[Premium-Rx] RA6830 digital noise source

[refmon]

Hi Frank and group,

I haven't studied the physical layout, so don't know if there is space
available for this system:  Watkins Johns 8618 - style shielding is as you
describe including the fact that the top cover does not, of its own accord,
contact the vertical intra-modular shield plates.   The WJ cover has an
interesting network of beryllium copper emi fingers that reach down and mate
with the intra-modular verticals.  At certain frequencies, I can see the
same effect you describe between top on/top off.

I have a few emi fingers I could donate to the "student fund".  They are
pop-riveted in place.

regards

John Collins
_______________________________________________
----- Original Message ----- 
From: "Carcia, Frank A. HS" <francis.carcia at hs.utc.com>
To: "Premium-Rx (E-mail)" <premium-rx at ml.skirrow.org>
Sent: Monday, September 29, 2003 05:43 AM
Subject: [Premium-Rx] RA6830 digital noise source


> Hi All,
> I was bringing a dead processor module back from hanger queen mode
> this weekend
> and found the digital noise source while testing it. I have 1
> chassis that has always
> been a problem with digital noise and has become a test box. I
> bought some modules
> from the famous Racal man in Md. and picked up a couple dead units.
> I added the extra ground on A11 which helped a little but the noise
> was hurting the
> MDS at 6 KHz. bandwidth.
> The 6830 modules are sitting on a ground plane mother board and the
> shield plates
> for each module are sitting in metal guides attached to the sides of
> the chassis.
> There is a vertical divider at the digital section around the
> synthesizer module.
> The radio is well shielded on 3 sides but the top is open.  I
> grabbed a piece of sheet
> metal and laid it across the tops of the modules A6 to A13 inside
> the chassis
> side rails and pressed it so it contacted all the module top
> shields. The two lit bars
> on the s meter display went out and the noise went away. The CPU
> module has
> two test points that can be shorted to throw it into reset holding
> the last settings.
> I tried it and there was only a slight change in noise between
> running and reset.
> The metal sleeve that is the radio cover does not contact the
> modules if they are
> pressed all the way in. There is about .05 inch between the top of
> the modules and
> the cover. The processor section  is about 1/8 inch.
> I'm thinking of a couple possible solutions for this problem and
> would love suggestions.
> 1. Make 3 shield plates to shield all the tops of the modules in the
> digital, synthesizer
> and RF sections. This could cause thermal problems if it is solid
> metal.
> 2. Shield the digital section. Then set the height of the RF modules
> ..05 inch higher
>     to contact the chassis sleeve. A number of holes could drilled
> into the sleeve
> and module shields. The modules could be tapped to accept small
> screws.
>
> I don't care if it takes a little more time to service the radio but
> there must have been
> a system spec Racal was stuck with that hurt the shielding.
> I've been chasing this one for a while and seem to have found the
> source. Now
> for a simple fix. Any ideas out there. Yup digital return currents
> on the chassis.
> fc
>     shields and
>
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