The RF Amplifier
The signal enters the RF amplifier from the aerial. This adds sensitivity to
Mixer / Local Osc.
The output from the RF amplifier enters the mixer stage where it mixes with the output from the local oscillator. This process produces the intermediate frequency (IF).
The detector changes the RF signal to an audio frequency signal. At this stage other circuits for different modes are switched in by a switch on the front panel. These are the Beat Frequency Oscillator (BFO) for CW, the Product Detector for SSB and the Ratio Detector for FM. The detector itself only works in AM mode.
Audio Frequency Amplifier
This amplifies the small audio frequency from the detector to a level where it can drive a loudspeaker. It is controlled by the receivers volume control.
The big advantage of the superhet receiver over the TRF receiver is that because the signal is converted by the mixing process to the intermediate frequency, high amplification and good stability are possible.
The Mixing Process
When you mix two signals, you get two outputs, one being the sum of the two frequencies and the other, the difference of the two frequencies. Because of this you can encounter an effect known as second channel or image interference. If the wanted signal is 1000 kHz and the local oscillator is 1500 kHz, you get an output from the mixer of 500 and 2500 kHz. The IF stage selects the wanted frequency of 500 kHz.
However, if a strong local signal is received on 2500 kHz, it may get through to the mixer where the 2000 will mix with the 1500 kHz from the local oscillator to give 500 and 3500 kHz. The IF amplifier will then select the 500 kHz signal as the wanted signal. Click [ HERE ] for a diagramatic explanation of Second Channel Interference.