First, select the line you wish to look at, using the LINE command in UIP.
If you are observing a source with large velocities slew to your source with the
OBS command before starting to tune the receiver.
Go the the receivers and check that the attenuator in the
is fully closed (anti-clockwise, all the way out) so
no power is incident on the multiplier.
The Gunn lock frequency is displayed on the Antenna display screen
and is called F-lock. Look up the
for this lock frequency. These are located on the tables
stuck to the sidecab wall near the entrance door or allidade.
Turn the SWEEP switch on the Gunn section of the
so you can get a clean lock (easier to get a good lock this way).
Now you MUST tune both the GUNN Tuning micrometer and Backshort to the
tuning chart values, and then tweak them (mainly the tuning micrometer) to
get the PLL to lock. Just tuning the tuner till it locks without adjusting the
power backshort to the correct position could result in you locking at the
incorrect frequency ...... and you would never know, except your lines wouldn't
show up.
When locked, check that the gunn control voltage centered in its range, and then
the green lock LED better stay on!
The sweep function of the PLL sweeps the bias of the Gunn and tries to keep
the Gunn Oscilator locked as it drifts with time.
Now begin to tune the
The general idea is to maximize Total Power and obtain a positive slope
on the IV curve: Start by setting Vsis=2.1mV, and by putting some
(20-40 mA) of magnetic field on the junction.
Open the attenuator, watching for
or by monitoring the
For all receivers the first step is to tune the
so that some power is visible on the
Adjust the
(except for the 850 GHz Rx which has no tuners), trying to peak up the Total Power (O-scope or handheld).
Turn the backshort right in to zero, then turn it out till you
see a peak in the total power and/or SIS junction current. This first peak is
likely to be the correct one for the backshort. If the junction current is very
high (above 13 microamps for the sidecab receivers), attenuate the LO signal.
Now adjust the
Again, turn it to zero, then start turning it out. You
will see the total power will not vary for
a long time - it is on a plateau. Then it will drop somewhat and rise sharply
to a peak, then drop again and rise to another peak. It is probably best on one
of these 2 peaks.
Iteratively tune the
micrometers to peak the
on the SIS junction, and then repeat tuning the Receiver.
Note
that Access to these tuners is quite difficult if the zenith angle is
less than around 30 degrees. Also please be very careful when tuning
the Backshort and E-plane tuners as they connect to
the mixer via a thin piece of
which is very fragile.
Once you have some sort of receiver response, you should, check the
magnetic field setting. Press the
this will start a
of total power versus junction bias on the
Turn the voltage on the magnet up from zero and watch the O-scope display.
You should see the curve become more
and the general amplitude can drop significantly. The aim is to set the
magnet current to minimize the total power, which means that the Josephson noise
is suppressed.
Continue tuning the receiver for optimum performance. After
peaking the total power, press the
softkey and observe the IV sweep on the
or the
Look to see if the slope is negative, flat or positive. If it
is negative, stay in this IV sweep mode, and gently adjust the E-plane (or
alternatively the backshort, thought the E-plane may work better) tuner until
the slope changes to flat to slightly positive.
(Refer also to the section on
or see the IVsweep menu or Receiver Control menu below for more detail on this.
Set the LO attenuator so that you get about
of current on the junction. If you draw too much current on the SIS
junction, the receiver may be unstable. If you see bad baselines, check the
current - if it is much above 12 microamps turn the
up to reduce the SIS current to something less than 12 microamps.
Finally it is best to put the SIS bias voltage on a flat spot
of the P/V curve (dP/dV=0). This is critical because in this way
the
Last but not least, please do remember to
the total power on the Sky when all is set and done and you are ready to observe! This guarantees
that the AOS are presented with a fixed IF power level.