al divider is not the only method of making a synthesizer using a phase locked loop. It is also possible to use a mixer in the loop as shown in Figure 5.24. By using a mixer in the loop, an offset to the reference frequency is generated. The way in which the loop operates with the mixer incorporated can be analysed in the same manner that was used for the loop with a divider. External signal (15 MHz) Output Phase n + f Reference Voltage Mixer controlled (10 MHz) parator (10 MHz) (25 MHz) oscillator filter Figures in brackets are those used in the example in the text Figure 5.24 A phase locked loop with mixer 140 Newnes Guide to Radio and Communications Technology When the loop is in lock the signals entering the phase detector are at exactly the same frequencies. The mixer adds an offset equal to the frequency of the signal entering the other port of the mixer. To illustrate the way this operates figures have been included. If the reference oscillator is operating at a frequency of 10 MHz and the external signal is at 15 MHz then the VCO must operate at either 5 MHz or 25 MHz. Normally the loop is set up SO that the mixer changes the frequency down and if this is the case then the oscillator will be operating at 25 MHz. It can be seen that there may be problems with the possibility of two mix products being able to give the correct phase comparison frequency. It happens that as a result of the phasing in the loop, only one will enable it to lock. However, to prevent the loop getting into an unwanted state the range of the VCO is limited. For loops which need to operate over a wide range a steering voltage is added to the main tune voltage SO that the frequency of the loop is steered into the correct region for required conditions. It is relatively easy to generate a steering voltage by using digital information from a microprocessor and converting this into an analogue voltage using a digital to analogue converter (DAC). The fine tune voltage required to pull the loop into lock is provided by the loop in the norm