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: http://www.naic.edu/~phil/hardware/iflo/if2monPortApr03.html
Дата изменения: Fri Apr 25 21:34:39 2003 Дата индексирования: Sat Dec 22 17:03:50 2007 Кодировка: Поисковые слова: http news.cosmoport.com 2005 04 22 1.htm |
The downstairs if/lo has an 8 way splitter/buffer
amplifiers for each of the 8 sub bands that can be generated (4 for polA,
4 for polB). The frequency range is 0 to 500 Mhz and can be sent to
7 back end devices. The eighth channel is a front panel monitor port that
is normally used for the spectrum analyzer. It is also used to take data
with the radar interface (ri) a/d system. The front panel output has caused
ripples in the spectra of the correlator if a cable/device was connected
or disconnected during an observation.
Lisa Wray modified the front panel output on 23apr03
to improve the isolation. The before and after setup is shown below...
The power for a transmitted and then 100% reflected signal was 11db
- 17db = 6db.
After the change the power was : -10db+24-3-3-30-10=-32 db. The difference
is 26 db (twice the attenuators added since the amps have the same ratio
of gain to reverse isolation).
The correlator is connected to the output adjacent to the front panel
(FP) output.
The decrease is not as large as the change in the power for a signal that traversed the path and then had a100% reflection.
addendum.. jon hagen pointed out that you have to follow the voltage through the path and then add this to the volaage that got sent directly to the correlator. The power difference along the path was 32 db. The voltage difference would have been half of this in dbs. Call this deltaV. If you add this to the value that went directly to the correlator (using units of Tsys=1) and then square it to get the voltage you get:
powerRipple(units Tsys)= (1+deltaV)^2= 1 + 2*deltaV + deltaV^2 .
Since deltaV is small you can ignore delatV^2. The ripple size will be 2*deltaV. The ratio of before and after should then be the 2*deltaV1/2*deltaV2 . This is 13 db (or one pass through the attenuation added). The measured value was about 12.
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