t the other end of the range the divider of the first loop is set to 28, giving a frequency of 28 MHz. The second loop has the divider set to 19, giving a frequency of 1.9 MHz between the mixer and divider. In turn this means that the frequency of the VCO must operate at 29.9 MHz. As the loops can be stepped independently it means that the whole synthesizer can move in steps of 100 kHz between the two extremes of frequency. As mentioned before this principle can be extended to give greater ranges and smaller steps, providing for the needs of modern receivers. Direct digital synthesizers Although phase locked loops form the basis of most synthesizers, other techniques can be used. One that is being used increasingly is the direct digital synthesizer (DDS). Although the techniques behind this method of generating signals have been known for many years, it was necessary for semiconductor technology to develop to a sufficient level before they became a viable option. In fact many synthesizers combine both PLL and DDS techniques to gain the best of both systems. As the name suggests this method generates the waveform directly from digital information. This is in contrast to a phase locked loop system 142 Newnes Guide to Radio and Communications Technology Phase increment between two points Direction of rotation Figure 5.26 Operation of the phase accumulator in a direct digital synthesizer which uses a signal on one frequency, i.e. the reference, to generate a signal on the required frequency by the action of the loop. The synthesizer operates by storing various points in the waveform in digital form and then recalling them to generate the waveform. Its operation can be explained in more detail by considering the phase advances around a circle as shown in Figure 5.26. As the phase advances around the circle this corresponds to advances in the waveform as shown. The idea of advancing phase is crucial to the operation of the synthesizer as one of the circuit blocks is called a phase accumulator. This is basically a form 