CCD-world: Influence of meteorological radar system on CCD electronics

[Tom Droege]

The following was posted to CCD-world:

There are a number of questions to ask before one can guess if you will
have any trouble:

1)  Are you inside a conventional metal dome, or in a roll off enclosure?
If directly exposed to the beam you have a larger problem than if you are
inside a metal  enclosure.  
2)  If in a dome, can the radar ever point into the dome slit?
3)  You never want to point your telescope near the radar.  Remember the
radar has "side lobes.  If you point at the antenna, it can get you even if
the antenna is pointing in another direction. 
4)  The wave length is short enough to allow significant energy to bounce
right down into the camera head.  
5)  On the optimistic side, if you have designed the camera electronics to
withstand a nearby lightning strike, then it will also stand a lot of
continuous RF.  
6)  Also on the optimistic side, if the radar beam does not cook you, it
will probably also not cook the electronics/CCD.   Possibly your officials
are not so safety conscious, so watch out for colleagues dropping dead on
rainy days. ;^(

We run a lot of electronics at Fermilab near very high power RF.  The
problem is not so much that things burn up, but that the RF gets into
everything at low level, gets rectified by PN junctions, and causes level
shifts in the electronics.  I would watch for background level shifts in
images that are read out as the beam sweeps by.  Note it is very difficult
to make quick fixes on this type of problem.  Any thing you do will change
the problem,  so half the things you do will tend to make an improvement.
But the improvement is not real and the problem returns.  The only thing to
do is to completely tear everything apart, and redesign the package for
better shielding.  I have often given this advice, it is never taken, they
just find a different consultant.  In the end more time is wasted than if
the device was completely rebuilt.  

If you do have the level shift type problem, the first thing to do is to
make an equivalent circuit diagram of the electronic system.  You need to
thoroughly understand how signals get from one location to another, what is
inside the ground shield, and what is outside.  Often just making a good
diagram will point at the problem and an easy fix.  This is already too
long.  I can discuss more about how to solve noise problems if anyone cares.  

Tom Droege


At 11:15 AM 9/18/99 +0900, you wrote:
>Is there anybody who has information about the influence on
>CCD electronics by meteorological radar nearby?
>
>The governmetal meteorological agency in my country is planning
>to setup a radar 500m to the west of our telescope.
>The freqency range will be 5.3 - 5.7 GHz and peak power is
>around 750KW with normal run at 110 - 570W.
>The direction of observation will be nearly horizontal,
>making 360 degree turn on rainy-cloudy days.
>
>They say that they will do every precaution to make no harm
>to our observing system caused by this powerful EM wave
>source. But I wonder whether this is really harmful or not,
>especially to our CCD electronics.
>
>Please send me any technical information or experience on
>similar situation.
>
>Thanks,
>Byeong-Gon
>
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