Reciprocal Mixing on Radio Receivers

- an introduction or tutorial about the essentials of reciprocal mixing and how it affects radio receiver performance.

Reciprocal mixing is a key radio receiver performance parameter. Although poor reciprocal mixing may not always be obvious, when radio receivers are operating in the presence of strong off-channel signals, the reciprocal mixing performance is a key parameter. This can affect many forms of radio receiver ranging from cellphones, through Wi-Fi access points to high performance radio communications receivers.

Reciprocal mixing results from the phase noise performance of the local oscillators within the radio receiver. In general the majority of the phase noise is generated by the main synthesizer within the receiver. As such its performance is critical to the reciprocal mixing performance of the equipment.

For receivers operating in the presence of local strong signals, the reciprocal mixing performance of the overall receiver is important. In turn this places requirements onto the synthesizers used within the receiver.

Reciprocal mixing basics

Reciprocal mixing occurs when the phase noise from the local oscillator signal is superimposed onto a strong off channel signal. This phase noise then masks out the much lower level wanted signal that is within the receiver passband.

To look at how reciprocal mixing occurs take the case of a superhet receiver tuned to a strong signal. The signal will pass through the radio frequency stages, and then in the mixer it will be mixed with the local oscillator to produce a new signal at the right frequency to pass through the IF filters. When the local oscillator is tuned away by ten kilohertz, for example the signal will no longer be able to pass through the IF filters. However it will still be possible for the phase noise on the local oscillator to mix with the strong incoming signal to produce a signal that will fall inside the receiver pass-band as shown. This could be sufficiently strong to mask out a weak signal within the receiver passband.

Reciprocal mixing measurement and specification

A number of different methods are used to define the level of reciprocal mixing. Generally they involve the response of the receiver to a large off channel signal. To perform a reciprocal mixing measurement is rarely easy. The signal generator must always be much better than the receiver, otherwise the performance of the signal generator will be measured! To overcome this many people use an old valve generator because their performance is often very good in this respect.

A measurement can be made by noting the level of audio with a BFO on from a small signal. The signal is then tuned off channel by a given amount, normally about 20 kHz and then increased until the audio level rises to the same level as a result of the phase noise from the receiver. As the noise level is dependent upon the bandwidth of the receiver this has to be specified as well. Generally a bandwidth useable for SSB is used i.e. 2.7 kHz.

For example a good HF communications receiver might have a figure of 95 dB at a 20 kHz offset using a 2.7. kHz bandwidth. This figure will improve as the frequency offset from the main channel is increased. At 100 kHz one might expect to see a figure in excess of 105 dB or possibly more.

Another way of measuring the phase noise response is to inject a large signal into the receiver and monitor the level needed to give a 3 dB increase in background noise level.

Summary

The reciprocal mixing performance of many radio receivers and radio receiver modules is seldom published. Nevertheless it is still an important characteristic that needs to meet the required levels if operation in environments where strong signals are present is envisaged. To ensure that the required reciprocal mixing performance is achieved, the local oscillator which is normally a frequency synthesizer must have a good phase noise performance.