Radio receiver design
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Radio receiver design includes the
Fundamental considerations
Design of a radio receiver must consider several fundamental criteria to produce a practical result. The main criteria are
Gain is required because the signal intercepted by an
Selectivity is the ability to "tune in" to just one station of the many that may be transmitting at any given time. An adjustable
Sensitivity is the ability to recover the signal from the background noise. Noise is generated in the path between transmitter and receiver, but is also significantly generated in the receiver's own circuits. Inherently, any circuit above
Stability is required in at least two senses.
The detector stage recovers the information from the radio-frequency signal, and produces the sound, video, or data that was impressed on the carrier wave initially. Detectors may be as simple as an "envelope" detector for amplitude modulation, or may be more complex circuits for more recently developed techniques such as frequency-hopping spread spectrum.
While not fundamental to a receiver,
Many different approaches and fundamental receiver "block diagrams" have developed to address these several, sometimes contradictory, factors. Once these technical objectives have been achieved, the remaining design process is still complicated by considerations of economics, patent rights, and even fashion.
Crystal radio
A crystal radio uses no active parts: it is powered only by the radio signal itself, whose detected power feeds headphones in order to be audible at all. In order to achieve even a minimal sensitivity, a crystal radio is limited to low frequencies using a large antenna (usually a long wire). It relies on detection using some sort of semiconductor
A crystal receiver is very simple and can be easy to make or even improvise, for example, the foxhole radio. However, the crystal radio needs a strong RF signal and a long antenna to operate. It displays poor selectivity since it only has one tuned circuit.
Tuned radio frequency
The tuned radio frequency receiver (TRF) consists of a radio frequency amplifier having one or more stages all tuned to the desired reception frequency. This is followed by a detector, typically an envelope detector using a diode, followed by audio amplification. This was developed after the invention of the triode vacuum tube, greatly improving the reception of radio signals using electronic amplification which had not previously been available. The greatly improved selectivity of the superheterodyne receiver overtook the TRF design in almost all applications, however the TRF design was still used as late as the 1960s among the cheaper "transistor radios" of that era.
Reflex
The reflex receiver was a design from the early 20th century which consists of a single-stage TRF receiver but which used the same amplifying tube to also amplify the audio signal after it had been detected. This was in an era where each tube was a major cost (and consumer of electrical power) so that a substantial increase in the number of passive elements would be seen as preferable to including an additional tube. The design tends to be rather unstable, and is obsolete.
Regenerative
The
Self-oscillation reduced the quality of its reception of an AM (voice) radio signal but made it useful as a CW (Morse code) receiver. The beat signal between the oscillation and the radio signal would produce an audio "beeping" sound. The oscillation of the regenerative receiver could also be a source of local interference. An improved design known as the super-regenerative receiver improved the performance by allowing an oscillation to build up which was then "quenched", with that cycle repeating at a rapid (ultrasonic) rate. From the accompanying schematic for a practical regenerative receiver, one can appreciate its simplicity in relation to a multi-stage TRF receiver, while able to achieve the same level of amplification through the use of positive feedback.
Direct conversion
In the
For receiving CW (morse code) the local oscillator is tuned to a frequency slightly different from that of the transmitter in order to turn the received signal into an audible "beep."
- Advantages
- Simpler than a superheterodyne receiver
- Disadvantages
- Poor rejection of strong signals at adjacent frequencies compared to a superheterodyne receiver.
- Increased noise or interference when receiving a SSB signal since there is no selectivity against the undesired sideband.
Superheterodyne
Practically all modern receivers are of the superheterodyne design. The RF signal from the antenna may have one stage of amplification to improve the receiver's noise figure, although at lower frequencies this is typically omitted. The RF signal enters a mixer, along with the output of the local oscillator, in order to produce a so-called intermediate frequency (IF) signal. An early optimization of the superheterodyne was to combine the local oscillator and mixer into a single stage called "converter". The local oscillator is tuned to a frequency somewhat higher (or lower) than the intended reception frequency so that the IF signal will be at a particular frequency where it is further amplified in a narrow-band multistage amplifier. Tuning the receiver involves changing the frequency of the local oscillator, with further processing of the signal (especially in relation to increasing the receiver) conveniently done at a single frequency (the IF frequency) thus requiring no further tuning for different stations.
Here we show block diagrams for typical superheterodyne receivers for AM and FM broadcast respectively. This particular FM design uses a modern
For single conversion superheterodyne AM receivers designed for medium wave (AM broadcast) the IF is commonly 455 kHz. Most superheterodyne receivers designed for broadcast FM (88 - 108 MHz) use an IF of 10.7 MHz. TV receivers often use intermediate frequencies of about 40 MHz. Some modern multiband receivers actually convert lower frequency bands first to a much higher frequency (VHF) after which a second mixer with a tunable local oscillator and a second IF stage process the signal as above.
Software-defined radio
Software-defined radio (SDR) is a
See also
Further reading
- Books
- Radiocommunication handbook (RSGB), ISBN 0-900612-58-4
- Patents
- U.S. patent 1,748,435 Crystal Radio Apparatus. H. Adams
Notes and references
- ^ Wes Hayward, Doug De Maw (ed),Solid State Design for the Radio Amateur, Chapter 5 "Receiver Design Basics", American Radio Relay League 1977, no ISBN
- ISBN 0-470-85164-3)