Radio receiver sensitivity
- an overview or tutorial about the basics of sensitivity in radio receivers
used in radio communications systems including signal to noise ratio, SINAD, and
noise figure.
Receiver sensitivity is one of the key specifications of any
radio receiver whether it is used for broadcast reception or professional use
within a fixed or mobile radio communications system or even within a two way
radio communications system. 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 of broadcast reception or within a radio communications
system of whatever description.
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 and can be seen in
the specification lists for radio receivers and radio communications systems.
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 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.
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:
- Determine the circuit topology required
- Choose a low noise device
- Determine the gain required
- Determine the current through the device
- Use low noise resistors
- Optimise 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 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.
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.
Summary
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.
|
