How to setup circular polarization with the L-band system :
- The tone port (A2 in rack 3) injects left-hand circular into the L band feed. Tune sig gen (the SMY) to 1/2 of central sky frequency (e.g. for a 1400 MHz centre frequency this would be 700 MHz).
- Select the L band receiver (http://ra-wiki.phys.utas.edu.au/index.php?n=MtPleasant26m.OTTER)) and start with agilent set to the standard frequency of 4.1 GHz (http://ra-wiki.phys.utas.edu.au/index.php?n=MtPleasant26m.OscillatorControl).
- Choose Agilent frequency for reasonably flat pass band on both linears over the bandwidth being observed.
- Make sure that for the Agilent setting used above that the SML required for the frequency translator is not within any of the undesirable LO ranges as listed on the yellow sheet attached to the frequency translator.
These undesirable LO ranges are (taken from yellow post-it note 08/03/2012):
- Balance the inputs to the quadrature hybrid :
- Using the spectrum analyser look at the generated tone using a narrow span (1–2 MHz).
- Using the OTTER interface (http://ra-wiki.phys.utas.edu.au/index.php?n=MtPleasant26m.OTTER), make sure that the quadrature hybrid is OUT.
- Toggle the switch between direct and crossed and adjust the attenuation in the IF strip so that the level of the tone on the spectrum analyser is the same when “direct” as “crossed” (http://ra-wiki.phys.utas.edu.au/index.php?n=MtPleasant26m.OTTER). It is important that these are as accurate as possible, so once you have it roughly right on 10 dB per division use 3 dB per division or 1 dB per division to get the tone level the same to better than 1 dB.
- When the switch is “direct” the attenuation changes the level of the signal on A4 (marked RHC)
- Switch the quadrature hybrid in. Now it shouldn’t make any difference whether the switch is set to “direct” or “crossed” (which presumably means that the cross-over switch is prior to the input to the Hybrid).
- Since the injected tone is left-circularly polarized, select which channel you want to be the right-hand circular and adjust the phase shifter position to minimise the tone in it. To do this go through the following steps
- Move the phase shifter in. While its moving watch the IF1 tone in the passband.
- If it reduces to the point where it is hard to distinguish from the background noise stop moving, increase the tone strength and start moving again.
- If the tone starts to increase in strength, move back. Once you have found the spot which is a minimum, stop.
- Move the shifter all the way in. Make note of how many steps to move from the minimum to the “IN” limit.
- Move the shifter out this many steps and stop. You may want to refine the minimum position by moving 10–20 steps either side of the minimum to see if you can find a better position. Keep note of the number of steps from the “IN” limit”.
- Change the level of the injected tone at this minimum point so that it just shows up. Now change to IF2 with the spectrum analyser. The level should be about 20 dB or more higher than IF1.
- It is worth letting the phase shifter travel through its full range as there may be multiple nulls, before tuning to a final value. This is quite a slow process as the phase shifter has a total of about 750 steps and moves slowly.
- Now try injecting into a broadband radiator at the vertex. You should have similar size tones in IF1 and IF2.
Last editted April 2010.
Settings for various L-band circular polarization experiments
NOTE: The phase shifter steps should be given from the inner limit.
|Frequency (MHz)||Agilent (GHz)||SMY (MHz)||Balancing Attenuation (dB)||Phase shifter steps||Tone isolation||Experiment||Person||Date|
|1400||4.15||802||9.25||418||18 dB||vt14g||John, Simon||26 Apr 2010|
|1400||4.15||802||12.375||418||28 dB||fringe check||Jay, Eric||29 Apr|