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Tuning tips for the new D-band mixers

Following the installation of the new D-band mixers we have noticed several quirks whilst trying to tune them. This page gives a few hints on how to deal with some of the problems we have encountered. You should probably continue to folow the old instructions first and only come back to this if you experience problems.

The end aim is to produce a nice stable looking I-V curve on the oscilloscope such as this one, you are advised to always check this before you press the remote button the first time:

If it doesn't look like this, but is more like a straight line, or has only a small knee then you probably have the current set too high. Switch the LO power control to manual and adjust the dial until the current readings are about 27 in both channels (this may involve turning the polarising turret as well). The line shape should then be as above.

Unfortunately various problems get in the way of achieving this. One of the most common is when the signal ends up looking like this:

Sofar this has always been caused by a problem with multiplier no. 1 and usually when it is suposed to be set to 110 (as given by the computer display). The solution is to wind the micrometer all the way in to its stop (this is only about half a turn from the 110 position). You should then see the 'jaggies' go away, now switch the sweep off and wind the micrometer back out and try and peak the current again. Turn the sweep back on and check the I-V curve, it should now be correct. You may however continue to get the 'jaggies'. If you can't get rid of them then leave the sweep on and wind the micrometer out (from its stop position) until just before the place where the 'jaggies' appear. You may have to repeat this several times to get the feel for it.

When checking the P-V curve, you are looking for something along these lines:

It may look much more 'stretched' in the vertical direction than this. If it looks nothing like this then check the I-V curve, if this doesn't look right then get that sorted out first.

When tuning for the first time in an evening you will probably get something looking like this:

This hump is caused by trapped flux in the junctions (Josephson currents). You need to supress this by setting the magnet currents. Start by turning the magnet current to zero (the peak will become huge). Then gradually turn up the magnet current, the correct position is the first position where the hump is completely (or close to completely) supressed, there may be several positions where it is only partly supressed before you get to the the one where it is completely supressed. You are looking for values between about 15 and 40 on the display (1.5 - 4 milli amps). If you need values in the 50s or higher then it may be worth doing a mini warm up.

To do this set the temperature indicator to position 1 or 2, this should show 3.3 or 3.4 K. Turn on the sweep and look at the I-V curve on the oscilloscope. Then open the JT-stage valve (large micrometer ontop of the dewar, very awkward to get at). You can open it up to about a reading of 300. you should immediately see the temperature start to rise, fast at first, then slower. You need to warm the junctions up to about 11K or when the I-V becomes a straight line (ie. has stopped superconducting). This should take about 5-7 minutes. Once this has happend dial the J-T valve back down to about 100 in one go. The temperature should rapidly drop. Once it gets down to about 8K dial the valve down to about 30 and wait until the temperature reaches below 6K. Then turn the valve until it is at 17. The temperature should rapidly drop back to 3.3-3.5K. The cooldown should take about 4 mins. You should now be able to supress the Josephson currents with much less magnet current.

Contact: Per Friberg. Updated: Thu Dec 23 19:40:49 HST 2004

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