Procedures for Manual Tuning of the Receiver
Procedures for Manual Tuning of the Receiver
Overview
Full manual tuning of the receiver is a complicated procedure. Normally,
if manual intervention is required, it will be for one or two of the
components. The principal components that can be relatively easily tuned
are the GUNN OSCILLATOR, the QUADRUPLER, the two MIXERS and the Half Wave
Plate.
All tuning takes place through the front panel of the RxB3 electronics rack.
There are 5 pull toggle switches on the GUNN, MODULATOR, QUADRUPLER and
the 2 MIXER modules. The position of these switches cannot be overridden
by the microcomputer. The purpose of this design is to ensure that, when
the situation warrants, these subsystems can be locked into Local mode and
adjusted only through the front panels. This mode would be desirable, for
example, if the motorized micrometers were unreliable or an attempt was
being made to preserve their longevity by restricting their movement.
The receiver can be used whether these 5 modules are in Local or Remote
mode. But if you leave any of these pull toggles in Local mode after
tuning, their settings cannot be changed either by the computer or through
the Display terminal menu.
If, after tuning, you set the (5) pull toggle switches
on the rack to Remote, control of those systems will be returned to the
microcomputer. The micro will read the values you have set on the front
panels and will use these as the requested values for the relevant parameters.
Consequently, the settings you have manually adjusted will be used until
a new frequency is entered or rxb_retune is issued when new values will
be computed for these parameters. If the pull toggle is in Remote these
new values will be used instead of whatever the front panel settings are.
If the module is still in Local mode, the newly calculated values for that
module will immediately be overwritten by the front panel settings which will
be used at the new frequency.
The other modules (apart from the 5 mentioned above) have Local/Remote
modes that will be toggled to Remote by the microcomputer whenever it needs to
control them.
Once back in the control room, you may wish to issue the rxb_auto
or rxb_retune> command to set any parameters
that may have been changed incidentally from their requested values by the
manual adjustments. The auto command will tune the receiver without
calculating any new demand parameter values. The retune command will
recalculate and use new parameter values for all modules that are not in
Local mode.
The Components
The Synthesizer Frequency
To ensure that the correct frequency harmonics are available for
the LO system, issue the fe] command from the TO's console to specify the
desired tune frequency and mode.
Gunn Oscillator
- If the "lock" LED beside the Backshort pot on the Gunn and Phase Lock
Loop Module is red, the Gunn is not locked.
- There are multiple locks possible for the Gunn, but only one of them
is correct for the frequency you want. Presumably this lock will be
acquired at tune and backshort settings close to those supplied by the
microcomputer. To read what those requested positions were, set the rotary
silver switch to tune (or backshort) and press the Set/Mon push-button. IN
displays the requested position, OUT the actual position.
- Set the pull toggle switch to Local
- Put the Micrometer Adjust, Osc and Sweep toggles up.
- With the silver knob at "Tune", slowly rotate the Tune Pot about
the requested position until the lock LED turns green. There are
several harmonics to which the Gunn can lock - but only one will give you
the right frequency. Presumably this is the one closest to the tune value
requested by the computer.
- Rotate the silver knob to Backshort position and adjust the Backshort
Pot through the range of values that the Gunn stays locked. Then reset it to
the middle of this range. ALternatively, you can peak the Gunn output by
monitoring Vdet on the MODULATOR as you adjust the Backshort.
- Inhibit the Micrometer Adjust toggle.
- There are two ways to check that the Gunn is locked to the right
frequency.
-
If you have an unmistakable spectral line, look at your source and see if
it's visible.
- An alternative procedure is to connect the EIP output from the upper
right of the RxB3 Receiver rack to the counter on RxC2. (You'll need an
EIP cable about 6 feet long). The value read by the counter should match
that shown as Foscillator on the PF1 Display terminal screen.
Quadrupler
- On the MODULATOR set the pull toggle switch to Local, the Loop
Control to Open Loop and the rotary silver knob to either Ia or Ib so that
you can monitor the junction current as an indicator of quadrupler output.
- On the QUADRUPLER set the pull toggle switch to Local, the Micrometer
Adjustment to uninhibited (up) and the rotary silver switch to INPUT.
- On the QUADRUPLER use the lower left pot to maximize the Ia(b) value shown
on the MODULATOR.
- On the QUADRUPLER set the rotary silver switch to OUTPUT and maximize
Ia(b) using the lower right pot.
- On the QUADRUPLER set the rotary switch to Vb and maximize Ia(b) using the
centre pot, but don't let the readout on the QUADRUPLER go below 3.0 or you'll
trip the multiplier interlock. (If the interlock light does come on, just
take the Vb value to 3.0 and it should reset to normal.)
- If there were large changes in any of the 3 multiplier parameters, you
might want to iterate through the optimization procedures again.
- Set the QUADRUPLER's Micrometer Adjustment switch down, the MODULATOR's
Loop Control to Closed Loop and the pull toggle switches on the MODULATOR
and QUADRUPLER to remote.
Optimizing the Mixer Backshorts
The procedure is the same for the two mixers.
- Set the pull toggle switches on the MODULATOR and the MIXER-CHAN A(B)
modules to Local. Set the Loop Control on the MODULATOR to Open Loop.
- Set the TOTAL POWER - CHAN A(B) module to Local.
- Set the SKY and LOAD CHOPPER Modules to Local and select the Cold Load.
Note that the current load is indicated by the LEDs underneath the elliptical
dial on the TOTAL POWER module.
- On the TOTAL POWER module module check that the black DVM switch beside
the elliptical display dial is set to Level. Adjust the attenuation do the
Level is about 200.
- On the MODULATOR set the silver rotary switch to Ia(b) to display the
mixer current. Adjust the Level Set pot so the current is around 15 uA.
- At the MIXER - CHAN A(B) module put the switch under the LED display
to the
backshort position and use the left hand pot to add about 30 units to the
displayed position (to compensate backlash before you seek the optimum
position).
- Adjust the backshort pot CCW (i.e. down) while watching for a
simultaneous peak in the Ia(b) (on the MODULATOR) and the TOTAL POWER Level.
You can go quickly at first as you're just taking up backlash in the drive.
Keep the Ia(b) value below about 30 uA and the Total Power on scale by
adjusting the Level Set pot on the MODULATOR and attenuation on the
TOTAL POWER module as necessary.
The peak position can be quite sharp. Once you pass through the peak you
must go back up about 30 units and come down to the peak again.
- Repeat this procedure for the other mixer if necessary.
- A useful function available on the TOTAL POWER module for each mixer
is a front-end Trx display. It can be used to display real-time
receiver temperature while adjusting bias voltage, mixer current and backshort position.
On the TOTAL POWER modules ensure that the IF level is mid-range on the elliptical
display. This can be adjusted by setting the modules to Local mode and keying in
appropriate IF level attenuation. Set the DVM toggles to Trx, put the Sky Chopper
on Load and the Load Chopper in Run mode. The receiver should now switch rapidly
between hot and cold loads, displaying the Trx values on the elliptical bar graphs.
Half Wave Plate
The Half Wave Plate is controlled from the MODULATOR module. Its function
is to adjust the mixer currents to the optimum values.
- On the MODULATOR set the pull toggle to Local, Loop Control to Open Loop
and the Half Wave toggle up to motor drive position.
- Set the toggles on the MIXER modules to monitor junction currents and
rotate the Plate in CW or CCW directions by toggling the direction
switch appropriately. As you do so watch the junction currents. This
switch does not reset itself, so you have to
manually recentre it if you want the plate to stop rotating. The junction
currents are quite sensitive to the orientation of the Half Wave Plate, so
quick flicks of the drive toggle away from the centre position will permit
final adjustments. The Level Control pot can be used during this procedure to
adjust the average junction current while the Half Wave Plate balances the
relative currents.
- When finished, the direction drive switch should be in the centre
position and the motor drive inhibited.
Lorne Avery
10 July 1997
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