al way. Multi-loop synthesizers Many high performance synthesizers use a number of loops, and use both mixers and digital dividers to obtain the required output. By combining a number of loops it is possible to produce high class, wide range oscillators with low levels of phase noise and very small step sizes. If a single loop is used then there are many short falls in the performance. There are a large variety of ways in which multi-loop synthesizers can be made, dependent upon the requirements of the individual system. However, as an illustration a two loop system is shown in Figure 5.25. This uses one loop to give the smaller steps and the second provides larger steps. This principle can be expanded to give wider ranges and smaller steps. The operation of the synthesizer is not complicated. The first loop has a digital divider. This operates over the range 19 to 28 MHz and has a reference frequency of 1 MHz to provide steps of 1 MHz. The signal from this loop is fed into the mixer of the second one. The second loop has division ratios of 10 to 19, but as the reference frequency has been divided by 10 to 100 kHz this gives smaller steps. The operation of the whole loop can be examined by looking at extremes of the frequency range. With the first loop set to its lowest value the divider is set to 19 and the output from the loop is at 19 MHz. This Receivers First loop Second loop (Output = 20.0-29.9 MHz in 100 kHz steps) Voltage (19-28 MHz) Voltage Reference Phase Programmable controlled Mixer controlled oscillator detector divider oscillator oscillator (1 MHz) (Divsion ratio 19-28) (1.0-1.9 MHz) (Divsion ratio Prog. filter 10-19) divider (100 kHz) Fixed (100 kHz) Phase divider detector filter (Division ratio = 10) Figure 5.25 An example of a synthesizer using two loops feeds into the second loop. Again this is set to the minimum value and the frequency after the mixer must be at 1.0MHz. With the input from the first loop at 19 MHz this means that the VCO must operate at 20 MHz if the loop is to remain in lock. A