Magnetron Source

A frequency locked microwave source is a critical part of any tests with a tuned cavity, because the higher the Q, the narrower the acceptable frequency spread.

Contents

Introduction

Commercial magnetrons produce a frequency of 2.45Ghz ± 10Mhz, which means the frequency could be as high as 2.460Ghz or as low as 2.44Ghz. Unfortunately, tests of the commercial magnetron we are using shows that the centre frequency wanders.

Test Setup

First, here is the initial test setup:

We tested to see what the attenuation was on the coupling port on the circulator and it turns out to be -62.6dBm. If the input power is 1000W or 60dBm, then the remaining power at the coupling port is roughly 0.6mW (-2.6dBm where -3 dBm = 0.5 mW) We then hooked up the magnetronas shown in figure 1.

And, as shown in figure 2, attached the coupling port to another 70dB attenuator for safety and then hooked the output into a power meter which read between -58dBm and -52dBm depending on how long we left the microwave on. We then hook the output up to the spectrum analyzer and got the results shown in figure three. The GIF contains about fifteen pictures taken over a thirty second test and here is a quick analysis:

  • The signal is intermittent – this is likely because the power supply is a voltage doubler which only has a %50 duty cycle, which are cheap to make but not useful for our tests.
  • The primary signal starts at roughly 2.466Ghz and moves downward – The cause is pretty simple, the magnetron is heating up, going from 33c to about 90c, most likely because of the short and reflected power. The center frequency is moving with temperature.

A commercial magnetron still may be useful, but we will need a water cooled one to keep the frequency stable and a continuous wave power supply.

Magnetron Frequency Locking

{Describe, using the William Brown paper as reference, how to frequency lock a magnetron}

Signalhound Spectrum Analyzer

An important part of the feedback loop is a spectrum analyzer. (See the Spextrum Analyzer page for more details)