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. The difference between a network analyzer and a spectrum analyzer is that a network analyzer has both a source and an analyzer which allows for the characterization of a passive device under test, like the cavity. A signal is injected into one port and the response recorded on the second and the network analyzer has special capabilities for calculating things like Q. The spectrum analyzer is useful for looking at a signal and recording things like power, frequency and frequency-spread and typical spectrum analyzers have no output capability.

The biggest problem for either is price. The yearly demand for analyzer is low, quite likely on the order of tens of thousands of units a year and they are complicated devices because they deal with Ghz frequencies. Up until a few years ago, analyzer have also always included a display, which adds to complexity and cost. Add “complicated” to “low volume” and in a mature market, you get astronomical prices as spectrum analyzers start at $10K and go up. The higher frequency, the higher the cost. Even second hand analyzers tens of years old, hold their value and there is a huge business selling and repairing second hand analyzers. Even if you do find one on the cheap, they weigh a ton, which means shipping is in the hundreds of dollars, not to mention the hundreds paid in GST and brokerage charges.

It is only within the last few years that it has been possible to purchase an analyzer that uses digital technology, DSPs specifically, and a computer to display the results.

Figure 4 : Signal Hound unboxed

The options were short listed to :

  • $4K for a second-hand spectrum analyzer available here in Edmonton, an Anritsu MS2602A for when the link goes dead
  • Or $1k to $2k for a USB driven, display-less spectrum analyzer that uses a computer to display the results and the two considered were these:
    • Signalhound (.com) – a small US based company that also repairs analyzers and does LCD retrofits
    • Spectran (aka Aaronia) – a larger German based company and they sell both a USB based version and a version with a simple LCD display.

The winner was Signalhound and was purchased for $1076CND (figure 4).

Signalhound won for a number of reasons:

  • Price – $919US (not including shipping, taxes, etc).
    • Aaronia has a better product that goes up to 6Ghz versus Signal hound’s 4.4Ghz, but costs $500 more
    • Aaronia doesn’t have an API or programmable interface (which is important and explained later)
    • Keeping the cost low will also help those who wish to duplicate our work.
  • Performance – up to 4.4Ghz
  • One important specification of a spectrum analyzer is how sensitive it is, however, in our case, because the magnetron signal has to be heavily attenuated before being injected into the spectrum analyzer, sensitivity isn’t an issue.
  • Custom programmable – This was the biggest reason Signalhound won because it can be used as part of the feedback loop!

I asked the designer to comment on using the Signalhound in our application and his response was “The Signal Hound API can stream 480K samples per second on a 240KHz bandwidth. It will be more than fast enough for the feedback loop you have described.”

Setup

{Describe how the frequency locking feedback loop works}