The narrow band 345 GHz and 650 GHz receivers in the sidecab and the 850 GHz receiver are tuned manually.
Always wear an antistatic wrist strap with a grounding wire when you touch receiver components.
UIP
LO
command. If you are observing a source with a large velocity, slew to your source with the OBS
command before tuning the receiver. For example, UIP> lo 12co3-2 /side up
UIP> lo /nogunn
Exit.
Gunn Supply
on the Gunn section of the PLL box should be 10.0 V.Gunn Monitor
signal with a spectrum analyzer. Center frequency = 20 MHz and bandwidth = 2-5 MHz.sweep
switch on the PLL box.sweep
switch. After locking the PLL, turn on (up) the sweep
switch. The PLL sweeps the Gunn oscillator bias to keep it locked as its tuning drifts with time.Loop Gain
knob on the PLL box is not too far clockwise (high gain). Once the PLL is locked, adjust the Loop Gain
so the loop corner noise is flat and not peaked. The loop corner noise appears on the spectrum monitor as bumps on either side of the main line. The bumps will move away from or towards the central line at 20 MHz as the loop gain is increased and decreased, respectfully. Try to make the bumps flatten out and disappear. If the loop gain is too high, then the PLL may oscillate and there will appear three lines of similar appearance instead of only one, which is normal.The general idea is to maximize the total power and obtain a positive slope on the IV curve.
normal
, adjust the voltage to 4.0 V. On the 345 GHz receivers, use the B-field display or Ocsiloscope (Total Power vs SIS bias) and/or the IV curve to check you are not getting the second photon step. Finally, check the IV curve to make sure the receiver is responding to the correct harmonic. (This is usually only a problem with the 345 GHz receiver).
Refer to Tuning Hints. to make sure that there is no second photon step!! 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 IF-fluxuations with time are minimized.
Caveat: This receiver has respectable performance above 700 GHz. The atmosphere, however, is opaque above 700 GHz. So if the receiver is mistuned, the system temperature will be very high and observing will be very confusing. Use, therefore, the correct columns on the tuning sheet to set the Gunn oscillator tuners. Presently the tuning sheet has red arrows to the correct columns. This has been a recurring error even committed by the local staff!
When the receiver is mounted, connect the the warm IF output to a labeled cable routed to the sidecab. This bundle also has the cable for the 109 MHz PLL reference. The IF cable in the bundle is connected to the old 492 GHz IF line at the dog house behind the side cab in the dome and is then routed through the IF matrix to the IF processor in the spectrometer room. The IF processor must be switched on or else a RF level too low
message will be seen.
This receiver has an internal calibration load. On the hand paddle, there is button labeled PHot
or PSky
depending on state of the load. If the button says PSky
then the receiver is looking into the ambient load. If the button says PHot
then the receiver is looking at the sky.
Caveat: Sometimes the internal load changes by itself. When this happens the receiver will not see the sky the system temperature will be very high system. Press TSky
on the hand paddle to correct.
There are Two Multipliers for this Receiver: 720-840 GHz, RPG Physics, x12 , Self biased (1998 model, 4 tuners, refer to data sheet for settings) 600-660 GHz, RPG Physics, x6 Self biased (2000 model, 4 tuners, refer to data sheet for settings) Note also: The 850 LO chain have no attenuators! LO Bias: Do not exceed 4 mA on Doubler Do not exceed 2 mA on Tripler
Note that all the multipliers are EXTREMELY sensitive to static discharges. These are delicate, expensive and hard to repair items. Please be very careful - wear a grounding strap, keep in contact with a chassis, do not power cycle anything unnecessarily. Also, be very careful when biasing the multipliers. Make sure you have the correct supply, that the current on the multiplier doesn't get too high (less than 2 mA for all), and that you do not ram the various tuners into their stops .
Also, please note that we now have just one kind of attenuator (Please note: NO attenuator has been implemented in the 850 LO chain. The Attenuator is a gold colored unit from Custom Microwave, and the attenuator is fully closed with the knob all the way out(anti-clockwise).
Multiplier Tuning instructions
Make sure the 230-690 GHz attenuators are fully attenuated (knob all the way out). Tuning Procedure: Set the doubler and tripler voltage to Zero, then put the manual shorting switch to normal, so that the multiplier can be biased as follows 280-330 GHz, Millitech, X4: 2 Tuners, Bias at 2-3V 320-380 GHz, Millitech, X4: 2 Tuners, (1990 model, Both 4th, 5th harmonic), Bias at 2-3V 320-380 GHz, Millitech, X4: 2 Tuners, (1999 model, no 5th ⇒ more RF Power for 4th harmonic), Bias at 2-3V 600-660 GHz RPG Mulitplier(X6): 4 Tuners, Doubler at 3-5V, Tripler at 2V. 660-720 GHz RPG Mulitplier(X6): 4 Tuners, Self Biased 780-840 GHz RPG Mulitplier(X6): 4 Tuners, Self Biased 840-920 GHz RPG Mulitplier(X12): 4 Tuners, Self Biased Switch in the line with the UIP command and try to lock the Gunn. Set the Gunn frequency Tuner and Backshort. Instructions are laminated on the sidecab wall near the door.
General multiplier settings:
Do NOT exceed 3 mA on the doubler and 2 mA on the tripler.
T1 is the doubler input tuner. Tune to maximize the negative current. If there is no current, lower the varacter bias voltage until rectified current starts to show. Be careful to Moniter the multiplier current at this stage. Since the 850 Multiplier chain has no attenuator, adjusting T1 is very likely to pull the frequency of the Gunn beyond the lock range of the PLL. Be aware of this!! You can still tune with the PLL unlocked because the Gunn frequency is only off by a few hundred MHz at most. Just lock the PLL when you are done tuning the receiver. T2 tunes is the doubler output power. Tune T2 to drop the doubler current by about 20-40%. This should increase the current on the tripler stage. T3 is the input tuner of the tripler, tune to maximize the current on the tripler. Note that T2 & T3 interplay. Moniter the current! T4 is the tripler output tuner. Tune T4 to drop the tripler current by 20-40%. This means that power is coupled out of the multiplier. However completely dropping the current to a local minima is most often NOT the best operation position. Now tune the receiver backshort to get some SIS current reading. From now on the process is iterative. That is, optize the SIS current by adusting the: Multiplier tuners Gunn backshort tuners Variable attenuator on the 230-690 Rx's Receiver tuners Bias of the multiplier The 850 has one extra adjustment, the focus of the multiplier. This micrometer moves the 850 LO chain normal to the dewar, and maximizes the coupling to the SIS mixer.
Overall and specific tuning instructions.
- UIP> za 44 (to allow the mirror to change the position for your receiver) - UIP> lo yourline /force /lock=default /if=1.5 (For side cab receivers, to set the line at the middle of the AOS) - check the tertiary mirror position.
- Don't go beyond Vop of greater than 10.5 V. If you go beyond that, you'll be in the over voltage protection mode. To get out of the mode, do following. The steps below will be done in the phase lock control panel in the sidecab.
i. Switch to “local” mode from “remote” mode.
ii. Turn the Gunn off. iii. Turn Vop down for 2-3 turns. At the same time, watch the Vop reading on the reader above the HP synthesizer. iv. Turn the Gunn on. v. Switch to "remote" mode from "local" mode.
And follow tuning instruction for tuning.
If you get a message SYNTH FAIL(06) on the antenna status monitor, powercycle the HP synthesizer (turn off and turn on) located just above the oscilloscope that allows us to check the Gunn lock status. You turn off, wait for 10 sec or so, and turn on the power of the HP synthesizer, and go back to UIP, reload the line (UIP> lo yourline/force) and see if the error message disappears or not. If this doesn't work, wait for 5 min. If 5min doesn't work either, wait for 20 min to let the synthesizer cool down completely. After 20 min, turn the power on and load some other line which is lower than your line frequency first, then load your line frequency next.
UIP> lo /nogunn