mess with the resonator-depended components.
2. If I couple the antenna (through an RF preamp for isolation) to the coil, then will demodulated audio present to the top of the 470pF? (collector of the second BJT)
Because there is a big C-B resistor there, the BJT may act as a detector. I may need to change this to 1M or so...
What do you think of these 2 points above?
Simulation of your Franklin oscillator shows an open loop gain of ~60 (~8x gain per stage), whereas one-transistor oscillators typically have single digit gain.
Replacing the 470p cap with a variable one can quench oscillation, but most of the control will be concentrated in the 0-5 pF range.
I suspect the difficulty in attenuating the Franklin's enormous gain is the reason why it is not traditionally used as a regenerative receiver.