[evlatests] 8192 MHz Birdie

[Rick Perley]

    To determine if the 8192 MHz birdie is getting in through the feed, 
or via the T304
module, we ran a short test on antenna 13, in a single-antenna mode, and 
Mike Revnell's
FFT program, to view the total power spectra.   David Sevilla's program 
to plot and display
was used.  (David is Walter Brisken's summer student). 

    The spectral resolution chosen was 7.8 kHz (the spectrum consists of 
2^17 - 131k points!). 
We obtained spectral from both 13A (where the T304 has internal 
absorber) and 13C (whose
T304 has none) for three setups:

    a) Cold sky
    b) Hot load (absorber over the feed)
    c) With the input to the T304 terminated. 

    The spectra were taken with an average of one minute of data. 

    Results:

    13A (whose T304 has internal absorber).

    a) Mean power level is 50 counts, peak-peak is about 20 counts, and 
no trace of the 8192 birdie is seen. 
    b) Mean power level is ~500 counts (i.e. - Y-factor is about 10dB), 
pk-pk is about 150 counts, and there is no trace of the birdie.  (No 
surprise, given (a))
    c)  Mean power level is 0.5 counts (so the internal noise is down 
about 20 dB from the input signal), spread is about 0.2 counts, and the 
birdie is seen at a level of 0.8 counts above the noise level. 

    13C (with no internal absorber). 

    a) Mean spectral power level is about 18 counts, and the RFI birdie 
is seen at 240 counts above the noise. 
    b) Mean level is 170 counts, spread is about 70 counts, and the RFI 
spike is seen at 250 counts above the noise.  *** Thus -- the 8192 
birdie is not coming in through the feed at any appreciable level***
    c) Mean level is very low (few counts -- scale is too large to see), 
RFI birdie is at 200 counts -- slightly less than in the two cases 
above.   The birdie is thus about 24 dB stronger than in 13A.