Automatic Frequency Control

In the 1930s, automatic tuning systems began to appear on many radio models. While initially regarded as little more than a gimmick, automatic tuning was gradually accepted by the radio-buying public and became standard on many models. These systems usually took the form of pushbutton tuning or motorized dials, and were designed to assist the consumer in quick selection of their favorite stations. Marketing usually stepped in at this point and gave the systems fancy names, such as the RCA “Magic Brain,” or the Philco “Mystery Control”!

As automatic tuning systems became more prevalent, a nasty problem soon reared its head. Technology of the day was good, but not good enough to ensure that once the system had selected the desired station, the station was tuned in accurately. Also, it was desirable to prevent the receiver from becoming detuned due to frequency drift.

To illustrate the problem, let’s assume that the center frequency (the intermediate frequency) in a superheterodyne receiver is 455 kilohertz, and that we wish to receive a station on 1230 kilohertz. In a superheterodyne, a locally-generated oscillator frequency will be produced, and mixed with the incoming frequency (1230 kilohertz, in this case) to produce the 455 kilohertz intermediate. Therefore, to tune the receiver in question to our station at 1230 kilohertz, our local oscillator should produce a signal of 1685 kilohertz, since 1685 – 1230 = 455 (this is the most common method of producing the intermediate frequency, the subtractive principle).

So far, the automatic tuning system in our example has roughly tuned the receiver, but now a touch-up must be applied. It’s easy to see that if our local frequency of 1685 kilohertz starts to wander in either direction (due to mistuning or drift), the difference frequency will not be 455 and the receiver is thus mistuned. The result, of course, will be a squawk coming out of the loudspeaker, which is not what we’re after.

Radio engineers solved the problem with the development of automatic frequency control (AFC). Simply put, AFC is a feedback loop that self-corrects the receiver to center its tuner on the desired station. The method is ingenious and normally only applied to superheterodyne receivers.

Here's how they did it. The output of 455 kilocycles was split (via a discriminator transformer) such that a deviation in one direction (by the oscillator) would produce a positive voltage, or if in the other direction, a negative voltage. This feedback voltage would then be applied to a vacuum tube control circuit placed in the oscillator circuit. Any deviation in tuning would nudge the oscillator in the necessary direction and return it to the exact frequency needed to produce our correct mixing frequency – and the receiver would be back in tune. The listener would never notice the process, since the correction happened within milliseconds. Thus, the receiver was self-correcting and would stay in tune.

As radio circuitry improved over the next few decades, AFC became less of a necessity and few manufacturers were willing to include it in receivers due to price considerations. With the advent of widespread frequency modulation (FM) broadcasting in the 1950s, though, AFC made a comeback. FM reception required that the receiver, once tuned to a station, remain accurately tuned. Again, the only way to guarantee that was with an automatic system. For the next few years, all the better FM radios provided some sort of AFC system to correct the drift usually associated with vacuum tube circuitry. Otherwise, the listener would be reduced to making frequent adjustments of the tuning knob!

Over the last two decades or so, solid state and digital designs have rendered AFC obsolete, apparently once and for all. Modern radio receivers are inherently stable and drift-free, and no outside assistance is needed with tuning.

SOURCES