Presentation on theme: "Summary of 74 MHz Test with “Power Down” at Parkes Site (2003-01-29) New Data file: 2003-01-29_1120_z20s.rpf (apparently no matched load data taken this."— Presentation transcript:
Summary of 74 MHz Test with “Power Down” at Parkes Site (2003-01-29) New Data file: 2003-01-29_1120_z20s.rpf (apparently no matched load data taken this time) Matched Load Data: for passband comparison was taken at an earlier session (2003-01-09) OVERVIEW & CONCERNS: Still pretty cluttered up with indications of spurious intermods being cycled in frequency with ~25 minute cycle Strongest RFI spikes at 2048, 4096, 6144 (and some intermediate at N*1024). Is correlator in good health? Total power passband on sky is much different than the passband with matched load (is the feed working right? We have been using the MITEC amps, and maybe Bill’s amps did more in the way of matching?) Description of Plots (that follow): 1. Comparison of spectra for one 5 second integration with scan average passband shape for a long integration on a previous date. 2. Total power versus time. Black lines are straight averages over frequency; red lines result from applying a median filter across each 5 sec spectrum first (to remove spikes) before averaging over frequency. 3. Another measure of Total Power variation with time -- obtained by choosing a narrow, apparently-empty frequency band between the spikes at channels 3280 and 3350.
Description of Plots (continued): 4. Dynamic spectra: First 50 minutes. Upper image is the low frequency half of the band 72-74 MHz; lower image is upper half 74-76 MHz. A passband shape was divided out; the passband was constructed by (1) median filtering each spectrum in frequency, (2) skipping those that jumped by 3% in total power, (3) averaging the spectra from the time step range 20 to 6000 (during which they appear to be well behaved in total power). 5. Dynamic spectra: 50 minutes near center of the recording. 6. Dynamic spectra: the entire range of the recording. Only 1 in 20 spectra are included. Note: sawtooths & gentle arcs. The total power doubles during the recording so that a smoothly varying total power level was subtracted to keep the image values within a useful grey scale… But the noise level increases with time through the image, and the passband calibration becomes inaccurate, since the sky power experiences a different passband than the amplifier/ground-pickup noise power (causing the white patches in the extreme freq channels at late times). 7. Example spectrum: Normalized to Tsys=1.0. Strongest spikes at N*2048 at ~35 Tsys 8. Example spectrum: Normalized to Tsys=1.0. Different gains and zooms. Lower panels show channels 2049:4096 (left) and 4097:6144 (right). Looks like N*1024 and possibly some N*512 coming through.
Description of Plots (continued): 9.Comparison Total Power vs. Time for site-powered (2003-01-09) and site-unpowered (2003-01-20). 10. & 11. Dynamic spectra: comparison of Site-Powered and Site-Unpowered on ~same stretch (normalized to unity Tsys). The other possibly important influence on the appearance of the dynamic spectra is whether the Parkes dish is allowed to move or not. When it changes its position with respect to the 74MHz feed (which is sitting on the ground), the reflected ground/sky radiation changes, and the total power and passband shape changes. This is probably the cause of the bright and dark bands in the image for 2003-01-09. The other possibility could be a time-variable source of broadband noise. 12. Comparison of Spectra from averages Time Steps 1500:2099 (Galactic Pole) and 5000:5599 (elevated total power as creep into Plane) [Galactic Center transited during the daytime, and RFI levels became prohibitive.] 13Difference between spectra in Fig 12. This is the ‘Gain(f)’ curve. 14Quotient of spectra in Fig 12… Flatness of curve again implies that the sinusoidal power level in spectrum is a gain of the feed effect. New data recorded by JR – analyzed by FB.
Comparison of Passbands measured with Matched Loads: Green and Red White with one 5 second integration on Sky: Blue and White
Total Power vs. Time Avg of entire band: Black Median filtered band: Red
Total Power vs. Time Step [ @ 100 sec/step ] * alternate approach * based on avg of frequency channels 3280:3350 from band of 8192 channels
Dynamic Spectra: first 50 min (time increases upward) 72 MHz 74 MHz 76 MHz74 MHz
Dynamic Spectra: 50 min (17:00-17:50UT - time increasing upward) 72 MHz 74 MHz 76 MHz74 MHz
Dynamic Spectra: entire dataset (11:20-23:00 UT - time increasing upward) * passband determined from average of median-filtered spectra from 11:23-20:00 UT * plots show 1 in 20 spectra * all dynamic spectra are Polarization 1 72 MHz74 MHz 76 MHz74 MHz