Transmitter circuit operation explanation

There is a parallel circuit with a several capacitors across the coil. The preset capacitor is to set the frequency to that required. It is FM modulated by the two variable capacitance diodes driven by the audio amplifier.
The fet is a source follower to drive the circuit and the gate is driven from a higher impedance tap to the circuit.

The oscillator is followed by two amplifiers with feedback resistors to set input and output impedance.

 

Actually Trevor, it's a classic Colpitts for sure but since the output signal is picked off the Drain I think I'd have to classify it as a Common Source.

Chris

 

There are many types of oscillator, given different names. This one would be a classic Colpitts without C3. It has a whiff of Clapp about it.

The best exposition of oscillators that I have seen is "Electronic Designers' Handbook" by Landee, Davis and Albrecht.

 

Sorry, my mistake, should have said C4. Will have to get my eyes amplified.

 

All components within the red border form the resonant tank circuit. R6 affects the "Q" of the tank as well as providing a DC path (that the gate needs) to GND. Neglecting the Gate to Source and Gate to Drain capacitance all remaining components affect the frequency of the oscillator.

C5, C6 and L2 form the positive feedback loop that all oscillators need to ...uh ... oscillate.

Chris

 

R5 reverse biases the two variable capacitors so they do not pass current. The audio amplifier swings this bias up and down so varying the capacitance and thus the frequency. These are diodes optimised for capacity change but in my day we used standard rectifying diodes such as the BY100.

Draw out the tank circuit and see that it has one inductor with several capacitors across it
CD1&CD2, C3, C4&C5&C6.
The fet drives the circuit across C6 and takes its input across C5.
C4 is perhaps there to isolate the DC short that the inductor would give. With a juction transistor this would generate a negative bias on the base but this is not the case here with an insulated gate.
The capacitors are sized to give the right frequency and the right impedance to connect the fet.

 

Well Trevor, you're not as bold as I am (birthday in profile) but your secret is now out.

Chris

 

Chris, you will need to pedal quite hard to catch me up.

Trevor

 

Ha, if I read that correctly, soon we'll be able to carbon date you like a fossil!
"Trevorsaur" (humanusbipodus) I like it! It rolls right off the tongue.

Chris

 

R6 100k holds the gate at around zero volts. Normally this would have a diode across it so that the gate would be biased negatively when the oscillator is working, giving a level control and reducing distortion.

 

Because it's not a MOSFET. It's a dual gate JFET. Unlike N Chanel MOSFETS JFETS conduct between the Source & Drain with no bias on the Gate. Their characteristics more closely resemble that of a Vacuum Tube than a MOSFET. R8 (as small as it might be) provides some negative bias between the Gate and Source.

Chris.

 

I looked up the BF982 and I found it is a MOSFET. It has protection diodes across the gates which will act as oscillation limiters and provide AGC action so no additional diode is necessary.

 

It also smells a bit like a tri-tet

 

Damn! As I typed that there was a little voice saying "Are you sure of this? Check this FET out before committing yourself ".

I should have listened to the little voice. Either that or the little voice should have been louder! Looking again at the schematic symbol I shouldn't have made such a nube mistake.

Chris