BFO (Beat Frequency Oscillator) in RF receiver How it works

Beat Frequency Oscillator(BFO) is an electronic oscillator used in the radio receiver to generate audio frequency signal. The incoming RF signal such as continuous wave Morse code signal or single side band(SSB) AM signal are mixed with the signal from the local beat frequency oscillator to generate signal in the audio frequency range(20 to 20KHz).

The circuit diagram of RF receiver with BFO is shown below.

In the above Beat Frequency Oscillator based receiver circuit diagram, the signal to be demodulated comes from antenna. The receiver has the Beat Frequency Oscillator(BFO) that generates signal that is mixed and demodulated using the RF mixer. The RF mixer can be simple as single diode modulator or ring diode modulator or BJT based or JFET based AM modulator. The mixer/demodulator generates output signal that has various frequencies- sum and difference frequencies of the signal coming from antenna and BFO and practically harmonics frequencies signals.

If the frequency($$f_{ANT}$$) of the incoming signal from the antenna is 100.5kHz and the frequency($$f_{BFO}$$) generated by the BFO is 100kHz then the output signal frequencies from the mixer/demodulator are:

$$f_1 = f_{ANT}+f_{BFO}$$ and $$f_2 = f_{ANT}-f_{BFO}$$

That is,

$$f_1 = f_{ANT}+f_{BFO}=100.5kHz+100kHz=200.5kHz$$

$$f_2 = f_{ANT}+f_{BFO}=100.5kHz-100kHz=500Hz$$

Both frequencies are present in the signal from the output of the mixer/demodulator.

The difference frequency f2 is audible, can be heard while the sum frequency f1 cannot be heard. The sum frequency f2 can be filtered using a LPF. A simple LPF with passband frequency of 100KHz and stopband frequency of 145KHz is shown below.

So after the filter the signal contains only signals with frequencies less than 100KHz. That is the sum frequency f1 is passed through the filter.

The signal amplitude is degraded while passing though the mixer and low pass filter circuits. So a power amplifier is used after the filter to amplify the signal. The amplification is high enough to drive a speaker. In the diagram above a 50Ohm load is shown which represents a speaker or earphones.

We can check with the Tektronix oscilloscope the output signal waveform and frequency.

The following shows the signal waveform at the output of the power amplifier on the tektronix oscilloscope.

So here it was explained what is BFO(Beat Frequency Oscillator) is and how it is used as an oscillator in the RF receiver circuit to get audio signal.

The following video shows simulation of the RF receiver circuit using BFO in Multisim circuit simulator.