[hpsdr] input protection

[Graham Haddock]

FRANCIS CARCIA wrote:
> We use a lot of micro semi parts. The Biased diode trick when the 
> circuit can't handle the high C of the transorb. I would prefer a 1.5 
> KW or a 3 KW part because the junction voltage takes off as soon as 
> current starts to flow. A low power part will need a lower junction 
> voltage to protect the input. I could easily see a 50 amp pulse coming 
> down a RX coax.
> Also you want a good ground on the shield so the common mode voltage 
> stays low.
> frank
Hi Frank:
The clamps I am looking at are 3 volt, and will hold 17 amps below 19 volts.
I can't imagine voltage clamped 1N4148s doing any better.

A 50 amp pulse across 50 ohms looks like 2500 volts to me.
That kind of surge really needs to be clamped at the building entrance with
ground plates and co-axial discharge tubes.

If that kind of pulse makes it all the way to the receiver, then parts 
will fail.

Part of the problem with ALEX is that there are seven possible inputs that
could be hooked to an antenna.  Each will have TVS protection, but I don't
have room to build seven battleships.

--- Graham

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