General description
L-band behavior of EVLA antennas at frequencies below about 1250 MHz
is unusual, in that the A and C IFs show highly variable sensitivity,
varying with frequency, while the B and D IFs degrade smoothly as the
frequency decreases.
The essential issue is that in order to fit the slice of frequency
space which we are going to send to the old (narrow-band) correlator,
the L301 and L302s must shift the spectrum so the desired frequency
slice fits into the fixed 64 MHz-wide window of Mike Revnell's D-to-A
box. This window is located at about 1830 MHz, so when we wish to
observe at a very low frequency, the overall front-end spectrum is
shifted way, way over. This means that the total power 'seen' by the
output TP meter in the T304 is very much reduced, as the front-ends
provide very little power at (real) frequencies below 1.2 GHz,
inducing the T304 to decrease the attenuation to keep the total power
at the agreed upon level. This is appropriate for the 8-bit sampler,
but is not good downstream, because the spectral power density has
been greatly increased. So when the signal passes through the D-A
window, the total power is now too high, and overdrives the backend
A/D whose job it is to keep the total power in the range the VLA's
3-bit sampler needs. This causes a dramatic loss in correlator
coefficient (readily seen).
For the B/D IFs, the situation is essentially reversed. Here there is
no power autolevelling -- levels are set by a table. As we
tune downwards in frequency, the actual power delivered to the 8-bit
sampler declines -- which is of little consequence to that device.
Apparently, the reduced power getting through the D-A's window is
within the range that the VLA's backend A/D can handle.
This is another 'transition' issue, but will only affect those brave
enough to observe at frequencies below 1.2 GHz. Currently, only
antenna 14 has an OMT which lets useful signals through to the
correlator at these frequencies.
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