[TowerTalk] Elementary Rx protection question

[Jim Lux]

On 1/13/13 12:57 PM, Pete Smith N4ZR wrote:
> Do back-to-back diodes across a receiving antenna degrade receiver
> performance before they begin to conduct?  what I'm considering is
> back-to-back series strings of diodes to limit RF voltage at the front
> end of an SDR.  My working assumption is that the conduction threshold
> could be set below the damage level but above the ADC clipping level,
> but if it affects performance before the ADC clips then that's no good.
>

The challenge is that diodes actually begin to conduct pretty early 
(it's sort of a square law leading to a linear) so it's not so much the 
clipping, but the IMD when the diodes are partially on.

Back to back diodes (zeners) work as a limiter to do damage protection, 
but reverse biased diodes make fine noise generators.  I think, if the 
knee is sufficiently far out, this might not be a problem: and if you've 
got your diodes in series, the forward conduction isn't an issue.