IFLO compression by the FAA 1350/1330 Mhz radar using lbw,lbn

     The compression of iflo/ lband receivers (lband wide , lband narrow) was measured on 29apr03. The setup was:

•  Center a band at 1415 Mhz and pass it through a 5 Mhz IF filter (centered at 260 MHz). Square law detect the signal with a 2 usec time constant. Sample the signal at 1 Mhz. Do this for pol A and polB. When this band goes negative during a radar pulse, you have compression.
• Center another band at 1350,1330 Mhz. Put in a large amount of attenuation (downstairs) so that the radar at it's peak does not go off scale on the a/d. Do this for polA and B
• Setup the ri to sample these signals simultaneously with 12 bits, 1 usec
• To decrease the i/o rate, set the radar blanker to a 100 usec window about the radar pulse. Use this as the ipp (trigger pulse) for the ri. Take 95 1 usec samples every ipp within this window.
• The azimuth was at 203 and the dome was at 9.5 degrees. (see az,za dependence of the radars for an idea of how much stronger it could get).

1. lband wide circular polarization, (hybrid in).
2. lband wide linear polarization, (hybrid out).
3. lband narrow.

   Data was taken for 100 seconds. There were 8+ complete rotations of the radar in this time (12 second rotation period). The 1415 Mhz samples were normalized to (sample/median(power) - 1)  where the median power was compute over the 100 seconds.

    The 1350 Mhz data was searched for the peaks when the radar was pointing at the observatory. These ipps (the 95 samples) were then plotted. The 1350 data was normalized to full scale while the 1415 Mhz data was normalized to medianTsys -1 (so the radar 1350 scale and the 1415 Mhz scale are not the same).  Each data set has two pages:

• Fig 1.. Plots the 1350 data (smooth lines) and 1415 Mhz data (histogram mode) for the peak ipps. Offsets have been added for plotting. When the 1415 Mhz data dips down during the positive swing of the radar, the system is saturated.
• Fig 2.  plots the 1350 Mhz data (dashed line) and the 1415 Mhz data with no offsets. You can read the value of the compression (the negative going lines) from here. When it reaches -1, the signal level in the 1415 Mhz band has gone to 0. -.5 would be 50% or 3 db compression.

lband wide circular polarization.

The compression went to about 80% (-7 db) with polA a little worse than polB.

lband wide linear polarization.

The compression went to about 80% (-7 db)  with polA a little worse than polB

lband narrow circular polarization.

The compression went to about 80% (-7 db)  with polA a little worse than polB. One ipp (the dark red) had  a large 1350 value and small 1415 value for the entire 95 usecs.

Conclusions:

1. The IFLO (connected to either lband system) is being driven into saturation by the 1330,1350 faa radar pulses when they point at the observatory (once every 12 seconds). The compression goes to 80% (-7 db). It is important to realize that it is the IFLO (first mixer stage) and not the dewar/postamp package that is being driven into saturation. The mixer chassis has about 40 db of gain and its amplifiers have the same dynamic range as those that precede it in the postamps of the lband receivers.
2. The system recovers rapidly from the compression (within the 2 usecs of our time constant)
3. There was one unexplained ipp (95 usecs) when the radar was high all the time and the 1415 Mhz was low. It did  not repeat.

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