I'm a new student in electrical engineering. I'm trying to make a BFO (Beat Frequency Oscillator) circuit for my university project. I didn't understand the duty of 3rd transistor and capacitors attached to it. When there are 2 oscillation voltage at different frequency, they must be connected to the mixer circuit and filtered through the low pass filter. But I cannot understand how this is done. Please can you help me?
Q3 is used for its nonlinear exponential base-emitter-diode behavior. You want a mixer, so Q3 provides the sum and the difference and the Osc1 and the Osc2 and many other distortion products, all on its output.
The 3 caps are a low-pass filter.
Notice more low pass filtering on base of Q5