Bandpass ripple in gregorian 430Mhz system

dec,2001

     Some time in dec01 a large ripple appeared in the gregorian dome 430 Mhz data. It did not look like interference because it was relatively stable. While debugging the problem, it was discovered that turning on the gregorian dome motors caused the ripple to appear. Turning off the dome motors converted the ripple into a large tone at  435.75 with a width of about 200 Khz. The figures show the birdie  and the ripple.
  • Top plot, dome motors off. Birdie at 435 is about 10 times tsys in polA and 3 times Tsys in polB.
  • Middle plot dome motors are on (but telescope is not moving). Ripples appear.
  • Bottom plot. 1 second dumps of the bandpass with .1 offsets added between each strip. Black lines have dome motor off. Blue lines have dome motor on.
    •     When the azimuth or carriage house motors were turned on, the narrow birdie spread out a little (say 400 khz) but no where near the modulation when the dome motors were on. We tried turning off 1 dome motor at a time but the modulation did not go away. With all of the power in the vertex drive shelter turned off, the large narrow spike remained. We worked on the shielding of the cables that ran from the vertex drive shelter (the motor amplifiers) down to the motors, but the problem persisted.
     
        And then we lucked out... The dome motors were off and we were monitoring the large spike at 435 Mhz with 1 second dumps. All off a sudden the narrow line changed to the modulated form for a few seconds and then hopped back to the large narrow birdie. After a little investigation we found that the operator had typed in the command to read the dewar temperatures and this caused the modulation to appear!!!
     
        Each dewar has a lakeshore temperature sensor that can read the dewar temperature. Cables run from each dewar to a multiplexor box in a separate rack situated on the rotary floor. This multiplexor can connect these signals one at a time to a digital volt meter that reads the voltage (temperature).  Turning the receiver monitor multiplexor box off and then on caused the birdie and modulation to go away. After cycling the power we could not make the tone or the modulation appear.
     
    It's still a bit of a mystery why the dome motors were able to cause the birdie generated by this box to switch from a tone to a modulated signal.  Turning the motors on may have caused some vibrations (although you didn't have to move the dome for the modulation to appear)  or the motors turning on could have caused the voltage to sag a bit causing the tone to switch to a modulated wave form.

        Whatever the reason, the problem has gone away but has not been fixed. If a birdie at 435.75 Mhz appears or you see large modulation in the dome 430 Mhz receiver, check out the receiver monitor multiplexor box.
     

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