The intercept point or more exactly the third order intercept point for a radio receiver is a measure of receiver linearity. It is a parameter that is used in specifying the strong signal handling capability of a receiver.
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.
Radio 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.
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.
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. Other radio receivers may have much lower intercept points to reduce the costs and also their requirements may not call for such good strong signal performance.
Intercept point specification problems
The intercept point on a radio receiver has a number of issues with its specification. These need to be understood when examining the performance of a radio.
- 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.
- 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.
- 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.
- 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.
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 pass-band.
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.
The intercept point and more specifically the third order intercept point for a radio receiver gives a good indication of its strong signal handling performance. However it is one aspect of this area of performance and also one specification in a variety of parameters that will specify the overall performance of the radio. It should therefore be taken along wit the other parameters in the datasheet and also interpreted in light of the difficulties of the measurement.