Wow, another status update in the same week means things are getting done! Summary:
An engineer friend from my university days has joined the team and will be designing and building the feedback control for the magnetron. A spectrum analyzer was purchased and the bent probes built.
You can read up about Neil Gruending in a recent EEweb article which mentions that he owns five multimetres – he is an engineer’s engineer. Neil and I worked on a project together many many moons ago and it was only in the last year that I reconnected with him through Facebook. After reading that article, I realized he was the perfect guy to build the feedback loop and the timing looks to be right as he has time and interest, yes!
Even better, Neil is a big supporter of open source hardware and has agreed to share his design, including software and any circuit layout. Sharing the design is important if other groups want to duplicate the work.
For most of this week, he and I have been talking about what equipment he needs and one of the first pieces is a spectrum analyzer. Which was something else I learned about this week, the difference between a network analyzer, which I use all the time in the lab, and a spectrum analyzer. The 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 (screen shots of which I have sent around previously) 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.
What took up the most time this week was looking at all the options available for getting a spectrum (or network analyzer). The biggest problem 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. For example, take a look at this ebay search which shows that if a spectrum analyzer is under $4K it is either 25 years old, or more likely, doesn’t go up to 3Ghz. 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. Meh.
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.
The options were short listed to :
- $4K for a second-hand spectrum analyzer available here in Edmonton [eBay.com – 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:
The winner was Signalhound and I purchased their spectrum analyzer for $1076CND today and it should be here next week. Here ‘s a pic:
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.” and the feedback loop I described was:
- “We are (quietly) duplicating the “demonstration engine” on the “EMDrive” (emdrive.com) in a university laboratory environment. The EMDrive is essentially a magnetron, the heart of a domestic microwave oven, dumping upwards of a 1KW at 2.45Ghz into a finely tuned closed resonating cavity. At the frequency the cavity will be tuned to, it will have a Quality factor in the tens of thousands, which means a bandwidth 2 to 6Khz wide. By “bandwidth” I mean that the input frequency will need to be centered at the resonating frequency and not wander outside that range. Commercial magnetrons are great at heating food, but not so great at delivering higher power in a narrow frequency range. Our plan is to create a feedback loop by first, reading the magnetron output frequency, attenuating it, measuring it, then controlling the magnetron modified with electromagnets instead of permanent magnets.”
Once the spectrum analyzer arrives, then I will be bringing it up to the university to test it against the current magnetron setup. I will then package everything up including the magnetrons, power supplies, circulator, tuning waveguide and coupler sections and get it all to Neil. The intention is to use, as a guide, the paper written by William C Brown “The Magnetron – A Low Noise, Long Life Amplifier” [PDF ]
Oh and I also got the bent probes finished today:
Whew! This project might get completed after all!
I will be up at the university this coming week to test the bent probes.