Sensitivity

• The sensitivity of a radio receiver is its ability to amplify weak signals. It is often defined in terms of the voltage that must be applied to the receiver input terminals to give a standard output power, measured at the output terminals.

• For AM broadcast receivers, several of the relevant quantities have been standardized. Thus 30 percent modulation by a 400-Hz sine wave is used, and the signal is applied to the receiver through a standard coupling network known as dummy antenna.

• The standard output is 50 milliwatts (50mW), and for all types of receivers the loudspeaker is replaced by a load resistance of equal value.

• The most important factors determining the sensitivity of a superheterodyne receiver are the gain of the IF amplifier(s) and that of the RF amplifier .

• It is obvious that the noise figure plays an important part.

• Receiver sensitivity or RF sensitivity is one of the key specifications of any radio receiver whether it is used for Wi-Fi, cellular telecommunications broadcast or any other form of wireless communications.

• The ability of the radio receiver to pick up the required level of radio signals will enable it to operate more effectively within its application.

• 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.

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 radio receiver 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 optimised 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:

  i. 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.

  ii.Determination of required gain

  • While it may appear that the maximum level of gain may be required from this stage to minimise the levels of amplification required later and in this way ensure that the noise performance is optimised, 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 optimise the noise performance of this stage, and secondly to ensure that later stages of the receiver are not overloaded.

  iii.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.

  iv.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 optimised to ensure the best performance for the whole radio receiver.

  v.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.

  vi.Optimise impedance matching

  • In order to obtain the best noise performance for the whole radio receiver it is necessary to optimise 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 optimisation to ensure the best overall performance is achieved for the radio receiver.

  vii.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.

  • Receiver sensitivity is one of the vital specifications of any radio receiver whether it is used on its own or within some form of radio communications system, either a two way radio communications system, or a fixed or mobile radio communications system.

  • The key factor in determining the sensitivity performance of the whole receiver is the RF amplifier.

  • By optimising 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.