In the 1955 edition of Electronic and Radio Engineering F. E. Terman wrote, "All radio receivers except a few designed to meet specialized needs are of the superhetorodine type." Today, all modern radio receivers are superhets.

As a model for this discussion, consider how a simple single-conversion superhet works, when there is only one over-the-air signal present at its antenna. The superhet inputs the over-the-air signal, and mixes (i.e. frequency subtracts) the over-the-air signal with a variable-frequency local-oscillator signal, to form a difference frequency signal. The receiver is so designed that when any over-the-air signal is "tuned-in", the absolute value of the difference between the over-the-air signal's assigned frequency and the local-oscillator's frequency always will be the same, known intermediate frequency (IF).

ANALOG TUNING

Analog receivers use inductance-capacitance tuned circuits to determine the local oscillator frequency. The frequency-tuning knob is turned to vary the shaft position of a rotary capacitor, and tune-in the desired signal. (You might consider a trombone player varying the length of his trombone's slide to change the note he plays a mechanical equivalent of what is happening.)

The tuning method is inexact, depending as it does on such things as the shaft position, the tuning skill of the listener, the air temperature inside the radio near the oscillator components, and the age and condition of the inductor and capacitor. A tuning dial is provided as a guide to where the listener should begin to look, while tuning-in the desired station.

DIGITAL TUNING

Digital receivers do not use variable-frequency local-oscillators. Instead they use precisely-ground pieces of quartz crystal [think tuning fork] to control the generation of an essentially frequency-invariant local oscillator, which may be thought of as a local frequency standard. Usually, the listener tunes a digital receiver by using keys to "punch in" the desired over-the-air frequency.

(Some digital receivers have been designed with an analog-like tuning dial. For example, the Panasonic RF-B65 has both keys to enter the frequency and a knob that rotates a shaft, whose digitally encoded position can be used to vary the frequency in 1 kHz steps. Another Example is the Lowe HF-150 whose shaft encoded tuning knob controls the frequency in 8 Hz steps; keys are available as an accessory.) Digital logic computes the difference between the over-the-air frequency and the intermediate frequency, and the logic synthesizes a difference frequency signal from the local frequency standard. The tuning is much more exact. An easily read liquid-crystal numeric display gives a very close, easily interpreted indication of what the tuned frequency is.

If quality and price considerations otherwise were equal, I would select a well designed, digital receiver because of the ease in tuning, the ease of interpreting the tuning displays, and the long-term tuning stability inherent in the design. Not too many years ago, when digital receivers were expensive (if not impractical) to build, there were well designed analog receivers that performed well and continue to perform well for their present owners. These days, analog receivers generally are designed to be low priced and are poorer performers than digital receivers that cost only a little more. (There are exceptions. The GE SR-III for AM-broadcast band (medium wave) reception has a reputation for good long distance (DX) reception at a reasonable (but not low) price. Digital receivers generate noise as an unwanted byproduct of the way they work. All noise, including digital noise, can be managed (although not completely eliminated) by careful receiver design. If your interest is listening to broadcasts rather than stalking rare (and presumably weak) DX signals, the noise may be a non-problem for you. If DX is your interest, a carefully selected digital receiver still may fill your need.

This article was last updated on May 11, 1999.

If you have any questions, feel free to Email me ce369@freenet.carleton.ca . I'll do my best to confuse you completely J (Comments or corrections also are welcome.)

This is hit on this document since May 11, 1999.



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