[evlatests] T304 Shielding Effectiveness

[Rick Perley]

    The T304 modules on IFs A and C in antennas 13 and 18 have received 
special internal shielding and absorber, in an attempt to reduce the 
strong 8192 harmonic of the 4096 LO, and the beat between the third 
harmonic of that LO and the L302 frequency.    There are two generations 
of these special modules -- a 'good' one, in IF A, and a 'better' one, 
on IF C.

    Earlier tests showed that these efforts have greatly reduced the 
internal coupling , but that the 'C' IFs seemed no  better than the A 
IFs in antennas 13 and 18.  A possible explanation is that the residual 
8192 MHz signal seen on IF 'C' is actually due to coupling with IFs B 
and D, where the signals are very strong -- there being no shielding of 
absorption in these modules. 

    A test was made today of this sugggestion.  The autocorrelation 
spectrum (with 1.5 kHz resolution) at 8192 MHz on antenna 18 was made 
with about 30 seconds' averaging, with all four T304s up and running, 
and with the B and D T304s in an unpowered state (Rob will have to 
supply details of how this was done).  

    This test support the suggestion of internal coupling.  On the 'A' 
IF ('good' shielding/absorber), the 8192 MHz signal was weakly seen in 
both halves of the test.  But on the 'C' IF, the weak signal visible 
when the B and D T304s were on disappeared when they were off.  It is 
suggested that the signal seen in 'A' is the residual native signal in 
the module, following 'good' absorber/isolation, while on 'C', the 
'better' absorber/isolation has indeed reduced the internal signal below 
the noise fluctuation level of this test, and that the signal seen is 
due to some coupling with the strong signals remaining in the B and D IFs.