gain variation with temperature

15sep01

     On 07sep01 blank sky was tracked rise to set  using the cband receiver. On 08sep01 another blank piece of sky was tracked with the lband narrow and lband wide receivers. Both of these observations were at night. Multiple 25 Mhz bandwidths (per receiver) were sampled at  1 second intervals using the correlator. A temperature sensor in the gregorian turret room was sampling the air temperature every 10 seconds while the measurements were being taken. The temperature sensor is situated in the center of the room between the IF/LO racks.
    The figures show the total power (for each 25 Mhz band) versus time. 4 bands with two polarizations each were measured simultaneously. The processing of the time series took 2500 second blocks at a time and computed  tp[i]/median(tp). A second order baseline was used to remove the zenith angle dependence.
  • Figure 1 has 4 25 Mhz bands at cband. Pol A and PolB are plotted in the same color. The temperature is varying by about .7 degF while the gain changes by about .8%. This occurs in all frequency bands and both polarizations.
  • Figure 2 plots 4 25 Mhz bands using lband narrow. There are discrete jumps that correlate with the temperature cycle (this is a separate problem.. see A1459 problems). If you undo the discrete jumps then there is  a .6% gain variation per deg F. Again the variation is in all frequency bands and both polarizations.
  • Figure 3 plots polA of lband wide with the hybrid out (pol B has a separate 100 second oscillation ..see lbw 1% oscillation). The gain variation is .5% per .5 degF. It is in all frequency bands. PolB probably has the same oscillation but it is difficult to see with its 100 sec 1% oscillation.
  •     All 3 receivers show a gain variation with the temperature cycle. It ranges from .5% to 1% per degF.
  • The same cband variation had been measured back in aug2000 .
  • The temperature dependence of lbw was measured on 12jun01 after an sband transmitter run and no gain variation with temperature was seen. The difference may be that the component that was varying had not yet reached equilibrium with the temperature that the temperature sensor was measuring.
  • lband narrow has a smaller variation than the other two. It is more difficult to measure it because of the jumps.
  • This problem is a common problem with multiple receivers so it is probably in the IF/LO. It is definitely something in the turret room since it correlates with the turret room temperature. It has been there for at least a year. It must be a component that is common to both polarizations. Things that come to mind are:
    1. The 1st lo synthesizer. Although the mixer output should not be sensitive to a small change in the lo levels.
    2. The zeeman box polarization switch. This is an active component that allows rapid switching of the two polarizations. It's initial purpose was for zeeman experiments (until the correlation method was implemented). It is now only used for diagnostic purposes.
    I've called this a 1% gain variation but i really don't know that. It could be the noise power of a device that is changing.
    processing: x101/010908/gaintempdepend.pro
     home_~phil