Analysis of Results & Chamber Complete!

It took years, but what a fantastic week!  Two events, long in the building stage, got completed this week – the vacuum chamber and cavity.

First the vacuum chamber and here is Kashish, one of the masters students, helping me pump it down for the first time:

The gauge with red numerals on the bottom right shows 49.0 microns (vs 760microns at atmosphere), which is the lowest we have pumped the cavity down to.  With a longer pump down, we should be able to get it close to our minimum, 18 microns.  We were using the timer on the iPhone, bottom left, to time how long the vacuum chamber could keep the vacuum.  Even with a temporary seal (the green thing on top) it was only losing about 40 microns of pressure in the first five minutes.  With a proper seal (shipped this past week), we should be able to extend the vacuum time.   Our experiments will be less then a minute long and even at the current state of leakiness, the chamber is useful.
Besides adding a better seal, another modification will be to put the vacuum gauge on it’s own port.  Currently it measures atmosphere (760microns) when the pump is on, which means we have to shut the pump off to see what the pressure is.

Getting a solid vacuum in the chamber is a big step because it verifies all the parts that have been bought off eBay, from the pump, to the cavity, to the strength (so far) of the cell-cast acrylic window.  Awesome!  $3500 and a year in the making! (It was September 8th,2009 when I first purchased the chamber)

The second big event is that it looks like the cavity is working as per our simulations!!

At first, we were getting this:

Really crappy!  High insertion losses (7db) and flat resonance spikes, which means low Q.
After mucking around, it turns out it was a problem with the probes.  In order to solder the probes to the probe mount, I take the bottom plate off to get inside.  After soldering the probe, I then test connectivity between the external probe mount and the cavity, which is a closed loop as it should be.  I then bolt on the bottom plate but unbeknownst to me, doing so warped the plastic just enough to disconnect the probe mounts.  The fix was easy enough, I used a lower power soldering iron to solder the copper mounting plates to the copper wrap!  I will get a picture at some point…

With the probes fixed, we immediately got much better results:

We haven’t calculated Q yet, but the results look very promising, an insertion loss of just 0.6dB and a beautiful narrow deep spike (the narrower the spike the higher the Q)  Below are the simulations results to compare – the “blue” line in the actual results corresponds to “red” in the simulated and yellow corresponds to green:

A quick analysis:

  • The range on the measured results is from 2.4Ghz to 2.5Ghz, the range on the simulation is 2.4274Ghz to 2.4632Ghz
  • Simulated insertion loss is 0.77dB (S2,1), actual is 0.63dB!
  • The measured results show a double resonance (the two blue spikes) as does the simulation (two red spikes)!
  • The maximum S1,1 simulated was -14dB, but actual is -28dB and you can tell the spike goes much much lower where, although I don’t have a picture, it measured -48dB!
  • If the actual results don’t change too much after we zoom in next week, the Q will certainly be lower then the 71K simulated which we can tell by how wide the spike is at the top.  The width of the spike at the top in the simulation is maybe 2 or 3 Mhz wide (take 2.455Ghz minus 2.445Ghz which is 10Mhz and the width looks to be about a fifth that wide), but the actual looks to be twice that (take 2.46Ghz  minus 2.44Ghz equals 20Mhz and the width is roughly fifth of that or, 5 or 6Mhz wide), which means a Q of probably 25K.  We will know more next week when we zoom in and calculate Q.
  • If the depth of the S1,1 curve is a measure of how pure the TE0,1,n mode is, then the actual results looks to be very good.  The difference between our actual (-28dB) to simulated (-14dB) might be explained by not having used enough tetrahedrons in the simulations and/or luck.  It is ironic, that even with our modern technology, we have no way to directly see the wave pattern inside the cavity, and can only strongly guess from indirect results that the cavity is resonating in the right mode.

Having played with the system a bit, a few things are apparent:

  • Tuning works very simply, you move the tuning plate up and the frequency of the resonances moves down.  Move the tuning plate down and the resonances move up.
  • Moving the plate doesn’t move the resonance frequency by very much, in other words, moving the resonance frequency up or down the band can be very fine with the mechanism we have.

As a side story – pictures of the results are taken with a VIXIA HF100 video camera and the lights off in the lab.  Picture a large room with five or six other people in it and every time we take a picture, we kill the lights.

Before going onto the next step, the probe solder points have to be cleaned up to handle high power and the copper mesh needs to be added around the bottom and top seams to ensure solid connectivity and no arcing.

Meanwhile we can move on with the following:

  • Check the attenuation of the reflection port on the circulator – Because resonance occurs at a single frequency with a very narrow bandwidth (the definition of a high Q), an ideal source would only transmit at that one frequency, no matter the condition.  Because we are using a real world and inexpensive magnetron, it will likely not transmit at the correct frequency all the time.  In order to tune the cavity to our magnetron, we have to measure what the magnetron’s center frequency is, and how badly it  wanders.  However, because the magnetron transmits roughly 800W of power, we cannot hook it to the network analyzer but need to attenuate the signal first.  The circulator, which dumps excess reflected energy into a water load, has what looks like a reflected measurement port, which we will see if the power is low enough to measure.  Worst case, the magnetron frequency wanders too much and a magnetron with a phase-locked-loop arrangement will have to be sourced/purchased.
  • Once the issues with the source are cleared up, we can then run the actual experiment and deal with heat problems.

Have a great long weekend! I am off to Calgary and will be back in the lab on Tuesday.