New Probes Tested

I rebuilt the probes using a proper metal bending tool ($100 from Princess Auto) and got to within 1mm accuracy.

(The probe on the far right was my first attempt and you can see it is a bit out-of-round.  The 3mm diameter wire is stiff and difficult to work by hand.  Luckily, 3mm wire is readily available at the local Metal Supermarket and relatively inexpensive.)

After mounting the probes in the cavity, they are between 28mm and 29mm in diameter:

I say “between” because of two things – I drilled the threaded holes a little too far from the probe entrances (the white dielectric) which leads to a slight twisting (they should be straight up and down) and widens them a bit.

However, the new probes did not help the measured S-parameters get any closer to the simulations.

Measured

Expected (simulated)

The resonances below the TE0,1 mode have quite distinct shapes.  The most uniquely shaped and easiest to find resonance is the one that shows up at roughly 2.285Ghz (reference the expected results above).  (By resonance, I mean any place where the S1,2 (blue line) rises and the S1,1 (red line) drops)  The 2.285Ghz resonance is a large broadband resonance followed by two sharp resonances at 2.32Ghz and 2.34Ghz and then, between 2.33Ghz and 2.42Ghz, a long dropping S1,2 curve.  That series of resonances and the dropping S1,2 curve, I arbitrarily call “Marker Mountain”, also shows up in the measured data.

The problem is that simulated resonances above 2.42Ghz are not matching the simulated curves.  Simulated data shows another wide steep S1,2 (blue) line, a couple more resonances and then the TE0,1 mode.  The measured data above “Marker Mountain” does have a bunch of resonances, but none of them are as strong as they should be nor in the right places.

The only other difference between the actual cavity and simulated cavity was the tuning plate, which was easy to delete in the simulation and rerun – Here is what resulted (compare it to the measured above!)

Simulated Results without a Tuning Plate

Closer!

Now the simulated and measure S-parameters are starting to look at least a bit more similar and more importantly, I think I can fix the problem!

A few asides:

  • Without the tuning plate, none of the four or five resonances above “Marker Mountain” are TE0,1 modes (I checked the field configurations for each and nada!)
  • I originally thought that removing the tuning plate would not change the S-parameter graph, except to move the resonances lower in frequency, but it would seem that is not the case.

My next task is to:

  • Mount the tuning plate and find the TE0,1 mode!

From last week:

  • Find a way to built mm accurate probes out of copper.  The good news is they are small and should be fairly inexpensive to get made.
  • Add the rest of the status updates to the blog. 82 posts now going back to 2007!
  • I will also run a monster simulation with the probe at 28mm, across a large frequency range that will probably take a day or so (likely running into hundreds of sample frequencies) .  With the resulting S-parameter chart, I can then compare it against the measured S-parameter chart and quickly find the TE0,1 mode.

Probe Diameter Sensitivity Analysis

I reran the probe dimension sensitivity analysis again with a much finer attention to detail and the results look good.  The first thing I did was to bump up the number of tetrahedrals to around 400K and also narrowed the range simulated to just around the TE0,1 mode,  2.435Ghz to 2.45Ghz (15Mhz spread).

Quick summary – A probe at 28mm in diameter is ideal

The results I sent around before with the 4dB insertion loss had a well meshed model but did not have enough samples in a tight enough frequency spread.  One realization over the past few days was that there are two types of precision in EM simulations, the first is the number tetrahedrons and the second is the number of samples.  I will describe the difference in the next two paragraphs.

When CST builds a model, the first type of precision is how many tetrahedral are necessary to accurately simulate the model at a single given frequency.  The larger the space or the higher the frequency, requires more tetrahedrals.  The primary method I use to control the number of tetrahedrals is by specifying the number of steps per wavelength, currently seven (default is 4).  I also increase the “curvature refinement ratio” to 0.25 (from 0.5) and the “Max number of steps from curvature ref:” to 150 (from 100) because our cavity is entirely made out of round parts.   The end result is that for the cavity which is 280mm in diameter and 390mm long simulated at 2.45Ghz, CST requires about 400K of tetrahedrals to meet an accuracy of 1% (it usually stops mesh refinement at 0.5%).  The trick is to get as much accuracy as possible without running out of memory or making the simulations take days.  That many tetradedrals uses about 4GB of RAM when simulating.

The second type of precision is that, depending on the width of the frequency range, CST runs the simulation at as many separate frequencies as necessary to make the interpolation of points on the S-parameter graph smooth.  In the S-Parameter plots below, the frequency width is only 15Mhz and CST sampled 15 different frequencies, each running with about 440K tetrahedrals.  In the previous S-parameter graph (showing 4dB insertion loss), the frequency range was 300Mhz and I stopped the number of samples at 60.

In short, to get an accurate S-parameter graph, it is necessary to both maximize the number of tetrahedrals and the number of samples in a tight a frequency range as possible.

Below are S-parameter graphs where I varied the probe diameter from 30mm (on the left, as they are currently constructed in the cavity) to 26mm (on the right).

It is clear from the graphs that the sweet spot is at 28mm with an S1,1 of -40dB and an insertion loss of -0.5.  It is also clear that having a probe a millimetre on either side will mean a drop of about 20dB for S1,1.

While the simulations were running, I also went back and have started putting all status updates going back three years (to 2009) into a blog you can get to here.   There are currently about sixty status reports up, but I should have them all up by the end of the week.

My next few tasks will be to:

  • Find a way to built mm accurate probes out of copper.  The good news is they are small and should be fairly inexpensive to get made.
  • Add the rest of the status updates to the blog.
  • I will also run a monster simulation with the probe at 28mm, across a large frequency range that will probably take a day or so (likely running into hundreds of sample frequencies) .  With the resulting S-parameter chart, I can then compare it against the measured S-parameter chart and quickly find the TE0,1 mode.

Machined Cavity Results – Meh…

Great progress has been made in the last week as I was up at the University and put the 2nd gen cavity under test for the first time.

Quick Summary -> The cavity needs better probes.

After hooking the cavity up to the network analyzer, the measured cavity produced about six different resonant frequencies between 2 and 3Ghz as expected.  We first zoomed into the most prominent resonance between 2 and 3 Ghz at 2.282Ghz as shown:

Notes:

  • The graph shows a resonance at 2.282Ghz, with a Q of 228 (calculated by using markers 2 and 3) and an insertion loss of 0.5dB (S2,1).
  • Although our target frequency is 2.45Ghz, the cavity does not have a tuning plate yet which means the cavity is longer then it needs to be and the TE0,1 mode is lower in frequency.

The problem is that because we can not see the e-field configuration inside, we were not sure if this was the right mode and can only guess based on indirect evidence.  For example, we can compare the measured S parameter results to the simulated S parameters and look for a mode, which, like the TE0,1 mode, should also have a low insertion loss (i.e. low S2,1).

Today, I went back to the simulations to confirm which resonance is the TE0,1 mode and the diameter of both probes.  Previous simulated results used two different probe diameters, 28mm and 30mm.  In early results, the 30mm probe diameter looked to produce the best result, but, just to confirm, I redid the simulations with a much higher tetrahedral resolution.  Our previous simulations had ~ 250K tetrahedrons done under CST MWS 2009 and I redid the simulations with 440K tetrahedrons with a newer version, CST MWS 2010.  (The larger simulation takes all day and pushes the limits of the computer even with 6 cores running at 3.2Ghz and 8GB ram)

First, the simulated results done before the cavity was built (note the marker at 2.45Ghz. showing an insertion loss of 0.766dB):

Here are the new results from the higher tetrahedron simulation:

There are three resonances in the simulated S-Parameter chart, from left to right, a broad one at 2.4383Ghz, the one noted at 2.4479Ghz and another one at 2.4553Ghz on the right.  After looking at the modes of each, the 2.4479Ghz is the TE0,1 mode we want (shown below).

(I used cut-planes and clamped it at 5000V/m to show the circular TE0,1 mode clearly)

Analysis:

  • As shown, with a 30mm probe and with a higher tetrahedron count, the insertion loss of the TE0,1 mode is 4db!! Yikes.   The calculated Q was still 67K, which is great.
  • The resonance we measured in the lab at 2.282Ghz is very likely the wide band resonance shown in our simulations at 2.4383Ghz and the mode looks like this:
     

    • Note the E-field starts and ends on the walls and is not the circular mode of TE0,1. (again, I used a cutting plane to make the e-field clearer)
  • When in the lab, the next resonance above 2.282Ghz was not very clear and the insertion loss was worse then simulated (6db or greater I think) – I will confirm this the next time I am in the the lab.

My next action is to do a simulation with a probe diameter of 28mm and change other variables (not the cavity dimensions of course) to see if  the TE0,1 mode can be made to have a lower insertion loss and perhaps a wider bandwidth.  If the modifications work, then I will match those conditions as closely as possible in the machined cavity.

Once I figure out if the probes fix the problem (or if the cavity is a write-off), then I will figure out a better way to make them.  Either using some type of jig to mold the wire in, or getting them machined directly. The probes I made are not very accurate (shown below) and likely affecting the results (they should be round and exactly 30mm in diameter, of which they don’t look to be either, LOL):

Until next week!

Here is a shot from the documentary footage as we were preparing the cavity for testing.

(Kevin, behind and left of us, is calibrating the network analyzer as Dr. Karumudi, myself and two masters students, Kashish and Gary (helping), bolt on the copper end cap.  You can see the 1st generation cavity in the foreground.)

Machined Cavity Probe Difficulties

Yikes, it has been a while since I got the cavity, but I am now one step away from getting it ready for testing. In the last update, I left you with pictures of the cavity a few days after I received it.  I attempted last week to solder the probes to the cavity wall, but even if the solder stuck to the copper plating, it was not strong enough to support the probe, which, when touched, broke the solder pad completely off.  Worse, the tinning process with the hot flame ended up taking some of the copper off with it.

Here is a screenshot from the documentary footage of the probe which was attached for all of a few seconds before I touched it again:

(The holes around the circumference are threaded to attach the copper end plate)

A better solution, although more time consuming, is to drill a hole, thread the hole, then add threads to one end of the copper probe and screw the probe into the cavity.  (The other end then attaches to the N-to-probe adapter).  I bought a tap set (shown below, including the screws) which was needed to mount the N-to-probe adapter but to thread the probe itself, I need two dies (one shown on the right).  I need a 2mm die for the measurement probe and a 3mm die for the power probe, and the dies should be here by Friday (nobody in Edmonton carries such small dies as regular stock).

The good news is that if this cavity shows promise, I can have it replated for about $500.  Being able to completely dissemble and resemble the cavity without soldering anything will be useful.  In the future, when working with cavities, my default will be to try and bolt things together instead of soldering them.

Here’s a cropped shot from the documentary footage as I drill holes to mount the N-to-probe adapter (you can see both the power adapter being drilled and the measurement adapter lower right side):

I have also been talking to vendors about getting a proper continuous power supply that runs on 110v and includes a feedback circuit to keep the frequency as tightly centered as possible.  At this point, I’m guessing it will cost anywhere from $5K to $10K for such a device (new), if I can’t make it myself.