Testing rf filters for the 327 receiver

13jan06 Links to plots:
Dynamic spectra no filter, then hipass filter in polA (.gif):
Dynamic spectra no filter, then hipass filter in polB (.gif):
Plots of the two candidates that came/went with the hipass filter (.ps) (.pdf):
Dynamic spectra with no filter, bandpass filter, and then no filter in polb (.gif):
Plots of the two candidates that came/went with the bandpass filter (.ps) (.pdf):

Introduction (top)

The 327 receiver has a number of birdies in the band. Some of these are harmonics of out of band rfi (ac birdies, fm station harmonics ..more info). It is possible that some of the birdies are intermods of out of band signals that are being created in the 327 receiver. If this is true, then filtering out the out of band signal should get rid of the intermods. To test this we placed filters after the hybrid and before the first amplifier:

29nov05 - hipass filters installed in both polarization's to cut off things below 300 Mhz.

Dome at az=285, za=10
25 Mhz band centered at 320 Mhz.
Data taken with filter out then filter in.

10jan06 - band pass filter installed in polb between the hybrid and the first amp.

Filter centered at 327 Mhz with a 20 Mhz bandwidth.
Dome at az=156.3, za=18
Data taken with filter out, filter in, filter out.

29nov05: the high pass filter.

Data was taken for 300 seconds with the filters out, then 300 seconds with the filter in. It took a few minutes to install the filter. With the filter in the system temperature increased by about a db so the filter in data will not be as sensitive as the filter out data.

For a birdie to be classified as an intermod in the 327 receiver it should pass the following tests:

It is present when the filter is out ( in either polA or polB or both).
It should not be in either polA or polB when the filter is in. A signal that just gets weaker is probably from the lower sensitivity of the filter in measurements (higher Tsys).
The signal should not be time variable. If it is then our filter out, filter in measurement may coincide with the birdie coming and going.

Dynamic spectra were made of the 600 seconds of data. A dashed horizontal line marks where the filter was installed. The images retain the full 2048 channel resolution so that narrow spikes can be seen.

A number of birdies got weaker when the filter was installed and some seemed to shift a bit in frequency. The birdies at 315.356 and 328.087 were the only two noticeable once that went away. It is interesting to note that the 321.9 birdie remained with the filter in. This is the 3rd harmonic of the 107.3 fm station. We even heard the radio station on the if signal at 321.9. This means that the fm station is creating the harmonic.

10jan06: bandpass filter installed.

Data was taken for 337 seconds with the filter out, 502 seconds with the filter in polB, and then 537 seconds with the filter out. The band was centered at 327 Mhz. The az,za position was different than the 29nov05 data. The filter increased the Tsys by about 20%.
Dynamic spectra were made with the 1200 seconds of data. Dashed horizontal lines mark where the filter was installed and then removed. The images retain the full 2048 channel resolution so that narrow spikes can be seen.

A number of birdies got weaker when the filter was installed and some seemed to shift a bit in frequency. The birdies on the edge of the 25 MHz band disappeared because the 20 Mhz bandpass filter cut them off, not because they were intermods. 2 signals were present, gone, present while the filter was (out, in, out). They were at 318.465 and 332.011. The second at 332.011 is definitely a harmonic. It is about 2% of tsys with the filter out. Signals weaker than this (330.34 and 330.75 1% of Tsys) were seen with the filter out and in. This birdie is outside the bandwidth used for the az swings of 29nov05.

Conclusion:

Installing the hipass and band pass filters did not remove a large number of birdies. At most 4 birdies were absent with the filter in. One of these looks definitely to be an intermod in the 327 system.

The 318.46 birdie was the only common frequency birdie of the 4. It was not obvious in the 29nov05 data. This may be do to the az/za dependence of the birdies.

107.3 3rd harmonic: The birdie at 321.9 was present with the filters in and out. The radio station 107.3 was heard when listening to this frequency in the 2nd IF. This means that the 3rd harmonic of the radio station 107.3 is coming from outside our receiver system (probably the station itself).

104.1 3rd harmonic: The birdie at 312.3 was there in polA of the 29nov05 data with the filter in. So the fm station 104.1 is probably generating this 3rd harmonic.

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