Receiver Intermodulation Performance

- radio receiver intermodulation performance, what it is, the factors that affect it and how it can be minimised.

Intermodulation distortion can be a particular problem for receivers.

Accordingly the radio receiver intermodulation performance is of great importance.

When a receiver, used for any purpose, has poor intermodulation performance, it can impair its reception, especially of weaker signals in the presence of stronger ones as spurious signals are generated which can mask out the wanted signal.

For applications from cellular to Wi-Fi, and PMR to other professional radio applications, the receiver intermodulation performance is of paramount importance.


Compression and receiver intermodulation distortion

The problems from compression arise as a result of the distortion which occurs to the signal when the amplifier runs into compression. The actual method which gives rise to problems may not be obvious at first sight. It can be viewed as the combination of two effects. However to see how it arises it is necessary to look at some of the basic effects of compression.

One of the forms of distortion which arises is harmonic distortion where harmonics of the wanted signal are produced. Depending upon the exact way in which the signal is compressed the levels of even order harmonics (2f, 4f, 6f, etc.) and odd order harmonics (3f, 5f, 7f, etc.) will vary. As a result of the production of these harmonics it is possible that signals below that being received could be picked up. However the RF selectivity is likely to remove these signals before they enter the first stages of the receiver.

Another effect which can be noticed is that the RF amplifier tends to act as a mixer. The non-linear transfer curve means that signals will mix together or modulate one another. This effect is known as intermodulation. It is unlikely that this effect on its own would give any problems. The mix products from signals close to the wanted one fall well away from the received signal. Alternatively, to produce a signal within the receiver pass-band, signals well away from the received one would need to be entering the RF amplifier. These would normally be rejected by the RF selectivity. Take the example of two signals on 50.00 and 50.01 MHz. These would mix together to give signals at 0.01 MHz and 100.01 MHz. These are not likely to give rise to any problems.

Problems start to arise when the two effects combine with one another. It is quite possible for a harmonic of one signal to mix with the fundamental or a harmonic of the other. The third order sum products like 2f1 + f2 are unlikely to cause a problem, but the difference products like 2f1 - f2 can give significant problems. Take the example of a receiver set to 50 MHz where two strong signals are present, one at 50.00 MHz and the other at 50.01 MHz. The difference signals produced will be at 2 x 50.00 - 50.01 = 49.99 MHz and another at 2 x 50.01 - 50 = 50.01 MHz. As it can be seen either of these could cause interference on the band. Other higher order products can also cause problems: 3f1 - 2f2, 4f1 - 3f2, 5f1 - 4f2, and so forth all give products which may could pass through the receiver if it is tuned to the relevant frequency.

Intermodulation distortion
Intermodulation products from two signals

In this way the presence of a strong signal can produce other spurious signals which can appear in its vicinity. The signals mixing with one another in this way may be of a variety of different types, e.g. AM, FM, digital modulation, etc., all of which may combine together to give what is effectively noise. This means that poor third order intermodulation performance can have the effect of raising the noise floor under real operating conditions. In turn this can appear to reduce the sensitivity of the radio receiver which in turn can degrade the performance of the overall radio communications system.


Receiver intermodulation specification

Specifications for receiver intermodulation performance normally specify the levels and frequency offsets of two input or test signals. It is then possible to measure the level of the resultant in-band distortion component.

It is possible to predict the intermodulation distortion level from a knowledge of the receiver third-order intercept point. For two equal-amplitude test tones, the power of the intermodulation distortion product is equal to three times the power of a single test tone minus two times the third-order intercept point, where all powers are in dBm:


Pim = 3 Ptone - 2 Ptoip

Where:
    Pim = power of the intermodulation product
    Ptone = power of a single test tone
    Ptoip = power of third-order intercept point
All powers are expressed in dBm

By Ian Poole


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