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The solution to creating a tunable receiver with
the bandwidth to get the whole signal, while rejecting the adjacent channels, is
the Superheterodyne receiver. Recall that the problem is, a filter that can
receive the carrier and the sidebands while rejecting the adjacent channels is
difficult to be made tunable. The solution is to make a receiver that is
fixed-tuned to a single frequency and changing the frequency you want to receive
to that fixed frequency.
The first stage of the Superheterodyne receiver is the RF stage. This is a radio frequency amplifier and filter. It contains a tunable broadband filter. This filter cannot reject the adjacent channels, but it rejects unwanted frequencies that the receiver would otherwise receive (image frequencies mentioned below).
The RF stage also determines the inherent noise of the whole receiver. Any noise generated in the RF stage is amplified in the following amplifiers. AGC, full quieting, squelch, desensitization -
The incoming radio signal is sent to the mixer along with the signal from the local oscillator. This causes beat frequencies that are at the sum and difference of the signals entering the mixer. Everything is filtered out but the difference signal, which is sent to the IF amplifier.
With an AM receiver, the local oscillator frequency is tuned to a frequency that is 455 kHz below the incoming radio signal. For example, if you want to listen to radio station KKOB in Albuquerque, NM (770 kHz), you will tune your local oscillator to 315 kHz. This mixes with the 770 kHz signal to produce signals at 1225 kHz and 455 kHz. Everything but the 455 kHz signal is filtered out and sent to the IF amplifier. The sidebands also mix with the 315 kHz signal from the local oscillator and produce their own frequencies. The result is that the signal leaving the mixer is a copy of the original signal, including sidebands, shifted to center on 455 kHz.
When you tune a Superheterodyne receiver, you are changing the frequency of the local oscillator so that different radio signals are converted to 455 kHz. The tuning knob of an analog receiver tunes a variable capacitor that tunes the local oscillator. This capacitor is ganged to another variable capacitor so that turning the tuning knob also tunes the RF stage filter.
When the radio is tuned to receive 770 kHz, it will also receiver any signal with a frequency of 140 kHz. This is because the local oscillator frequency of 315 kHz will mix with 140 kHz to create 455 kHz. (140 kHz + 315 kHz = 455 kHz). This is called an image frequency. However, 140 kHz is blocked by the filter in the RF stage.
Modern digital radio circuits use frequency synthesizers for oscillators. This is particularly so for the local oscillators of Superheterodyne receivers. You are expected to review Phase-locked Loops (including Frequency Synthesizers) in Analog Circuits as well as Registers (particularly Digital Counters) in Digital Circuits as part of your study of communications circuits.
The next stage is the intermediate frequency
stage or IF amplifier. This is the actual radio receiver. The IF filter of a
broadcast AM receiver is tuned to 455 kHz and is designed to have a flat
response across the entire signal then drop-off sharply at the ends of the
sidebands. Several filter designs can achieve this. Tuning these filters is a
painstaking process, but it only has to be done once. For an AM broadcast
receiver, the bandwidth of this filter is 10 kHz. Since regulations limit the
sidebands to 4 kHz above and 4 kHz below the carrier, this gives the AM
broadcast signal a bandwidth of 8 kHz. The broadcast channels are placed 10 kHz
apart. This leaves a 2 kHz guard band between adjacent channels. An IF filter
with a 10 kHz bandwidth accepts the 8-kHz-wide signal but cuts off its response
in the guard band. It accepts both sidebands but not anything from adjacent
channels.
The IF stage of a broadcast FM receiver must have a bandwidth
of 200 kHz. This is because a broadcast FM signal must contain two high-quality
audio channels plus other signals that are not heard on standard receivers. The
IF stage of an analog television receiver must have a bandwidth of 6 MHz. The
television receiver would need a bandwidth of 8 MHz. However, approximately half
of the lower sideband is filtered out to make room for more channels in the
television bands.
Radio receivers of bands other than the AM broadcast band may have IF amplifiers that work on frequencies other than 455 kHz. For example, an FM receiver may an intermediate frequency of 10.7 MHz.
Some radio receivers have two mixer/IF combinations to improve image frequency rejection. For example, an FM receiver may have its first IF state with a frequency of 10.7 MHz with a second IF stage of 470 kHz. This is called double conversion.
Most AM receivers use 455 kHz for the intermediate frequency. However, this is not always the case.
The final stage is the audio frequency stage or the AF amplifier. This is an audio frequency pre-amp and power amplifier to drive the speaker.
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| A block diagram of a Superheterodyne receiver. |
The human auditory system is capable of hearing
frequencies up to 20 kHz. However, a maximum of 4 kHz is adequate for voice
broadcasts. Therefore, the AF amplifier for an AM broadcast receiver must pass
frequencies as high as 4 kHz (i.e., have a frequency response of 4 kHz).
However, to reproduce music with adequate sound quality, frequencies up to at
least 15 kHz must be passed. Therefore, the AF amplifier for an FM broadcast
receiver must have a frequency response of 15 kHz. Analog video requires a
frequency response of 4 MHz. The last stage of an analog television receiver
called a video amplifier because of its high-frequency response, must have a
frequency response of 4 MHz to handle the video signal. In the case of a
television receiver, this stage is called a video amplifier. (The IF stage
of a television receiver must have a bandwidth of 6 MHz because of the way the
sidebands are filtered. Television receivers are covered below.)
Double and trible conversion -
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