A radio receiver is an apparatus designed to intercept a radio wave, remove the information carried by that radio wave and then present that information in the proper format.
There are several parameters used to judge the performance of a receiver. The two most important are sensitivity and selectivity.
Sensitivity measures the ability of a receiver to respond to extremely weak signals. Sensitivity is often expressed as the minimum signal input required to produce a given signal to noise ratio (SNR) at the receiver's output. Sensitivity values range from ~ 1mV for a 10 dB SNR for an inexpensive AM radio to < 100 nV for a 10 dB SNR for specialized high performance receivers.
Since sensitivity is generally quoted with reference to a SNR, the noise floor of the receiver is an important factor. The receiver's noise floor is the noise that exists at the output of the receiver when there is no input. A sensitive receiver must have an extremely low noise floor, which often necessitates the use of exotic components in the first stage of the receiver. (recall Friis's formula from the unit on noise, which shows that the noise figure of the receiver is determined chiefly by the noise ratio of the first stage).
Selectivity measures the ability of the receiver to discriminate between the desired signal and other undesired signals on nearby frequencies. Selectivity is generally measured in terms of bandwidth. If two signals are separated by a frequency greater than the quoted selectivity, it is possible to receive one without interference from the other.
Let us now look at some typical receiver systems to see how they compare in terms of sensitivity and selectivity. The earliest receivers, known as crystal sets, (see figure below) consisted of a tuning circuit, typically an LC tank circuit with either L or C variable, that was used to select the desired reception frequency.
The output of the tuner went directly to a detector. In the early receivers, the detector was a Schottky diode formed by placing a thin metal wire ("cat's whisker") in contact with a crystal of a semiconducting mineral such as galena. (hence the name crystal set). The output of the diode went to a pair of high impedance headphones ( Z > 1000 ohms). This receiver was very simple and required no external power, but it suffered from serious drawbacks:
1. There was no amplification of the signal, which seriously limited the sensitivity. In order for a signal to be heard, it had to be powerful enough to drive the headphones.
2. The selectivity of the receiver was very broad because only one tuned circuit was used and that tuned circuit was loaded down by the headphones.
The first attempt to improve the performance of the basic receiver was to add several stages of RF amplification between the antenna and the detector. This type of receiver was known as a TRF (Tuned Radio Frequency) receiver. A block diagram of a TRF receiver is shown below.
The addition of the amplifiers ahead of the detector increased the sensitivity of the TRF receiver greatly over the crystal set. However, the multiple stages of RF amplification brought some new problems:
1. The gain of the amplifiers generally decreased as the frequency increased, thus the sensitivity of a TRF receiver varied greatly across its tuning range.
2. Designing the tuning circuits for TRF receivers was a mechanical nightmare. Each stage of amplification had a tuned circuit, all of which had to track; that is all tuned circuits had to be tuned to the same frequency at the same timel. If the dial were set to 600 KHz, all amplifiers must be tuned to 600 KHz. If the dial is then turned to 1500 KHz, all amplifiers must now be tuned to 1500 KHz.
3. The selectivity of a TRF receiver was not constant. As the frequency increased, the selectivity became broader. This happened because the tuning circuits used in a TRF receiver had a fixed inductor and a variable capacitor. The Q of this circuit, which determines the bandwidth, is constant as C is varied. When a parallel LC circuit has a constant Q, its selectivity (bandwidth)is determined by its resonant frequency. As the resonant frequency increases, so does the bandwidth.
The selectivity problem was particularly troublesome for TRF receivers. The AM broadcast band extended from 540 KHz to 1500 KHz during the 1920's, when TRF technology was used. If the tuned circuits in the receiver were designed for the proper bandwidth (10 KHz for an AM broadcast signal) at 540 KHz, the bandwidth at 1500 KHz was almost 30 KHz, which was nearly 3 channels on the AM band. As more AM stations began to crowd the band in the late 1920's a new receiver design was needed. That new design, the superheterodyne, will be discussed in the next module.
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