Between observing runs, it is advisable for the receiver performance
to be checked and documented using an X-Y plotter. To see the junction's
IV curve, connect 
and 
outputs on the back of the
SIS bias box to the X and Y inputs, respectively, of the XY plotter.
The sensitivity knobs on the plotter should be set to 0.05 V/cm on
both the X and Y channels. We found that for the X channel the OFFSET
should be set to 0. The Var. knob should be in the 10 o'clock position.
Likewise for the Y channel the OFFSET should be set to -1 and the Var.
to about the 5 o'clock position. To get the IV curve on scale, some
adjustment of the zero position will probably be required.
As you sweep the SIS bias voltage, you will see photon steps in the
IV curve. For quasiparticle (single electron) tunneling, which is what
we are primarily interested in, the width of the photon steps are
. The photon steps with smaller voltage widths of 
indicate the presence of 2 electron (known as a Cooper pair) tunneling.
Cooper pair tunneling adds noise and instability to the receiver and
can be reduced by adjusting the current to the superconducting magnet
in the dewar. There is a bias box for this. A good working current
is 35 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.
Adjust the receiver backshorts to maximize the L.O. power seen on the XY plotter. Ideally, there should be enough L.O. power so that the DC LO IV curve crosses the unpumped (non-LO) IV curve at a point one third to one half the height of the unpumped curve.
Now let's peak the I.F. power. Leaving the X input of the XY plotter
connected to the of the SIS bias box, connect the DC output
of the total power box to the Y input of the XY plotter. The polarity
of the signal from the total power box is reversed, so you will need
to rotate the banna plug 180 
when connecting it to the
Y channel input. You may have to adjust the sensitivity of the Y channel
to keep the plotter on scale. Now as you scan the SIS voltage, you will
see a peak in the I.F. power just below the knee of the unpumped SIS
IV curve. Usually this peak occurs in the middle of the first photon
step below the knee. With the bias voltage set so that the plotter
pen is on the I.F. power peak and the bias box set to constant
voltage mode (indicated by a ``V'' on the bias box), slowly adjust
the receiver backshorts to maximize the I.F. power. Begin with the
junction backshort, and then move to the E-plane backshort. As you
turn the bacshorts from there full in position outward, you will
notice several peaks in the I.F. power. The best performance is usually
obtained at the first I.F. power peak encountered from the full-in
position.
Once you have maximized the I.F. power with the backshorts and have biased the junction on the peak of the I.F. power curve, you are ready to perform a Y-factor measurement. First, fully attenuate the I.F. by increasing the attenuation on the I.F. power box. Now trace a line on the XY plotter by scaning the SIS bias voltage. This line marks the zero level for I.F. power. Now return the I.F. attenuators to their original setting. While holding an ambient temperature load in front of the beamsplitter, scan the SIS bias voltage and record the curve on the plotter. Now do the same procedure again, but with a nitrogen dipped load in front of the beam splitter. In both instances an I.F. power peak will be observed. However, the ``cold'' I.F. power peak should be more than a factor of two lower than the ``hot'' I.F. power peak. A sample set of IV and IF power corves are shown in Figure A-1. These curves were made during the first commissioning run for the receiver in April 1995.