I am a beginner hobbyist form Brasil, posting here for the first time.
I am trying to understand the schematic of an old Transglobe radio that my late grandfather gave me. It motivates me to learn more of analog electronics.
After searching the internet for oscillators, it appears to be something like a Colpitts Oscillator, but not quite.
If any of you could tell me what oscillator it is, it would be of great help.
The Signal comes from T101 and goes to T103, which is the mixer.
From what I understand T102 gives the gain to maintain the oscillation, but where is the Tank circuit?
Where is the feedback? C112 maybe? But from where does it come?
Why is t102's base a constant voltage?
Really appreciate any help! Sorry if it was too much of a beginner's question.
Best Answer
At first glance T102 looks like a common-collector colpitts oscillator but it isn't. It has split capacitors (C108 and 109) on the emitter but these do not feed to the base so that is ruled out.
On further inspection and noticing D101 (a varactor diode) it's clear that it is a tuned-collector-oscillator with capacitors C126||CT103||CV103||[C113+D101] in parallel with inductor TR103. C111 is the feedback circuit from the tank to the emitter. That feedback is attenuated by C108
C109 takes the oscillator signal at the emitter of T102 and "adds" it to your received signal from T101. This addition of signals is non-linearly amplified by T103 and you get an "addition" turning into a "mix" (multiplication) with sum and difference frequencies produced at T103's collector.
Here's the closest diagram I could find of the type of oscillator you have: -
See figure 8 in this link.