Cleaning up the 327 rcvr band

    In 2005 we have made some attempts at cleaning up the rfi in the 327 Mhz receiver band. The cause of interference are:

• Alfa motor controller. (sept05)It produces combs at:
• 40 Mhz comb (probably from the dsp clock). It is 20 to 30 db above the noise floor. The 8th harmonic (320Mhz) falls in the 327 rcvr band.
• A 16 Khz comb from the current and commuation frequency of the amplifier. It is center centered on the 40Mhz comb (at 320 Mhz in the 327 band). It is strongest near 320Mhz and gets weaker as you move away.
• 2 Khz comb that is a frequency used by the amplifier.
• A 7.5 Mhz comb was also seen when the motors were turned on/off

See: the 16Khz comb is coming from the alfa motor contoller:
          Resolution:  The current fix  is  to turn off the alfa motor controller when doing 327 measurements.
 
• Air conditioning units in the dome:  (oct05) They produce:
• A 1 Mhz comb. It is not resolved in a  95 hz wide channel. It drifts  up to 30 khz in 1 hour. It can get up to 8 times the noise floor in a 95 hz channel.
• The birdie will go away if you turn off the breaker to the ac units. Turning off the ac using the remote control does not get rid of the birdie (it is coming from the digital board in the unit which stays on when you push the off button on the remote).
• There are two combs that drift at different rates. Each ac unit (two in stalled) produces its own comb.

See: 1 Mhz comb in 327 receiver (coming from AC units in dome)
Resolution:
• We will try to move the ac units out of the dome (maybe in a shielded room by the compressors on top of the dome). This will take a while.
• Turning off the ac control downstairs with the remote does not help.
• There is a switch (next to the ac remote downstairs) that will turn off the old ac unit (over the tertiary). This will make one of the birdies go away. It will also make the room heat up.
• We tried putting capacitors in the line that went down to the compressor (on the service platform). This did not improve things.
• more info on dome ac Rfi.
• Azimuth dependence of birdies in the 327 receiver (more info).
• There are 17 birdies that show a clear azimuth dependence (other may have an az dependence but they are time variable.
• Dynamic spectra show that there are many frequencies that are turning on/off together. They may be intermods or harmonics.
• Hipass, bandpass filters placed in front of  1st amp to check for intermods. (more info)
• Saw 4 birdies that went away with the filter. At least one of them was surely an intermod
• The 3rd harmonics of 104.1 and 107.3 did not go away when the filter were inserted so they are coming from the fm stations.
• 3rd harmonics of fm stations are showing up in the band(14jan06):
• The harmonics are not being created in the receiver (more info).
• Birdies with 1 minute period seen in 430 and 327 Mhz.
• Birdies with periods of 1 minute were seen in the 430 and 327 Mhz bands. The 327 Az swings showed that the 327 Mhz birdies have an azimuth dependence (which means they are coming from outside the dome).

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