[evlatests] L-band performance

[Bob Hayward]

Just a reminder that the LNAs used in the L-Band receiver are balanced 
amplifiers. Unlike the single-ended amplifiers used in all the other 
EVLA receivers (except S-Band), a balanced amp may still work function, 
more-or-less, when one of its active elements fails.

In a single-ended amp, if the transistor dies or a wire bond pops loose, 
the amplifier may well turn into an attenuator, or at the very least, a 
low-gain, noisy amplifier. In a balanced amp, the architecture (which 
splits the signal with a 180 deg hybrid coupler, amplifies each path 
separately, and recombines them in a 2nd coupler), gives you a much 
better input return loss (essential for us since 1-2 GHz cryogenic 
isolators don't exist) plus 3 dB more headroom. A failure in one of the 
two paths will essentially double the noise figure but reduce the gain 
by 3 dB.

The L-Band uses two types of balanced amplifiers in each polarization. 
The 1st gain block is a low-noise (4K), low-power amplifier with 18 dB 
gain. The 2nd gain block is a higher-noise (20K), higher-power 
amplifier, also with 18 dB gain. If we have a failure in the low-noise 
amp, it may increase the receiver temperature by something less than a 
factor of 2 yet drop the gain by only 3 dB. If it happens in the 
high-power 2nd stage amp, it may only increase the receiver temperature 
by a few Kelvin and also drop the gain by only 3 dB.

We are attempting to determine if the amplifier failures are electrical 
in nature (i.e., fried devices) or are a mechanical problem (i.e., 
popped wire bonds). There is circumstantial evidence that most of the 
failures are in cryostats with the new beefier Model 1050 refrigerator. 
Accordingly, we are also considering doing some vibration tests using 
accelerometers to compare the magnitude of the mechanical impulse shock 
from the thermal cycling of the cold-head seen on the Model 1050 and 
compare it to that seen with the Model 350 fridge that was used - 
successfully - on all the Interim L-Band (1.2-2.0 GHz) receivers.

-Bob


Rick Perley wrote:
> As part of debugging the tendency for my scripts to collapse the 
> correlator, another 'flux density' test script was run today.   This
>  script includes L-band, so I took a careful look at the data, given
>  Chuck's report this morning of some serious issues with the L-band 
> receivers. The situation isn't nearly as dire as Chuck's report
> seemed to indicate:
> 
> 1) Antennas 11, 17, and 19 were out of the array. 2) ea18 gives
> essentially no fringe power at any of the four subbands. 3) ea26 is
> similarly weak, on RCP only. 4) ea23 is also very weak, on LCP only.
>  5) ea13's fringes are down by about half in RCP. 6 Three other
> antennas give weaker-than-expected fringes on single IFs:  ea07A,
> ea04A, and ea15C.  However, these might perhaps be low power levels
> specific to a single IF. 
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