A simple way of phase locking microwave local oscillators

  
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The circuit is presented as a starting point - there will be some experimentation needed for each individual case. For the popular microwave bands the LOs are usually generated from overtone crystal oscillators followed by multipliers, with the following table showing the usual LO frequencies for the narrowband segments along with the associated crystal frequency. The final
A simple way of phase locking microwave local oscillators - schematic

columns show the highest frequency that is a submultiple of both this and a 10MHz reference, the highest common factor or HCF, and the associated division. This HCF can become the comparison frequency in a phase locked loop, and is the highest frequency that is possible here. All the comparison frequencies can be derived from a 10MHz reference by making use of simple logic divider chips to give the divide by R function (all could be derived from 2MHz in fact) All the comparison frequencies are over 200kHz, so phase locked loops can be made with wide loop bandwidths. For those marked with a *, an even higher comparison frequency is possible, but the values stated keep the frequencies within a narrower band for a common design. So now the only difficulty is providing the divide by N from the crystal frequency. An off the shelf synthesiser chip such as the MC145170, or those from other manufacturers, would make an easy job of this but there is an even simpler solution providing you are prepared to do a bit more adjustment and optimisation. Anyone who has studied the brick` range of microwave sources will have seen how a high Q cavity oscillator is locked to a reference oscillator in the 100MHz region by a sampling phase detector. This device combines the functions of frequency multiplier and phase in one network. In the bricks, a snap varactor diode is hit with about 200mW of reference signal and so generates sharp...



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