Two Basic Receiver Designs

A wireless or radio receiver converts EM waves into the original messages sent by a distant transmitter. Let’s begin our study of receivers by defining a few of the most important criteria for operation, and then we’ll look at two common designs.

Specifications

The specifications of a receiver indicate how well it can do the functions it is designed to perform. Sensitivity The most common way to express receiver sensitivity is to state the number of microvolts that must exist at the antenna terminals to produce a certain signal-to-noise (S/N) ratio or signal-plus-noise-to-noise (S+N/N) ratio in decibels (dB). Sensitivity is related to the gain of the front end (the amplifier or amplifiers connected to the antenna), but the amount of noise this stage generates is more significant, because subsequent stages amplify the front-end noise output as well as the signal output.
Selectivity: The passband, or bandwidth that the receiver can hear, is established by a wideband preselector in the early RF amplification stages, and is honed to precision by narrowband filters in later amplifier stages. The preselector makes the receiver optimally sensitive within a range of approximately plus-or-minus 10 percent (10%) of the desired signal frequency. The narrowband filter responds only to the frequency or channel of a specific signal to be received; signals in nearby channels are rejected.
Dynamic range: The signals at a receiver input vary over several orders of magnitude (multiples or powers of 10) in terms of absolute voltage. Dynamic range is the ability of a receiver to maintain a fairly constant output, and yet to maintain its rated sensitivity, in the presence of signals ranging from very weak to very strong. The dynamic range in a good receiver is in excess of 100 dB.
Noise figure: The less internal noise a receiver produces, in general, the better is the S/N ratio. Excellent S/N ratio in the presence of weak signals is only possible when the noise figure, a measure of internally generated receiver noise, is low. This is paramount at VHF, UHF, and microwave frequencies. Gallium-arsenide field effect transistors (GaAsFETs) are well known for the low levels of noise they generate, even at quite high frequencies. Other types of FETs can be used at lower frequencies. Bipolar transistors tend to be rather noisy.

Direct-Conversion Receiver

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Block diagram of a direct-conversion receiver.
A direct-conversion receiver derives its output by mixing incoming signals with the output of a tunable (that is, variable frequency) local oscillator (LO). The received signal is fed into a mixer, along with the output of the LO. Above figure is a block diagram of a direct-conversion receiver.
 
For the reception of on/off keyed Morse code, also called radiotelegraphy or continuous wave (CW), the LO, also called a beat-frequency oscillator (BFO), is set a few hundred hertz above or below the signal frequency. This can also be done in order to receive FSK signals. The audio output has a frequency equal to the difference between the LO and incoming carrier frequencies. For reception of AM or SSB signals, the LO is set to precisely the same frequency as that of the signal carrier. This condition is known as zero beat because the beat frequency, or difference frequency, between the LO and the signal carrier is equal to zero.
 
A direct-conversion receiver provides rather poor selectivity. That means it can’t separate incoming signals very well when they are close together in frequency. This is because signals on either side of the LO frequency can be heard at the same time. A selective filter can theoretically eliminate this. Such a filter must be designed for a fixed frequency if it is to work well. But in a direct-conversion receiver, the RF amplifier works over a wide range of frequencies.

Superheterodyne Receiver

A superheterodyne receiver, also called a superhet, uses one or more local oscillators and mixers to obtain a constant-frequency signal. A fixed-frequency signal is more easily processed than a signal that changes in frequency. The incoming signal is first passed through a tunable, sensitive front end. The output of the front end is mixed with the signal from a tunable, unmodulated LO. Either the sum or the difference signal is amplified. This is the first intermediate frequency (IF), which can be filtered to obtain a high degree of selectivity.
 
If the first IF signal is detected, the radio is a single-conversion receiver. Some receivers use a second mixer and second LO, converting the first IF to a lower-frequency second IF. This is a double conversion receiver. The IF bandpass filter can be constructed for use on a fixed frequency, allowing superior selectivity and facilitating adjustable bandwidth. The sensitivity is enhanced because fixed IF amplifiers are easy to keep in tune.
 
A superheterodyne receiver can intercept or generate unwanted signals. False signals external to the receiver are called images; internally generated signals are called birdies. If the LO frequencies are carefully chosen, images and birdies do not cause problems during ordinary operation of the receiver.
 
Following Figure is a block diagram of a generic single-conversion superheterodyne receiver. (Individual receiver designs vary somewhat.) Here’s what each stage does.
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Block diagram of a single-conversion Superheterodyne receiver.
Front end: The front end consists of the first RF amplifier, and often includes LC bandpass filters between the amplifier and the antenna. The dynamic range and sensitivity of a receiver are determined by the performance of the front end.
Mixer: A mixer stage converts the variable signal frequency to a constant IF. The output is either the sum or the difference of the signal frequency and the tunable LO frequency.
IF stages: The IF stages are where most of the gain takes place. These stages are also where optimum selectivity is obtained.
Detector: The detector extracts the information from the signal. Common circuits are the envelope detector for AM, the product detector for SSB, FSK, and CW, and the ratio detector for FM.
Audio amplifier: Following the detector, one or two stages of audio amplification are employed to boost the signal to a level suitable for a speaker or headset. Alternatively, the signal can be fed to a printer, facsimile machine, or computer.