With the advent of RF tuned junctions it is now quite realistic to get
mixer conversion gain. Although this will give very sensitive performance it
is not neccesarily a stable mode of operation and should be avoided. Ideally the receiver
should be run with near unity conversion gain (or loss). Conversion gain
regions(on the 230-492 Rx's) can be spotted by negative to infinate slopes on the LO pumped I/V curve.
The LO pumped slope should ideally be slightly positive (nearly flat)
, and never negative! Typically, after
peaking the total power, go into the IV sweep mode, and look to see if the slope is
negative, flat or positive, and whether there is a
second photon step.
If the slope is negative, stay in the IV sweep mode, and gently adjust the E-plane (or
alternatively the backshort) tuner until the slope changes to flat to slightly positive.
The second photon step can be seen both in the IV sweep, as a
second step to the left of the main one, and also in the B-field display
mode, as a second bump to the left of the main one.
NOTE: A numeric value for the slope around the bias point can be abtain from the SLOPE button in the
Rx control window.
There are two ways to display the SIS current as a function of bias.
- Use Disp to display the SIS current as a function of bias on the handheld display.
The frequency is calculated from the width of the photon step (+- 5 percent) and should
confirm that you are on the correct multiplier harmonic. This is mainly a problem with the 345, where our
current multiplier occasionally puts out more of the 5th harmonic than
the 4th. Please note however that if the Josephson currents are not adequately suppressed,
the frequency calculation can be thrown off. (Return to B-field mode in this case).
- use Scope to display it on the Osciloscope. The signals from all
four receivers are now automatically routed via an electronic switch to the O-scope.
