Receiver Operation

Question File Number 17



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Receiver Fundamentals

The basic operating functions of radio receivers have been covered in Receiver Blocks Diagrams.

The details of the various signal modes are treated in Signals

Here we look at typical specifications for receivers and at some of the features found to improve operating convenience.

Frequency Stability

The ability of a receiver to stay tuned to an incoming signal for a long period is related to the frequency stability of its local oscillator. This same requirement applies to transmitters. Oscillators

Metal shielding is used around oscillator coils and the components used may be especially selected for high frequency stability.

Sensitivity

The sensitivity of a receiver is its ability to receive weak signals. Selectivity is more important than sensitivity.

Noise

The first stage in the receiving block-diagram chain, the RF amplifier, sets the noise characteristics for a receiver. The RF amplifier should use a low-noise device and it should generate very little internal noise. Measurement of sensitivity requires test equipment, equipment able to measure the signal plus noise audio output from the receiver and the noise alone with no signal being received.

The ratio (S+N)/N (i.e. signal plus noise to noise) is often used with this test for comparing receivers.

There is far more to measuring the sensitivity and other characteristics of a receiver than is often realised! Please refer to standard textbooks on the subject.

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Selectivity

The ability to separate two closely spaced signals is a receiver's selectivity. The characteristics of the filter in the IF amplifier determine the frequency response of the IF stages and the selectivity.

The narrower the filter pass-band, the higher the selectivity.

The receiver pass-band should be tailored to the characteristics of the incoming signal. Too wide a pass-band and unwanted noise is received which detracts from the reception of the wanted signal.

We use bandwidth to measure selectivity. This is how wide a range of frequencies you hear with the receiver tuned to a set frequency. Filters can often be selected by a front-panel switch to provide different receiver bandwidth characteristics.

The Audio Stage

The audio stage of a receiver amplifies the signal from the detector and raises it to a level suitable for driving headphones or a speaker.

AFamp

A typical speaker is a load impedance of about 8 ohm. A transformer is generally used to match this low-impedance load to the impedance level required for the best performance of the amplifier.

There are many types of audio amplifier. The circuit shown here is to show the principles. It is typical of that in a very simple radio - with a very small speaker and low audio output.



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Compiled Sun Nov 28 2010 at 8:41:44pm


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