straightforward way, and one that does not involve any construction changes in the tube, is to change the anode current flow. This phenomenon is known as ‘frequency pushing' to the magnetron engineer. The tuning approach that has been investigated experimentally is changing the anode current in the tube by using the error signal from a phase comparator to change the current in a small buckboost coil, as shown schematically in Fig. 7. The ability of this circuit to provide both phase lock and high gain simultaneously is shown in Fig. 8 where it is compared with the traditional arrangement for frequency locking a magnetron. Over 30 db of gain was achieved with the freedom to change the drive frequency over a relatively wide range of frequency while closely holding the phase lock. Although the use of a buckboost coil in many applications would be highly desirable, its use in the SPS transmitter has already been pre-empted for control of the output of the magnetron. For SPS application, it has therefore been proposed that magnetron tuning be accomplished by fitting the magnetron with a tuner mounted on a diaphragm at the end of the tube. The mechanical tuner will be driven by a ‘voice coil'. The time response for this arrangement is expected to be similar to that of the buckboost coil.
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