Measured Intermods in ALFA/pdev from radars.

Links:

Intro
Setup for data taken 30apr09
Plots of the measured intermods
Summary
Table of mixer intermods

Intro:

   The radar rfi seen in the a2133  data during apr09 showed radar like rfi (time periodicity that was synchronized to the radars) spaced over parts of the band.

• 30apr09: Step the first LO by 1 Mhz steps.
  
•  On 30apr09 a check was done to see if there are intermods being created in our first mixing stage.  The first LO was stepped from 1625 to 1620 in 1 Mhz steps (skycfr 1375 to 1370). If the rfi is  intermods in the first mixer with LO order  greater than 1, then the rfi should move by steps greater than 1* 1stLoOrder Mhz.
• 04may09: step the 2nd lo by 1 Mhz.
• The second LO was stepped by 1 Mhz. If the intermods are generated in the second mixer, then the rfi should move by  lostep * intermod 2ndLO order.
  

    The intermods in the 1st and 2nd (base band) mixers were then computed for intermod orders of 1 to 10. The standard alfa/pdev search setup of 1375 SkyCfr, 1300,1450 subband centers was used.

The setup for data taken 30apr09.

• The telescope sat at:
• az,za=285.4,10.1 for sky  freq=1375,1374,1373
• az,za=180,3.4 for sky freq 1372
• az,za=180,10 for sky freq 1371,1370.
  
• Alfa (beams 0-5) and the pdev spectrometer were used for data taking. Beam 6 was ignored because bm6Polb is dead.
  
• The sky center frequency was stepped from 1375 to 1370 in 1 Mhz steps using the first LO (1625 -> 1620).
  
• 60 seconds of .1 second integrations was taken at each step
  
• Two 172 Mhz bands were taken with pdev. The bands were centered in the IF 175 Mhz(1450 skyfreq) and 325 Mhz (1300 skyFreq). Stepping the first LO centered these two bands at:
  
• 1450,1300 sky cfr=1375
• 1449,1299 sky cfr=1374
• 1448,1298 sky cfr=1373
• 1447,1297 sky cfr=1372
• 1446,1296 sky cfr=1371
• 1445,1295 sky  cfr=1370
  

Plots of the 30apr09 intermods:

• 1475->1450 5 Mhz steps.
• These move down by 5 MHz for every 1Mhz decrease in the first LO.
  
• This was time correlated with the FAA radar (1330,1350) Mhz.
• 1450-1445 in 1 Mhz steps.
• These are the DC spikes in the spectrum.
• 1375-1365 (then off the plots) in 5 Mhz steps.
• These do not show up in the overlapping 1300 Mhz band.
• They must be created in the baseband mixer 175 Mhz cfr to baseband.
• 
  

• these plots show a blowup of the first 1450 Band around the rdr rfi: 1440-1480 and 1355-1385
  

• 1300->1295 in 1 Mhz steps.
• this is the dc spike of the spectra
• 1270->1260 in 2 Mhz steps.
• There is an image of the 1330 radar that starts at 1270 (1300 - (1330-1300).
• When we step the cfr by down by 1 each step (1300,1299,1298..), the distance (1330 -cfr) moves up by 1 Mhz steps.
• The Image for the 1330 Mhz will be 1 Mhz farther from the center each step, and the center is moving down by 1 Mhz each step, so the image will move by 2 Mhz steps.
• 1375->1365 5 Mhz Steps.
• These are not present in the 1300 Mhz band.
• This means that they are not being created in the first mixer.

04may09 . step 2nd lo by 1 Mhz.

• file 0000:
• sky cfr 1450, 2nd lo's 175.
  
• rfi at 1475 and 1375
• file 00100:
• sky cfr 1450, 2nd lo's 174.
• rfi still at 1475, 1375.. so the rfi is not being created in the 2nd mixer.
  

SUMMARY:

• A table of the observered intermods is shown below.
• A table of  computed intermods (to 10th order) created in the alfa/pdev mixers can be found (here).
• The 1475 radar rfi:
• It was time correlated to be from the faa 1350/1330 radar.
  
• It moves by 5 times the change in the 1st lo.
  
• None of the computed 1475 intermods moves by 5*lo.
• When the 1st lo is moved by 1 Mhz,    the location of the radar moves by 1 Mhz in the IF.
• 5*rfi - 3*lo at 175 does match what we see so this is probably an intermod in mixer 2 .
• When the 2nd lo was moved by 1 Mhz the intermod didn't change??
  
• 1403 rfi is from punta salinas.
  
• I'm not sure how this is getting from the punta salinas freq to 1403.
• A2010 reported the same frequency (1403) as the mock spectrometers.
• A2010 and a2133 use different rf frequencies: 1385 MHz and 1375 MHz) so the lo's are different. This make it look like the 1403 is before the first mixer.
  
• 1424.9, 1425.1 are also from punta salinas
• 1313.5/1298.6, 1304.5/1289.3 are each spaced by 15 MHz so they must be from punta salinas.
• 1422: this was seen in the galfacts data.
•  It is time correlated with the 1270 remy radar.
• It could be 4*1269.5 - 3*1625. The radar looked to be centered closer to 1269.5 than to 1270.
• 1399: This is time correlated with the aerostat radar when it was broadcasting at 1246/1261
• it could be 8*1246.7 - 6*1625.
• Probably need to transmit at some of these frequencies and see if we can generate the intermods. Then we can change the lo and the input rf..