Communications Receivers.

Introduction.

Strictly speaking a receiver is any device that will recover the information which has been modulated onto a radio signal and make it available for use. That means that such things as a crystal set, an AA5 and an AJ6 can qualify as a receiver. The 3 items sighted above are most often called radios but it is still correct to refer to them as receivers.

There is a subset of receivers that would be highly insulted if you called them radios. Those are the ones in which the noun receiver is modified by the adjective communications. What defines the difference between a communications receiver and a radio? A number of things any one of which alone might not qualify it as a C R but when taken as a group, will.

A communications receiver must;

1. cover more than just the AM broadcast band,
2. have a band spread dial with accurate calibrations for ham or other communications bands. Or its frequency coverage may be restricted to those bands only.
3. have a switch for changing the IF bandwidth.
4. have a crystal filter or built in Q multiplier to further narrow the bandwidth.
5. have a beat frequency oscillator (BFO) for reception of CW and SSB signals. Or separate detectors for AM and SSB/CW.
6. have a switch for selecting fast or slow AGC action.
7. have separate controls for RF (Radio Frequency) gain and audio gain (volume control).
8. have a crystal calibrator that produces marker frequencies at regular frequency intervals in the communications bands.
9. have a signal level meter (S meter),.

While some of these features may be missing on a true C R most of them must be present to qualify. A box with short wave bands but none of these features is really just a radio. In the 1950s top of the line receivers started to appear that dispensed with the AM broadcast band and all frequencies between the ham bands. These ham band only receivers were decidedly not radios.

Prerequisites.

Like any good course this one has prerequisites. One of the most important is Simple Superhet. The reader should also study the pages under the heading "Basics" on AM, SSB, and FM. The information on the pages under the heading "Radio and Detector Circuits" is somewhat simplified and, if desired, may be used as introductory material for the pages under the heading of "The basics."

Introduction. You are here.
Image Rejection, IF Bandwidth, and Number of Conversions
Block Diagram
Construction Tips
Power Supply
Audio Amplifier
RF Amplifier and Preselector
Converters, Mixers, and Local Oscillators
IF Amplifiers and Band-pass Filters
Detectors
AGC (Automatic Gain Control), and S Meter (Signal Meter)