 to the value in the phase accumulator each time, whereas the next frequency will be obtained by adding 5068. All the time the clock remains the same, and it is normally a crystal oscillator which may be oven controlled to ensure the frequency stability of the whole system. From this it can be seen that there is a finite difference between one frequency and the next, and that the minimum frequency difference or frequency resolution is determined by the total number of points available in the phase accumulator. A 24-bit phase accumulator provides just over 16 million points and gives a frequency resolution of about 0.25 Hz when used with a 5 MHz clock. This is more than adequate for most purposes. It is worth noting that to change frequency the synthesizer simply starts adding the new number onto the total held in the phase accumulator. There is no need for a reset. This means that the synthesizer can change frequency virtually instantaneously. This can be a major advantage of this type of synthesizer in some applications. Phase locked loop systems have a distinct settling time which slows down the rate at which they can change. This may be a critical parameter when receivers are required to scan. The other advantage of direct digital synthesizers is that they can operate over a very wide range. The major limitation is the top frequency of operation that is governed by the integrated circuits. These synthesizers do have some disadvantages. They are currently more expensive then their phase locked loop counterparts, but this may change with further developments in semiconductor technology. The 144 Newnes Guide to Radio and Communications Technology other disadvantage is that they produce small spurious signals close to the wanted signal. Normally these can be reduced to acceptable levels, but they may pose a problem in some applications. Receiver circuitry Today integrated circuits are widely used in receivers, and indeed there are many ICs that contain complete receivers. However, few IC circuits are shown here as