1. 100-250 MHz with LO at 175 MHz
2. 250-400 MHz with LO at 325 MHz

• The red noise source was sent to Box 103.
• Separate measurements were done for each 150 MHz subband and polarization.
• The signal goes through the IQ  mixer, 75 Mhz bandpass filters, spliters, amps , and then two the digitizers.
  
• The I signal was sent to Re(polAinp). 0 was sent to Im(polAinp) (done digitally in the pdev device).
• The Q signal was sent to Re(polBinp). 0 was sent o Im(polBinp).
• Full stokes data was taken and then angle between polA, and polB (atan(U/V)) was plotted. This is the phase difference between the I and Q inputs.. with one minor detail....
• The minor detail..
  
• Let Enp be the signal in channel LO + n
• Let Enm be the signal in channel LO - n
• At baseband, the individual I and Q spectra have the + and - frequencies folded on top of one another.
• In = (Enp - Enm), Qn=(Enp90 - Enm90).. and Enm90  has a negative rather then positive going 90 degree shift.
• The output signal is not a function of the gain variation in the band between the Plus and Minuse image.
• To make this work we need to filter out 1 of the image bands (below or above the LO) prior to the downconversion.
• For the High band (250-400) we used an IF filter covering 220-320 and an lo of 325. This gave data from 240-325 (the lower half).
• For the low band (100-240) we used an IF filter covering 140-200. Data sets were taken with an LO  of 205 and then 215 to separate out the phase offsets in the 75 Mhz baseband filters and the I,Q mixer.
  

• Page 1: The "Stokes" parameters for the I,Q spectra.
  
• Page 2: The phase angle between "polA,polB" which is measuring the phase difference between the mixer I and Q outputs. The top plots are versus IF frequency while the bottom plots are at baseband.
  
• The phase in both cases is within 2 degrees of 90 degrees with a deviation acrros the band of about a 1 degree.
• The  120 to 140 MHz portion of the lower band had little power (because of the IF filter) so the excursions from 90 deg should be ignored.
• The angle between I,Q for the lo band has both the 205 and 215 Mhz LO data.  The structure in the phase vs frequency at baseband is the same for both LO's so this is coming from the baseband filters.
  

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