Pages: (1) (2) (3) (4) (5) (6) (7) (8) Modulation page // Oscillator Page

The Oscillator and Beat frequency

Oscillator is Australian for "I'll see you later." :-) Actually, an oscillator or does what its name implies, it sets up an electrical current that races back and forth, oscillating, until it produces a radio frequency within the radio. The mixer then combines the two radio signals, the incoming frequency, and the one produced by the local oscillator, into a single, intermediate frequency. In many radios that frequency is 455,000 cycles per second. The radio then processes that so called IF frequency with steps we will see later. But back to the oscillator.

We saw on the modulation page that old radios used direct conversion, whereby anything sent from the antenna got passed on to the amplifier. No finishing or real control like in the superheterodyne circuit we've been discussing. Direct conversion isn't too efficient because it deals with a huge range of frequencies. Whatever the radio is tuned to, up or down in the radio band, the radio must accommodate. This makes it tough for a radio engineer to design a circuit for the best sound quality and reception. In a superhet circuit the radio receiver deals with just this one frequency, purposely made with the help of the oscillator. It's like the difference between driving a car with a manual transmission instead of an automatic. The automatic deals with the changing road conditions, gearing the car appropriately for the conditions. An oscillator likewise helps gear down and smooth out the raw signal coming from the antenna. Let me try one more overly simple explanation.

Remember what I said about the loading coil on an antenna and how it was a compromise?, that ideally we would like the antenna length to match the length of the radio wave it is supposed to pick up or transmit on? Well, the superheterodyne receiver we're discussing is also a compromise. Ideally we would like a receiver tuned for each frequency it is supposed to pick up. That isn't practical, with most radios needing dozens, and in the case of cellular radio, hundreds of frequencies, to receive on. So we design a circuit like this where we change the frequency of the the current flowing through our receiver to a common, pre-determined frequency for our radio to amplify.

Although beyond this discussion, a beat frequency oscillator is a sort of super oscillator, a variable type you can control. It's in addition to the normal oscillator that is always present in modern radio. A BFO circuit allows really fine tuning of a signal, so much that you can listen to what's called single side band transmission or SSB. It's the most efficient way to transmit A.M. signals but you need a special receiver to make the signal intelligible.

Back to the article -->