Radio receiver design
Encyclopedia
Radio receiver design includes the electronic design of different components of a radio receiver which processes the radio frequency signal from an antenna in order to produce usable information such as audio. This article only concentrates on the historical configurations leading up to and including the modern superheterodyne receiver design. The complexity of a modern receiver and the possible range of circuitry and methods employed are more generally covered in electronics and communications engineering. The term radio receiver is understood in this article to mean any device which is intended to receive a radio signal in order to generate useful information from the signal, most notably a recreation of the so-called baseband
signal (such as audio) which modulated the radio signal at the time of transmission in a communications or broadcast system.
such as the original cat's-whisker diode discovered long before the development of modern semiconductors.
(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 1960's among the cheaper "transistor radios" of that era.
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 seen as a major cost (and recipient 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 The design tends to be rather unstable, and is obsolete.
The regenerative receiver also had its heyday at the time where adding an active element (vacuum tube) was considered costly. In order to increase the gain of the receiver, positive feedback was used in its single RF amplifier stage; this also increased the selectivity
of the receiver well beyond what would be expected from a single tuned circuit. The amount of feedback was quite critical in determining the resulting gain and had to be carefully adjusted by the radio operator. Increasing the feedback beyond a point caused the stage to oscillate at the frequency it was tuned to.
Self-oscillation reduced the qualitity of its reception of an AM (voice) radio signal but did ironically make it useful as a CW (morse code) receiver inasmuch as 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 an annoying 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.
where they are mixed with a signal from a local oscillator
which is tuned to the carrier wave
frequency
of the transmitted signal (unlike the superheterodyne design, where the local oscillator is at an offset frequency). The output of this mixer is thus audio frequency, which is passed through a low pass filter into an audio amplifier which may drive a speaker.
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."
, 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. 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 selectivity of 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 phase locked loop detector, unlike the frequency discriminator or ratio detector used in earlier FM receivers.
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.
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
signal (such as audio) which modulated the radio signal at the time of transmission in a communications or broadcast system.
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 diodeDiode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
such as the original cat's-whisker diode discovered long before the development of modern semiconductors.
- Advantages
- Simple, easy-to-make. Here we see a classic design for a clandestine receiver in a POW camp.
- Disadvantages
- Insensitive, it needs a strong RF signal and/or a long-wire antenna to operate.
- Poor selectivitySelectivitySelectivity may refer to:* Selectivity , in radio transmission* Binding selectivity, in pharmacology* Functional selectivity, in pharmacology* Socioemotional selectivity theory, in social psychology...
since it only has one tuned circuit.
Tuned radio frequency
The tuned radio frequency receiverTuned radio frequency receiver
A tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
(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
Envelope detector
An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the "envelope" of the original signal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls...
using a diode, followed by audio amplification. This was developed after the invention of the triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
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 1960's among the cheaper "transistor radios" of that era.
Reflectional
The reflectional receiverReflectional receiver
A reflectional radio receiver is a radio receiver design in which the same amplifier is used for both the high-frequency radio and low-frequency sound signals. The radio signal from the output of the amplifier passes detection and then re-enters the input of the amplifier...
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 seen as a major cost (and recipient 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
Selectivity
Selectivity may refer to:* Selectivity , in radio transmission* Binding selectivity, in pharmacology* Functional selectivity, in pharmacology* Socioemotional selectivity theory, in social psychology...
of the receiver well beyond what would be expected from a single tuned circuit. The amount of feedback was quite critical in determining the resulting gain and had to be carefully adjusted by the radio operator. Increasing the feedback beyond a point caused the stage to oscillate at the frequency it was tuned to.
Self-oscillation reduced the qualitity of its reception of an AM (voice) radio signal but did ironically make it useful as a CW (morse code) receiver inasmuch as 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 an annoying 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 Direct conversion receiver, the signals from the antenna are only tuned by a single tuned circuit before entering a mixerFrequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
where they are mixed with a signal from a local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
which is tuned to the carrier wave
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the transmitted signal (unlike the superheterodyne design, where the local oscillator is at an offset frequency). The output of this mixer is thus audio frequency, which is passed through a low pass filter into an audio amplifier which may drive a speaker.
For receiving CW
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
(morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
) 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 SSBSingle-sideband modulationSingle-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
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 figureNoise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
, although at lower frequencies this is typically omitted. The RF signal enters a mixer
Frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
, along with the output of the local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
, in order to produce a so-called intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
(IF) signal. 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 selectivity of the receiver) conveniently done at a single frequency (the IF frequency) thus requiring no further tuning for different stations.