[TowerTalk] Fwd: Elementary Rx protection question

[Hans Hammarquist]

Do not use zener diodes across the input. They have a high capacitance, which would degrade the signal. Using "small signal" diodes would be OK but you may experience a degradation due to IMD.


Better would be to put a bridge consisting of 4 "small signal" diodes "feeding" a zener diode in parallell with a small capacitor. That way your diodes will be back-biased with less IMD problems.


Hans - N2JFS



-----Original Message-----
From: Jim Lux <jimlux at earthlink.net>
To: towertalk <towertalk at contesting.com>
Sent: Sun, Jan 13, 2013 7:34 pm
Subject: Re: [TowerTalk] Elementary Rx protection question


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.

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