Radiocommunications Agency  EMC Awareness

Transmitter frequency control

Introduction

Radio transmitters are licensed to use an assigned frequency in the spectrum with a particular occupied bandwidth around it. Successful spectrum management relies on this frequency being properly and accurately maintained when the transmitter operates; if it strays outside its allotted slice of the spectrum, it may end up on a frequency allocated to another user who will then suffer interference. The radio spectrum is so highly populated that there is a high probability of this happening.

Controlling the transmitter output

There are three aspects to exercising control over the transmitter output to maintain efficient spectrum use:

• The transmitted power should be no more than is necessary to achieve the required signal to noise ratio on the desired link, whether this is for broadcasting, communications or some other use;

• The intended transmit frequency should be maintained to a high degree of accuracy and should not drift with time or electrical or environmental conditions;

• The transmitted signal should consist as far as possible only of this intended frequency, and emissions at all other (spurious) frequencies must be strictly limited in level.

All radio transmitters are covered by European regulations (encompassed in the Radio & Telecom Terminal Equipment Directive) which apply requirements for efficient spectrum use. For a transmitter to be legally placed on the market it has to meet appropriate standards for spurious emissions and frequency stability. A transmitter which doesn’t meet these requirements, as well as being a danger to other spectrum users, is likely to be illegal.

Key issues in employing this technique

Stability

In most modern radiocommunication applications, the transmit frequency is determined by a quartz-crystal-based frequency synthesizer. This has excellent frequency stability, of the order of parts per million, but needs care in design – or extra filtering – to keep down the spurious emissions generated by the synthesizer technique.

Some types of transmitter, particularly those used for low cost low power data or telemetry, or for high power material treatment, may use oscillators that are not crystal controlled. In these cases you need much greater care in design to make sure that electrical or environmental changes – in load, supply or temperature, for instance – don’t change the transmitter frequency outside the allowed parameters. Testing of such products should aim to prove their stability over the whole range of such influences.

Spurious emission

Unwanted, spurious frequency components are either due to by-products of the frequency generating process, or to non-linearities in the power amplifier chain. An under-specified power amplifier may be able to deliver the required power, but will operate in a non-linear mode, so that the RF output becomes distorted. This means that it contains harmonics of the intended frequency, and if the RF is amplitude modulated (to carry speech or data) then this modulation will also be distorted, which will create spurious emissions near to but outside the intended occupied channel. Non linear power amplifiers can only be used if there is good bandpass filtering of the output before it is fed to the antenna, and if the modulation technique involves no changes to the amplitude of the transmitted signal – i.e. it is phase or frequency modulation.