Radio receiver sensitivity

Radio receiver sensitivity

Receiver sensitivity is one of the key specifications of any radio. The two main requirements of any radio receiver are that it should be able to separate one station from another, i.e. selectivity, and signals should be amplified so that they can be brought to a sufficient level to be heard. As a result receiver designers battle with many elements to make sure that these requirements are fulfilled

A number of methods of measuring and specifying the sensitivity performance of radio receivers are used. Figures including signal to noise ratio, SINAD, noise factor and noise figure are used. These all use the fact that the limiting factor of the sensitivity of a radio receiver is not the level of amplification available, but the levels of noise that are present, whether they are generated within the radio receiver or outside it.

Noise

Today technology is such that there is little problem in being able to achieve very large levels of amplification within a radio receiver. This is not the limiting factor. In any receiving station the limiting factor is noise - weak signals are not limited by the actual signal level, but by the noise masks them out. This noise can come from a variety of sources. It can be picked up by the antenna or it can be generated within the radio receiver.

It is found that the level of noise that is picked up externally by a receiver from the antenna falls as the frequency increases. At HF and frequencies below this the combination of galactic, atmospheric and man-made noise is relatively high and this means that there is little point in making a receiver particularly sensitive. Normally radio receivers are designed such that the internally generated noise is much lower than any received noise, even for the quietest locations.

At frequencies above 30 MHz the levels of noise start to reach a point where the noise generated within the radio receiver becomes far more important. By improving the noise performance of the radio receiver, it becomes possible to detect much weaker signals.

Design for noise performance

In terms of the receiver noise performance it is always the first stages or front end that is most crucial. At the front end the signal levels are at their lowest and even very small amounts of noise can be comparable with the incoming signal. At later stages in the set the signal will have been amplified and will be much larger. The same levels of noise as are present at the front end will be a much smaller proportion of the signal and will not have the same effect. Accordingly it is important that the noise performance of the front end is optimized for its noise performance.

It is for this reason that the noise performance of the first radio frequency amplifier within the receiver is of great importance. It is the performance of this circuit that is crucial in determining the performance of the whole radio receiver. To achieve the optimum performance for the first stage of the radio receiver there are a number of steps that can be taken. These include:

• Determine the circuit topology required

• Choose a low noise device

• Determine the gain required

• Determine the current through the device

• Use low noise resistors

• Optimize the matching

• Ensure that power supply noise entering the circuit is removed
  Determination of circuit topology. The first step in any design is to decide upon the type

of circuit to be used. Whether a conventional common emitter style circuit is to be used, or even whether a common base should be employed. The decision will depend upon factors including the matching input and output impedances, the level of gain required and the matching arrangements to be used.

Choice of active device. The type of device to be used is also important. There are generally two decisions, whether to use a bipolar based transistor, or whether to use a field effect device. Having made this, it is obviously necessary to decide upon a low noise device. The noise performance of transistors and FETs is normally specified, and special high performance low noise devices are available for these applications.

Determination of required gain. It may appear that the maximum level of gain may be required from this stage to minimize the levels of amplification required later and in this way ensure that the noise performance is optimized, this is not always the case. There are two major reasons for this. The first is that the noise performance of the circuit may be impaired by requiring too high a level of gain. Secondly it may lead to overload in later stages of the radio receiver and this may degrade the overall performance. Thus the level of gain required must be determined from the fact that it is necessary to optimize the noise performance of this stage, and secondly to ensure that later stages of the receiver are not overloaded.

Determination of current through the active device. The design of the first stage of the radio receiver must be undertaken with care. To obtain the required RF performance in terms of bandwidth and gain, it may be necessary to run the device with a relatively high level of current. This will not always be conducive to obtaining the optimum noise performance. Accordingly the design must be carefully optimized to ensure the best performance for the whole radio receiver.

Use of low noise resistors. It may appear to be an obvious statement, but apart from choosing a low noise active device, consideration should also be given to the other components in the circuit. The other chief contributors are the resistors. The metal oxide film resistors used these days, including most surface mount resistors normally offer good performance in this respect and can be used as required.

Optimize impedance matching. In order to obtain the best noise performance for the whole radio receiver it is necessary to optimize the impedance matching. It may be thought that it is necessary to obtain a perfect impedance match. Unfortunately the best noise performance does not usually coincide with the optimum impedance match Accordingly during the design of the RF amplifier it is necessary to undertake some design optimization to ensure the best overall performance is achieved for the radio receiver.

Ensure that power supply noise entering the circuit is removed. Power supplies can generate noise. In view of this it is necessary to ensure that any noise generated by the radio receiver power supply does not enter the RF stage. This can be achieved by ensuring that there is adequate filtering on the supply line to the RF amplifier.

Summary

Receiver sensitivity is one of the vital specifications of any radio receiver. The key factor in determining the sensitivity performance of the whole receiver is the RF amplifier. By optimizing its performance, the figures for the whole of the receiver can be improved. In this way the specifications for signal to noise ratio, SINAD or noise figure can be brought to the required level.