Radio Receiver Intercept Point

- radio receiver intercept point / third order intercept point: what it is, the factors that affect it and how it can be improved within a receiver.

The receiver intercept point is a measure of receiver linearity, and it is used in specifying the receiver performance.

The receiver intercept points - typically the second and third order intercept points are widely used in determining the overall dynamic range of a radio receiver, along with indicating other aspects of the performance.

Receiver intercept point basics

The intercept point is a useful theoretical concept for looking at the rise in levels of the intermodulation products with increasing levels of signal. It is found that the level of the intermodulation products rise faster than that of the required signal at the output. As a result there is a point at which the level of a specific intermodulation product reaches that of the required signal - it intercepts the level line for the required signal.

Receiver intercept point
Receiver intercept point

It should be noted that the intercept point is a theoretical point as the amplifier or other circuits will run into saturation before this and could not handle the power levels.

It is found that the intermodulation products rise very fast. The higher the order, the faster they rise, although their levels start much lower. For a 1 dB increase in wanted signal levels, third order products will rise by 3 dB, and fifth order ones by 5 dB. Different plots can be made for the different order products and second, third, etc order intercept points can be deduced. The higher the level of the intercept point, the better the performance.

Second and third order intercept points
Second and third order intercept points

As an example of the sorts of figures that may be obtained, a very high quality professional communications radio receiver may exhibit a third order intercept point of possibly 25 dBm.

Intercept specification problems

A number of problems exist with intercept point measurements and specifications. These need to be remembered when interpreting the performance of a receiver:

  • Intercept points not directly measurable:   The receiver intercept points are extrapolated from data obtained at lower signal levels and this means that their accuracy is dependent on the assumption that the curves of second and third-order distortion are described by straight lines with slope values of two and three, respectively. For this to be true, the relationship must hold good over the usable dynamic range of the receiver.
  • Distortion curves not valid as receiver approaches overload:   This effect can be overcome by measuring the distortion products at low input levels. It is found that the intercept measurements are best made at levels where the distortion products are around 60 dB less than the input signals. This can call for high performance test equipment.
  • Values may be frequency dependent:   Second-order distortion produce distortion components at twice the frequency of a single input signal (second harmonic distortion) and at the sum and difference frequencies of two input signals. With such a wide selection of frequencies involved the second order products may be affected by frequency response variations. Third, fifth and similar products can be much closer in frequency and are therefore not subject to the same frequency response limitations.
  • Incorrect distortion slopes:   While most components seem to provide intermodulation products with the "normal" slope. Some components such as ferrites and also GaAsFETs do not full obey these curves. Accordingly the intercept points can be measured at greatly different levels and the results compared to check the specification validity.

Receivers with poor intercept performance

One of the major problems of receivers with a poor intercept performance is that when the third, fifth, seventh and higher order products are generated, these can fall within the receiver passband.

These new signals generated by the intermodulation process can be received. It may therefore, appear that a receiver is very sensitive because of the number of signals that appear. Instead the signals will be "phantom" signals that would not be received on a better receiver.

The major problem occurs when an intermodulation product is stronger than a real signal, and located on the same frequency. Under these circumstances, the real signal will be masked out by the intermodulation generated signal. The intercept point gives an indication of this type of problem as well as many other issues.

By Ian Poole

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