IAN LANG ELECTRONICS
No 2,3 & 4.The RF Stage, Mixer and Local Oscillator. (Front End)
In a superheterodyne receiver we mix two signals together, one from the tank circuit which responds at the frequency of the broadcast signal, and one from the local oscillator. Both have a variable capacitor and they are either electronically or mechanically ganged together so that each turns with the other. We mix them through a non-linear device to obtain both original frequencies plus a sum and difference frequency. To comply with British Standards, the difference frequency needs to be 455 kHz for AM. For instance, if we are tuned to receive a broadcast at 530 kHz we need the local oscillator to be working at 985 kHz. This is because the difference frequency is equal to the frequency of the local oscillator minus that of the tank, ergo 985 - 530 which is 455. In rarer cases we could use the sum : in this case it would be 1515 kHz. In commercial sets the difference frequency is always used, as it sits outside the band used for AM transmissions. Whichever frequency, sum or difference, we may use becomes what is termed the intermediate frequency. The reason for this will become clear shortly.
The use of 455 kHz is deserving of a sentence or two. This is an agreed standard as it is kept clear of any other sort of transmission by regulation. It sits well outside the long wave band (150-285 kHz) and the medium wave (525-1605 kHz) and so outside interference by strong signal is unlikely. The equivalent on the FM broadcast bands is 10.7 MHz for much the same reasons, the 31 metre band stopping at 9.75 MHz and the 25 meter band beginning at 11.7.
The drawing below shows a typical RF stage for a receiver made of discrete components:
The local oscillator is shown in green, the tank in red. The variable capacitors are linked, and the local oscillator has an unlinked trimmer for adjustment. The local oscillator is amplified by two NPN BJTs,the first for signal amplification, the second for a buffer. Both tank and LO input to a dual gate N channel MOSFET, which is the sole component for the mixer, and which outputs the intermediate frequency as well as the sum and the two original frequencies. The filtered circuits allow only the intermediate to the IF amplifier. This constitutes the entire front end of the superhet.