Overview

Low-priced shortwave receivers often suffer from performance problems that are a direct result of the trade-offs that the manufacturers make to produce a low-cost model. In addition, older receivers often suffer the same problems, as do many homebrew radio receiver designs. Chief problems are sensitivity, selectivity, and image response.

Sensitivity is a measure of the receiver s ability to pick-up weak signals. Part of the cause of poor sensitivity is low-gain in the front end of the radio receiver, although the IF amplifier contributes most of the gain.

Selectivity is a measure of the ability of the receiver to (a) separate two closely spaced signals and (b) reject unwanted signals that are not on or near the desired frequency being tuned. The selectivity provided by a preselector is minimal for very closely spaced signals (that is the job of the IF selectivity in a receiver), but it is used for reducing the effects (e.g., input overloading) of large local signals so fits the second half of the definition.

Image response affects only superheterodyne receivers (which most are) and is an inappropriate response to a signal at twice the receiver IF frequency that the receiver is tuned to. A superhet receiver converts the signal frequency (RF) to an intermediate frequency (IF) by mixing it with a local oscillator (LO) signal generated inside the receiver. The IF can be either the sum or difference between the LO and RF (i.e., LO + RF or LO ? RF),...