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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?

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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

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  • \$\begingroup\$ Is there no need for resistance to filtering? \$\endgroup\$
    – k.bucaga
    Commented Mar 15, 2019 at 11:39

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