jan16: 327 rfi from the turret control box
On 27jan16 Felix and i found strong rfi
coming from the turret control cabinet (huffman box in the gregorian
receiver room). Checking the rfi gasket on the inside of the huffman
box door, we found high impedance along the gasket (this was
the old gasket with rubber impregnated with silver.. we've had
problems before with this gasket material oxidizing..). Willie
and Osvaldo replaced the gasket with a wire mesh gasket.
On 28jan16 we tested the rfi shielding of the
The setup was:
- 327 receiver 300 to 350MHz
- Mock spectrometer
- 7 bands, 8 MHz/band 8192 channels/band (giving 976 channel
resolution), 1 second integrations.
The datataking procedure was:
- do tests after the repairs to the gasket
- start taking data with the huffman box closed, the turret
- open the huffman box door..
- this took about a minute
- take data for a few minutes with the door open
- Turn off all power to the turret control box. Take data for a
- Power the turret box back on
- Close the door (this took about a minute)
- continue taking data..
The dynamic spectra:
The dynamic spectra were made by:
- Computing median bandpass over when cabinet was off
- fitting a 20 order harmonic to this median bandpass
- flatten each of the spectra with this.
- remove tsys from each spectra using the median
- average the two pols.
- I placed horizontal lines where the cabinet was closed,
opened, pwroff , pwron, closed
- Some of the steps took a few seconds to complete.. So you
need to look around the horizontal lines to see where things
Some comments on the rfi:
- There were other sources of rfi present when this data was
- sband console (before it was repaired)
- dome cameras, alfa motor controller (i forgot to turn them
- These other sources would not turn off, on when the power in
the turret power was cycled..
- The 306 MHz sweeping rfi was also seen in the sband
- When the turret door is open, we can see a comb (about .5 MHz)
sweeping in frequency.
- Not sure where the sweeping rfi at 306 MHz is coming from ..
- it's not the sband console..
- looks like some of the sweeping rfi we had at lband long
ago, when the receiver cal drivers were oscillating.
- I forgot and left the alfa motor and the cameras on during
- some of the solid vertical lines will be coming from them..
- Some combs can be seen at:
- 303.8,309.8,314.25,320.25.... it continues all the way up..
Comparing cabinet Door closed with the cabinet power off:
For each band i averaged when the power was
all the way on, and when the power was on and the door was
closed.. i tried to leave out the transition periods.
The plots show the
rfi when the cabinet was PwredOn/Closed and PwredOff/open (.ps)
- Each page shows 1 8MHz band.
- Top frame: vertical scale is 0 to 1.5 Tsys
- Bottom frame blows up the vertical scale to 0 to .2 Tsys
- The doorClosed pwrOntime avg is 280 seconds. the
powered off time avg is 120 seconds.
- Black is with the cabinet Pwr off, red is with the cabinet
powered on and the door closed.
- I plotted the red (on,doorclosed ) and then over plotted the
- If you see a red line, with a black line over it, then that
rfi did not come from the turret cabinet.
- Red lines sticking up by themselves are most likely coming
from the turret cabinet.
I then plotted the
rfi peaks coming from the turret cabinet (.ps) (.pdf)
- looking at the dynamic spectra, the rfi peaks at:
- Are not coming from the turret cabinet.
- They are time variable and were just not present when the
cabinet power was off
- The 4 rfi spikes that were found are less then .03Tsys..
- This is too weak to see in the dynamic spectra (1 second,
976 Hz is about .03Tsys).
- So i can't say whether these are time variable or actually
coming from the turret box.
- Prior to fixing the turret box shielding, there were
lots of rfi spike coming from the turret control box.
- The little star clock is supposed to be 18.432MHz
- The measurements below show that the clock is actually
Strongest rfi Spikes with cabinet door open
Tsys in 976Hz chan
- so if you want to test for leakage from the turret cabinet,
i would use 331.909 MHz
- After repairing the cabinet shielding, 4 spikes were found
- All of them were less that .03*Tsys (in 1kHz rbw) so they
were not visible in the 1 second dynamic spectra.
- so i can't say whether they are really coming from the
turret or not...
- The only way to tell would be to do long integrations with
multiple on,off cycles.. and then check to see if the rfi
spike are only in the power off cycles.