Receiver sensitivity

- notes and a tutorial overview about the basics of sensitivity in radio receivers RF circuits detailing the various methods used and why sensitivity is essential.

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


Methods of specifying sensitivity performance

As the RF sensitivity performance of any receiver is of paramount importance it is necessary to be able to specify it in a meaningful way. A number of methods and figures of merit are used dependent upon the application envisaged:

  • Signal to noise ratio:   This is a straightforward comparison ratio of a given signal level to the noise within the system.
  • SINAD:   This receiver sensitivity measurement is slightly more formalised, and it also includes distortion as well as the noise.
  • Noise factor :   This RF receiver measurement compares the noise added by a unit - this could be an amplifier or other unit within the system or it could be a complete receiver.
  • Noise figure:   The noise figure, or NF of a unit or system is the logarithmic version of the noise factor. It is widely used for specifications of sensitivity and noise performance of a receiver, element within a system, or the whole system.
  • Carrier to noise ratio, CNR:   The carrier-to-noise ratio is the signal-to-noise ratio (SNR) of a modulated signal. This term is less widely used than SNR, but may be used when there is a need to distinguish between the performance with regards to the radio frequency pass-band signal and the analogue base band message signal after demodulation.
  • Minimum discernable signal, MDS:   The Minimum detectable or minimum discernable signal is the smallest signal level that can be detected by a radio receiver, i.e. one that can be processed by its analogue and digital signal chain and demodulated by the receiver to provide usable information at the output.
  • Error vector magnitude, EVM:   Error vector magnitude, EVM is a measure that can be used to quantify the performance of a digital radio transmitter or receiver. There various points on the constellation diagram set to identify various digital states. In an ideal link, the transmitter should generate the digital data such that it falls as close to these points as possible - the link should not degrade the signal such that the actual received data does not fall onto these points, and the receiver should also not degrade these positions. In reality, noise enters the system and the received data does not fall exactly onto these positions. The error vector magnitude is a measure of how far from the ideal positions the actual received data elements are. Some times EVM may also be known as the Receive Constellation Error, RCE
  • Bit error rate, BER:   Bit error rate is a form of measurement used for digital systems. As the signal level falls or the link quality degrades, so the number of errors in the transmission - bit errors - increases. Measuring the bit error rate gives an indication of the signal to noise ratio, but in a format that is often more useful for the digital domain.

All the receiver sensitivity specification methods 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.


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 or radio communications system, 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.


Note on Noise:

Noise is present in all electronic and RF circuits. It presents a limitation on many aspects of performance. Noise arises from many causes and sources. Understanding what forms of noise are present and enables the system performance to be tailored to ensure the effects of the noise can be minimised.

Click for more information on Noise.



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:

  • 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.
  • 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.
  • 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 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.
  • 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.
  • 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.
  • 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.

By Ian Poole


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