Resonances in the New lband wide receiver

apr 03

  Links to plots:
Plots of the 3 resonances that were found:
Plots of on/off -1 for the entire frequency range:
Plots of on, off for the entire frequency range:

    The new lband wide receiver was installed in feb03. One of its features is that it has many fewer resonances than the old lbw receiver (see resonances in the old lbw receiver for a list of the old receiver resonances).
    The resonances result in losses in the OMT. A simplified model of the system temperature including these resonances is:
    Tsys= Tomt*alpha + (1-alpha)*Tsky + Tamp.
Where Tsky includes the sky and scattered radiation, Tomt is the physical temperature of the OMT and Tamp is the amplifier temperature referenced to the input of the amplifier. alpha is the fractional loss of the sky signal caused by these resonances. Rewriting the equation:
    Tsys= Tsky + Tamp + alpha*(Tomt-Tsky)
This shows that the spectral density  at the resonance will increase or decrease (relative to the adjacent frequencies) depending on whether or not (Tomt-Tsky) is positive or negative. If you are on cold sky then Tsky may be 6+ 14(scattered)=20 K while the OMT temperature is about 85 K. If you look at a 10 jansky source then Tsky= 20+12*11k/Jy= 152K is greater than Tomt so  Tsys will have a negative going bump at the resonance frequency.

    On 17apr03 1 minute on off position switching was done on the source 3C138 (J0521+166). This source   is 9.2 Jy at 1400 Mhz. The correlator was setup to run 4 by 25 Mhz bands. After each on/off pair, the frequency was stepped by 100 Mhz covering 1100 to 1700 Mhz. The entire range was covered in 6 steps. The frequency range was repeated with a 10 Mhz offset at each frequency setting to cover the frequency where the subcorrelators overlapped. The lbw receiver was set  to native linear polarization (the hybrid was not inserted).

    The ons and offs were searched for patterns that had positive bumps in the off and negative bumps in the on. The rms for each 60 sec off was also computed to see if there was any rfi in a channel. Three resonances were found (there is a fourth resonance above 1720 which wasn't covered). Since 3C138 is about 7.5% polarized at 1400 Mhz, polA and polB were averaged. This gave a consistant source strength versus parallactic angle.

Plots of the 3 resonances that were found:

    The figures show the 3 resonances that were found.  The median value of each spectra was removed for plotting purposes. The colors are:
The cals were used to convert the data to Kelvins.

    Plots were made of on/off-1, rms by channel, and the individual spectra of the offs where the resonances occur. The resonance are flagged with a green vertical line.

Plots of on/off -1 for the entire frequency range:

    The plots show on/off-1  for the entire frequency range 1100 to 1700. Each plot covers 100 + 10 Mhz. The colors are:

Plots of on, off for the entire frequency range:

    These plots show the individual 25 Mhz band passes for the on and off scans for polA. The median system temperature has been removed for plotting. There are two sets in each plot. The second graph has  a 10 Mhz frequency offset. The colors are:

Table of resonances:

    The table below lists the 3 resonances that were found. There is a fourth above 1720 Mhz.
Frequency of resonances Mhz
processing: x101/030417/