Dept of Astrophysics
University of NSW
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Tuning the 3mm Reciever

Step 1: Setting the LO Frequency

  • Select the polarisation to be tuned from the top menu bar (1)
  • Bring the "LO PHASE LOCK CONTROL PANEL" (2) window to the to the front.
  • Select the desired line within the "Rest Frequency" box (3).
  • Set the LO frequency by clicking the "SET LO FREQUENCY" button (4) in the pink LO panel. The fields containing the frequency details should change to reflect values associated with the the chosen line and the "f ref SET" indicator light should change to green.
  • Note which sideband (LSB or USB, default = USB) your line is tuned in.
  • Ensure the "IF MON", "GUNN" and "SWEEP" switches (5) at the bottom right of the LO control panel are set to on.
  • Load the preset "Attenuator" and "Gunn Control" values by clicking on the lime green "Auto Preset" button (6). Note that this only loads the presets into the software- the drives on the reciever need to be activated before the values are transfered to the hardware. The indicator lights next to the "LO ATTENUATORS" and "GUNN CONTROL" dials should change to red when you load the presets..
  • Transfer the settings to the reciever hardware by clicking each of the "Drive" toggle switches (7) next to preset dials to the "on" position. Physically this action activates servos in the receiver to change reciever cavity shape etc. This will take a few seconds to complete after which time all indicator lights will change to green. Toggle the drive switches to off, as unplesant resonances can take place within the servo circuits, making tuning difficult.
  • If you are lucky the green "PH LOCK" light will be lit at the bottom left of the panel, indicating the Gunn oscillator phase lock circuit is locked. Even if the "PH LOCK" light is green, check that the circuit maintains a phase lock when the sweep is turned off. If not, see the panel below for what to do.

Tuning the LO Manually

Sometimes the presets values for a line do not lock the LO. If this happens you are required to fiddle with the Gunn controls yourself until you get a phase lock.

  • In the 'Spectrum Analyser Input' window flick the slider switch to the polarisation to be tuned and the 'IF Select' switch to 'LO'.
  • If the Reciever Tuner GUI has just been started it may be necessary to toggle these switches once for the software to pick up the correct settings.
  • Turn off the sweep. Turning off the sweep makes it more difficult for the LO electronics to acquire a lock but means the tuning is more reliable in the end.
  • The spectrum analyser to the left of the mmdrive PC should be displaying a green trace on its screen consisting of a number spikes. The largest spike should be at or near the centre if the presets are almost right.
  • Place the cursor in the box next to the 'Gunn Frequency' Control and use the arrow keys to adjust the value up and down. The largest peak on the spectrum analyser screen should move in response.
  • Your goal is to move this peak into the centre of the spectrum analyser screen.
  • When a lock has been aquired the 'PH Lock' light will turn green
  • Switch back on 'Sweep' to prevent drift.

Problems:

  • Sometimes the peak will never lock in but jump aound the centre position. It may be that you have the peak for the wrong sideband. If this is the case:
    • Move the peak off the left hand side of the screen. Another peak will be observed to 'cross over' from the left. Try acquiring a phase lock with this.
  • All of the preset values were saved after tuning from a higher value. The presets may work if you go to a higher value and then drive down.

Step 2: Setting the Side Band Rejection

In single sideband (SSB) tuning it is necessary to achieve a measure of rejection of the unwanted sideband, be it the upper (USB) or lower (LSB). The rejection of the unwanted sideband can be checked by injecting synthisised tones into the reciever and measuring the power difference between the two sidebands.

  • Bring the "SIS MIXER BIAS - POLARISATION X" (8) window to the to the front.
  • Toggle on the "Drive" switches (9) next to the "TUNER" preset dials and wait for the red indicator lights to wink out.
  • Select the "Hop" position on the "SPECTRUM ANALYSER INPUT" window (10).
  • Set the "tone arm" into the path of the beam by clicking on the "SB Tones" button in the "SIS MIXER BIAS" window (11). The "SB Tones" button should flash red and the green "ARM OUT" indicator light should extinguish. When the tone arm is positioned correctly the red "ARM IN" indicator light should come on. The tone arm will take a few seconds to move into place during which time the system is vunerable to crashing should someone try to issue any other commands.- Patience!
  • In the "spectrum analyser input" window (10) set the "IF SELECT" slider switch to the "HOP" position and choose the relevant polarisation.
  • The trace on the spectrum analyser screen will describe two tones alternately jumping up from a flat baseline. The left-most tone is the LSB while the right-most is the USB. Which tone you want maximised will depend on which which sideband you specified in the "LO PHASE LOCK CONTROL PANEL" earlier. The degree of rejection is the difference in height between the two tones and each division is ~ 10dB.
  • A rejection of 25dB is considered an excellent tuning at Mopra.
  • If you are not happy with the rejection provided by the preset values you can opt to try to improve it as outlined below otherwise click the 'SB Tones' button again to retract the tone arm and the tuning is complete.

Setting the Sideband Rejection Manualy

With the tone spikes visible on the spectrum analyser:

  • On Polarisation A the rejection is controlled via "TUNER 2" and on Polarisation B via "TUNER 1".
  • Conversely, on Polarisation A the sensitivity is controlled via "TUNER 1" and on Polarisation B via "TUNER 2 ".
  • Turn the associated drive switch on and try different values above and below the preset value for each tuner.
  • There is a delicate balance to be struck between maximum rejection and maximum sensitivity- the best rejection settings may result in reduced sensitivity.
  • The rejection is given by the height difference in the tone spikes and the sensitivity is the level above zero of the flat line from which the spikes are jumping. Maximise this level for better sensitivity.
  • While adjusting the rejection keep an eye on the following parameters and their values:

    Parameter Acceptable Range Ideal
    Mixer Current (JI) 0.70 - 1.20 1.00
    Modulator Current (OMI) 1.00 - 2.00 1.60

  • Move the tone arm back out of the beam using by clicking the "SB Tones" button when you are happy with the result.

Problems:

  • All of the parameters controled by the SIS mixer panel are interdependant and much effort is required to get a 'perfect' tuning. If you get a good tuning try to resist the urge to tweak!

Step 3: Optimising Performance (Optional)

General Tips
  • The values in the preset lookup table associated with each line were found by approaching from higher numbers. Some backlash may exist withing the drives so when altering any values manualy it is best to approach from higher numbers to be consistent.
  • It is probably easiest to change values by clicking in the displayed value box and then using the up/down arrow keys. For coarse control one can click and drag the knob, but care must be excercised when approaching zero or ten as the scale changes as you pass the ends. (decimal pt shifts one place)

The following are a list of ideal values for various parameters associated with the reciever.

Parameter

Range

Adjust using:

Mixer Current (JI)
Select at the top left of the SIS Mixer Bias window

 0.70 - 1.20 LO Level dial in the "LO Control Panel"

Modulator Current (OMI)
Select at the top left of the "SIS Mixer Bias" window

 1.00 - 2.00 (1.6 ideal)

ATT X dial in the "LO Control Panel"

Improving System Sensitivity

A guide to the system sensitivity can be gained by conducting a so called "y factor" measurement from the "POWER METER" window.

  • Check that the paddle is set to out before you start- the "PADDLE" button in the "SPECTRUM ANALYSER INPUT" window.
  • From the "DISPLAY" menu on the top menu-bar select the "POWER METER".
  • Within the power meter window clicking on "Y FACTOR" does the test and returns a value in the "Power dB" field. This value should be compared to previous values at the same sky frequency. Note: this value is based on a comparison between sky and ambient load temperatures and will return different values depending on the weather conditions. A cloudy sky will return a lower y-factor.
  • Terminate the test by clicking again on the "Y Factor" button. De-select the power meter window in the display menu again to hide
  • The sensitivity may be tweaked by adjusting the Vj Set control on the green SIS Mixer Bias panel.
  • The "y factor" test may also be done manually using the power meter on top of the spectrum analyser.
  • Press the "REL" button on the face of the power meter to temparorally zero the display.
  • Set the paddle into the beam using "PADDLE" button on the "SPECTRUM ANALYSER INPUT" window. The change in power should now be displayed on the power meter. This value should be ~ 40
  • When the measurement has been completed set the power meter display back to normal by pressing "SHIFT" and then "REL".
  • Best sensitivity has been attained when the value on the power meter is least negative.

 

Page created by Cormac Purcell and maintained by Nadia Lo. If you have any comments and suggestions, please direct your email to nlo@phys.unsw.edu.au.