Receiver Tuning:

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.

Here is the procedure for powering-up the LO chain.

1. Make sure the LO power attenuator is fully-out (extended toward the telescope flange).
2. Apply +0.2 V of bias to both the doubler and tripler. Remember to turn the power knobs fully clockwise.
3. Turn on the power to the Phase Lock Box. Flip-up the Oscillator power toggle switch located on the Phase Lock front panel. The Gunn is now energized.
4. Slowly turn in the LO attenuator until you see negative current being drawn on the doubler. This will happen when the LO attenuator micrometer is nearly all the way in. Once you start to see negative current, slowly turn up the doubler bias voltage until the current is reduced to 0.0 mA. Also increase the tripler voltage to  0.4 V. Continue to slowly turn in the LO attenuator knob and zeroing out the current by increasing the doubler bias voltage. Also, slowly increase the tripler bias voltage until it reaches  1.5 V. Do not exceed 4 V bias on the doubler.
5. Now, slowly turn back out the LO power attenuator until the SIS bias current at 2.08 mV reads about 22 microamps.

Once the LO is on, the following procedure can be used to optimize the receiver performance at a given frequency.

1. The most difficult part of tuning the receiver is adjusting the multiplier for maximum output power. Multipliers can be blown-up in several ways: a) by static discharge, b) by applying too much voltage, or c) by applying too little voltage. Spikes on the AC power line can also kill or reduce the efficiency of a multiplier. One rule to follow is that the bias current on the doubler should always be negative. The bias voltage on a multiplier will change as the incoming Gunn power changes. So when tuning the Gunn or adjusting the LO attenuator, monitor the multiplier bias voltage and current. If the bias current goes beyond the  mA. increase the bias voltage. Do not let the bias voltage on the doubler go above  V.

   The best way to tune to a new frequency is to ``walk'' the multiplier from a frequency where its output power is peaked. The target Gunn oscillator and synthesizer frequencies can be computed from the following expressions.

   

   

   where,

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

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

   Try not to adjust the Gunn tuning so far that you no longer see LO power on the SIS junction current. You can monitor the SIS junction current either by looking at the SIS bias box current meter or the junction IV curve on the X-Y plotter. To see the junction's IV curve, connect the and outputs on the back of the SIS bias box to the X and Y inputs, respectively, of the XY plotter. These cables are labeled and should be located near the X-Y plotter. The junction should be biased in constant voltage (V) mode (indicated by the central toggle switch on the SIS bias box). At each frequency step, optimize the multiplier and receiver by adjusting the tuning micrometers. Each of the multipliers have two tuners. Begin with the frequency doubler (the one closest to the Gunn). Adjust the doubler's backshorts (there is one on each side) to maximize the multiplier current. Watch the tripler bias voltage and current also, since by adjusting the doubler, you are varying the input power to the tripler. After performing these adjustments, there should be some LO power present on the junction's DC IV curve. Adjust the receiver backshorts to maximize the LO power seen on the XY plotter. Ideally, there should be enough LO power so that the DC LO IV curve crosses the unpumped (no-LO) IV curve at a point one third to one half the height of the unpumped curve. As you sweep the bias voltage, you will see photon steps in the IV curve. For quasiparticle (single electron) tunneling, with which we are primarily interested, the width of the photon steps are . The photon steps with smaller voltage widths of indicate the presence of 2 electron (known as Cooper pair) tunneling. Cooper pair tunneling adds noise and instability to the receiver and can be reduced by increasing the current to the superconducting magnet in the dewar. There is a bias box for this. A good working current is about 25 mA. Cooper pair tunneling is also responsible for the current spike at 0 V seen in the unpumped SIS IV curve. With proper adjustment of the magnetic field this ``Josephson super-current'' will be largely eliminated.

2. Once you have tuned the receiver to the target frequency, you must phaselock the Gunn. 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. Now adjust the Gunn tuning and backshort micrometers slightly. 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.

3. Now the I.F. power should be peaked. A DC voltage proportional to the total I.F. power available to the AOS is displayed on the front of the total power box. These voltages are also available on BNC connectors on the front of the total power power box. Now as you scan the SIS bias voltage, you will see a peak in the IF power at a voltage just below the knee of the unpumped IV curve. Usually this peak occurs in the middle of the first photon step below the knee. Now, slowly adjust the receiver backshorts to maximize the I.F. output power. Begin with the junction backshort and then, once you have peaked the IF power with it, do the same with the E-plane tuner. As you turn the backshorts you will notice several peaks in the IF power. The best performance is usually obtained at the first IF power peak observed as one turns out the backshorts from their full-in position. Turn the backshorts in and out slowly. One of the primary failure modes of receivers is broken backshort drives!

4. Once you have maximized the I.F. power with both backshorts and have biased the junction on the peak of the I.F. power curve, you are in a position to perform a Y-factor measurement. To perform a Y-factor measurement, alternatively place a hot and cold load in front of the beamsplitter and record the total power output voltage of the receiver. The ratio of to is the Y-factor. You should be able to get Y-factors of around 1.6 to 2.0 with the current system. If your Y-factor isn't so good, try reoptimizing the backshorts or adjusting the LO power.

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.