Receiver Tuning Guide for Observers

Receiver tuning can be performed in a straightforward, systematic fashion. If you take things slow and think about what you are doing, there is no need for undue anxiety. Read through the following steps before beginning the tuning process. Whenever tuning the receiver, be sure to wear a grounded, antistatic wrist strap. The best place to ground the wrist strap is on one of the dewar's many screw heads. Table 1 summarizes the receiver's tuning settings from 215 to 270 GHz.

1. Calculate the required Gunn oscillator and synthesizer frequencies. This can be done in an approximate way by using the following formulas.

   

   

   where,

   ``+'' should be used for lower sideband operation (LSB)

   ``-'' should be used for upper sideband operation (USB)

   A more exact calculation for the synthesizer frequency (which includes the source velocity relative to Earth) can be obtained using Bill Peter's program SYNCALC located in the directory [SMT.TEST].

   Enter the required frequency on the HP sythesizer using the tuning knob on the righthand side. Push buttons under the frequency display allow you to set the number of decimal places to which you want to enter the frequency. Once the frequency has been entered, hit the HOLD button. This will keep the frequency from being accidentally changed. Look up the required Gunn frequency on the attached Gunn tuning sheet. Adjust the Gunn tuning and backshort micrometers accordingly. When adjusting the Gunn, keep an eye on the spectrum analyzer monitoring the 100 MHz phaselock I.F. signal. When the Gunn is locked, you will see a signal at 100 MHz. Also, a red LED on the Phaselock Box will shine reassuringly. The Gunn operating voltage should be around 9.9 V. This voltage is adjusted using the right most vernier knob on the phaselock control box. The numbers on the knob correspond to the Gunn voltage.

2. Now decrease the L.O. attenuation until you see a rise in the junction current. At this point you are ready to adjust the receiver backshorts. (If no rise in junction current is observed, you should try optimizing the receiver backshorts anyway. You can get an idea of where the backshorts should be from the accompanying tuning sheet. Please note the optimum receiver settings may change over time due to wear and tear on the backshort drives, etc.) The junction and E-plane backshorts are accessed from the back of the receiver. The junction backshort is on the left and the E-plane backshort on the right. If you are moving to higher frequencies, the backshorts should be turned in, the opposite is true for going to lower frequencies. To move the junction backshort in, turn the knob counter-clockwise. To move the E-plane backshort in, turn it clockwise. The backshorts should be easy to turn. If you find they are getting a bit stiff, it means you have reached the hard stop or they have been damaged. Try moving them in the oppposite direction. If they are still stiff notify the SMTO staff member on duty. As you get near the optimum settings, you should see a peak in both junction current and total power. The total power level can be read from the front of the total power box. A voltmeter can be connected via a BNC cable to the front of the total power box so the power level can be seen by the person turning the backshorts. Turn the backshorts very slowly. If you go too fast you may miss the tuning peak altogether. If you pass through the peak, no problem; simply reverse the direction you are turning the knob. Due to backlash in the gears, you will probably have to turn the knobs further backward than you might think to find the peak again. At this point you are probably saturating the junction with L.O. power. Increase the L.O. attenuation until you read a current of   A.

3. Next adjust the SIS bias voltage for maximum I.F. power. The best SIS bias voltage should be between 2.0 and 2.2 mV (see tuning chart). If the magnetic field is not completely optimized, you may find that you can get higher I.F. power at lower bias voltages. These higher power levels are the result of Josephson noise, and should be avoided. To eliminate unwanted Josephson noise, you may initially need to turn the magnet bias up to  mA. After a few seconds, try reducing the magnet current back down to 32 mA or so. Once you have peaked the SIS bias voltage, adjust the L.O. attenuator once more to get the junction current down to around 25  A.

4. Once you have maximized the I.F. power with both backshorts and the junction bias voltage, you are in a position to perform a Y-factor measurement. To perform an accurate measurement, be sure to first fully attenuate the I.F. (using the adjustable attenuators on the total power box) and zero-out any DC offset using the DC offset and gain controls on the total power box. Once the DC offset is zeroed out, return the I.F. attenuators to their original settings. To compare your Y-factors to the ones in the provided table, place an ambient temperature piece of absorber in front of the beamsplitter and write down the total power voltage. Next, place a piece of absorber that has been immersed in liquid nitrogen in front of the beamsplitter and note the total power voltage. The ratio of the ambient to cold load temperatures is the Y-factor. If your Y-factor turns out to be on the low side, check to see if the Gunn is still locked and the junction curent is still around 25  A. If all they look OK, you should try reoptimizing the backshort positions and/or the SIS bias voltage.

   If your Y-factor is acceptable ( ), you are ready to observe!

   A sample set of IV and IF power curves are shown in Figure 3. A look-up table of receiver tuning parameters is attached. Due to backlash, the receiver backshort (BS) and E-plane (EP) micrometer values may differ from the exact values listed.