FAA radar cause spectral birdies

• The baseband mixer would go into compression when the a/d converter got to about 700 counts.
• After the change the bbm would start to go into compression when the a/d was around 1600 counts.
• Prior to the change the radars never caused an a/d overflow (since the mixer could not put out enough power to get to 2048 counts).
• After the change the a/d converters would overflow every time a radar beam would pass by the AO. This was seen in the 1290 radar and the FAA 1350 MHz radar.
• A radar acts like a sine wave in the band.
•  Clipping a sine wave creates spectral artifacts by:
• clipping gives harmonics at 1,5,9 times the radar frequency at baseband
• DC causes mixing products at  the radar harmonic +/- the radar frequency at baseband
• clipping asymmetries causes images reflected about the center of the band.
• When a large sine wave is inserted in the bbm a dc offset is measured in the a/d rms values. This may be coming from the bbm chassis or the a/d's

• 11jun08 sitting at az,za = 288.8,11.3  .1 second sampling, 8192 channels, 172 MHz band centered at 1325 MHz.
  
• 12jun08 sittting at az,za=278,8m13, .01 second sampling, 8192 channels, 172 MHz band centered at 1345 MHz.

• Page 1: 11jun08, .1 second spectra averaged over 1 second.
• The Band is centered at 1325 MHz. The 1350 radar is offset 25 MHz from the center of the band.
  
• Top 30 1 second spectra plotted with offsets.
• The faa radar points at ao during spectra 11 and 23.
• The dashed vertical lines are spaces every 25 MHz from the center of the band
• Bottom: spectra 11 when faa points at AO
• Red line: the 1350 radar
• Green: the 5th harmonic of the radar (+/-) folded at the edges.
• blue: The 7th harmonic of the radar (+/-) folded at the edges.
• Page 2:  .01 second spectra averaged over 1 second
• The band is centered at 1345 MHz.
• Top: 30 1 second averages plotted with offses. The dashed green line is the center of the band.
• Bottom: The 1 second average for spectra 12 when the radar was strong.
• Black line: 1350 radar
• green lines: the 5th harmonics
  
• blue lines: the +/- 5 MHz spurs caused by DC.
• Page 3: 12jun08. 10 millisecond spectra.
• Top:  14 10 millisecond spectra when the faa beam sweeps by AO.
• It saturates for 4 spectra or 40 milliseconds.
• The FWHM of the FAA radar is about 40 milliseconds.
  
• The signal probably goes 3 to 6 db above the clipping level.
• You also see a birdie at 1340 MHz when the radar isn't strong. This is the  complex image of the 1350 radar (reflected about the 1345 center).
• Bottom: A single 10 millisecond spectra when the radar was strongest.
• The signal is 33 db above the noise floor with a FWHM of 200 KHz
• The peak system temperature increase would be this value divided by the ratio of the Bandwidths, divided by the radar duty  cycle:
• (.2/172MHz)*(2800Ipp/5usecsPulse) + 33db= 31db
  
• The A/D converters were set to a 1 sigma rms of 30 counts. The dynamic range of the A/D is then:
• (2047/30)^2 = 37 Db.
  
• This says the A/D has enough dynamic range for this signal....
• BUT... We Know the signal strength was compresses so the 31db  increase of Tsys is probably low..

Summary:

• The change in the a/d voltage range 1.5 V to .78 is causing overflows in the a/d with the radars.
• The radar acts like a sine wave and caused spectral artifacts when the a/d is clipped.
• This has been seen in the faa radar and the 1290 remy radar.
• The overflow last for about 40 milliseconds which is equal to the beam width of the radar.
  
• The overflow must be in the 3 to 6 db range since it goes away around the fwhm of the radar.
• Assuming no compression the measured signal would raise the total power 31 db. The digitizers set at 30 counts rms should have 37 db dynamic range. Since we saw the compression the 31 db is low due to the compression.  
  

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