Radio receiver design

Radio receiver design is a complex topic which can be broken down into a series of smaller topics. A radio communication system requires two tuned circuits each at the transmitter and receiver, all four tuned to the same frequency. [Cheney, M., Uth, R., & Glenn, J. (1999). Tesla, master of lightning. New York: Barnes & Noble Books. Page 71.] The term radio receiver is understood in this article to mean any device which is intended to receive a radio signal and if need be to extract information from the signal.

Basic designs

Crystal radio

* Advantages
** Simple, easy to make. This is the classic design for a clandestine receiver in a POW camp.
* Disadvantages
** Insensitive, it needs a very strong RF signal to operate.
** Poor selectivity, it often only has only one tuned circuit.

Direct amplification

The directly amplifying receiver contains the input radio frequency filter, the radio frequency amplifier (amplifying radio signal of the tuned station), the detector and the sound frequency amplifier. This design is simple and reliable, but much less sensitive than the superheterodyne (described below).

Reflectional

The reflectional receiver contains the single amplifier that amplifies first radio, and then (after detection) sound frequency. It is simpler, smaller and consumes less power, but it is also comparatively unstable.

Regenerative

The Regenerative circuit has the advantage of being potentially very sensitive, it uses positive feedback to increase the gain of the stage. Many valved sets were made which used a single stage. However if misused it has the great potential to cause radio interference, if the set is adjusted wrongly (too much feedback used) then the detector stage will oscillate so causing the interference.

Tuned radio frequency

the RF interference that the local oscillator can generate can be controlled with the use of a buffer stage between the LO and the Detector, and a buffer or RF amp stage between the LO and the antenna.

Direct conversion

In the Direct conversion receiver the signals from the aerial pass through a band pass filter, and an amplifier before reaching a non-linear mixer where they are mixed with a signal from a local oscillator which is tuned to the carrier wave frequency of an AM or SSB transmitter. The output of this mixer is then passed through a low pass filter before an audio amplifier. This is then the output of the radio.

For CW morse the local oscillator is tuned to a frequency slightly different from that of the transmitter to make the received signal audible.

*Advantages
**Simpler than a superhet
**Better tuning than a simple crystal set

*Disadvantages
**Less selective than a superhet with regard to strong in-band signals
**A wider bandwidth than a good SSB communications radio, this is because no sideband filtering exists in this circuit.

uperheterodyne

Here are two superheterodyne designs for AM and FM respectively. The FM design is a cheap design intended for a broadcast band household receiver.

For single conversion superheterodyne AM receivers designed for mediumwave and longwave the IF is commonly 455 kHz.

For many single conversion superheterodyne receivers designed for band II FM (88 - 108 MHz) the IF is commonly 10.7 MHz. For TV sets the IF tends to be at 33 to 40 MHz.

FM vs. AM

To make a good AM receiver an automatic gain control loop is essential; this requires good design. To make a good FM receiver a large number of RF amps which are driven into limiting are required to create a receiver which can take advantage of the capture effect, one of the biggest advantages of FM. With valved (tube) systems it is more expensive to make active stages than it is to make the same number of stages with solid state parts, so for a valved superhet it is simpler to make an AM receiver with the automatic gain control loop while for a solid state receiver it is simpler to make an FM unit. Hence even while the idea of FM was known before World War II its use was rare because of the cost of valves - in the UK the government had a valve holder tax which encouraged radio receiver designers to use as few active stages as possible, - but when solid state parts became available FM started to gain favour.

Automatic gain control

Further reading

Radiocommunication handbook (RSGB), ISBN 0-900612-58-4