345 GHz Tuning Instructions

Avoiding the 5th Local Oscilator Harmonic

There is a well--known problem with tuning our 345 receiver. Whilst the LO chain is supposed to produce the 4th harmonic of the Gunn frequency, it is very easy to tune up to the 5th harmonic. The Y--factor on the 5th harmonic seems to always be lower than what can be acheived with the 4th harmonic (less than 2 versus well over 2), but that knowledge is of only limited help when you're trying to tune. Similarly, the new SIS computer's can tell you whether you're on the 4th of 5th harmonic, but it doesn't always show it clearly enough to see.

Luckily, the 5th harmonic gives a very poor Y--factor on the sky, because the resulting sky frequency lies above 400 GHz. So if you tune to ``Y-on-the-sky'' (YOTS), then you will be pulled towards the 4th harmonic. It is fairly easy to do this. First, put the calibration chopper blade into spin mode. This can be done by issuing the following easy to remember commands:

The chopper blade (above the Cass focus, which is used for CALS) will now spin. Ensure that the Y factor mirror is retracted from the beam. Plug the total power output of the receiver into the oscilloscope in the sidecab. Using the oscilloscope, you should be able to see a square wave, with the period of the chopper (which can be seen be looking from the Cass. platform towards the hole in the primary). If no square wave is seen, you are badly mistuned. Play with the tuning until a square wave with decent signal/noise is seen on the scope. Once you've got a reasonable square wave, plug the total power output into a DVM, and put it on the AC voltage setting. You now have a YOTS meter. You can tune up the receiver and LO using on YOTS, and be reasonably sure that you've optimised things for the 4th harmonic.

If you're tuning to a frequency where the sky transmission is varying rapidly as a function of frequency, then the YOTS meter could cause you to tune up with a poorly balanced sideband ratio. So if you are tuning to such a frequency, after maximizing YOTS, you should maximize total power on a hot load.

To stop the chopper from spinning, issue the command: XQ \#S,
then type control-backslash, to exit from SET HOST/DTE GALIL The chopper will now spin down, and should come to rest at the sky position. You should look at the chopper blade, to make sure that it has come to rest at the proper position. If it has not, then cycle power on the Galil motor controller (near the top of the sidecab rack, with a big green power button). The Galil should home itself when power is re--applied.