- 04 Nov 2008
Date: 10/28/2008, Simon
From Langston's GBT observations ,we are trying to understand why Glen reported higher than expected system temps. See: * log08oct12.doc
From previous lab testing, we know that LO1 drops off rapidly above ~32Ghz, see: * LOdoubler.xls
Due to the slope of LO1's response, we have have determined the LO1 input power needed to provide optimal power at the downconverter input, see:
For GBT observations and lab testing, a w/g high pass filter is installed between the doubler output and downconverter input. Use numbers from column "With high pass filter between doubler and down converter"
Glen used a constant LO1 power of +10dBm for all his observations, regardless of Fsky.
For lab testing, we used a constant LO1 of +6dBm which is equivalent to what Glen used when observing on the GBT, LO2 is 8900Mhz at +4.2 dBm. Single pixel is cold in lab, 300k w/g load on feed input. We used the R&S spectrum analyzer and looked at both the RCP and LCP IF. Data was taken at four different LO1 frequencies: 15900, 16400, 16900, and 17400Mhz, corresponding to a sky frequency of 23, 24, 25, and 26Ghz. Here are the IF plots, note that Fsky=23 and 26 Ghz are clean, and Fsky=24 and 25 clearly shows a spur. Due to LO1 rolloff, data taken at Fsky 25 and 26Ghz is very low. Increasing LO1 power didn't change the response significantly at 25 and 26Ghz. The following measurements were taken from channel LCP, 600 to 3600Mhz. Data for channel RCP was taken, but not shown in the following: * spurs.xls
Next, we used a power meter on the IF at the downconverter output, and recorded Pout with the 300K and 80K absorber over the feed. The following gives Trx for RCP and LCP:
From this data, we can only conclude that Trx is lower in LCP than RCP. Although Trx is somewhat reasonable at Fsky=23 and 24Ghz, it is not at Fsky=25 and 26Ghz. This could be attributed to LO1 dropping out, drift/non-zero in the power meter, and spurs in the IF. More testing is needed to fully understand why the GBT measurements are so different than
we took in the lab.