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Re: [TowerTalk] receiver protection

To: Pete Smith <n4zr@contesting.com>, towertalk@contesting.com
Subject: Re: [TowerTalk] receiver protection
From: Jim Lux <jimlux@earthlink.net>
Date: Mon, 28 Mar 2005 14:02:36 -0800
List-post: <mailto:towertalk@contesting.com>
At 01:16 PM 3/28/2005, Pete Smith wrote:
>I am looking at a design for receiver protection in an SO2R environment
>that involves a series resistance (#49 bulb) and stacks of back to back
>diodes shunting the receiver input.  The objective is to protect front-end
>components against strong out-of-band signals, but in the process I don't
>want to incur an intermod penalty under normal operation.
>
>If I stack 3 diodes in each direction, the forward conduction point will be
>2.1 volts.  Is that high enough to avoid generating intermodulation, except
>under circumstances where I wouldn't be able to hear anything anyway?


That notional 0.7V Vf is at some DC current and is the sum of the IR drop 
across the diode plus the forward drop of the junction.  However, the diode 
DOES conduct all the way down to zero (otherwise you couldn't use a diode 
as a detector for microvolt signals, or as the sensing element in a power 
meter).  You'll be in the "square law" region.

There's also the problem that the reversed biased diode has some non-zero 
capacitance.  That C will vary as the voltage (not as much as a varactor, 
but varies none-the-less).

The overall circuit will look remarkably like a crystal radio (with the 
diode shunted across the antenna input).

I assume you're looking for something that has infinite impedance until the 
voltage exceeds some threshold? Like a neon bulb that fires at a couple 
volts.   Zener diodes don't have much leakage current when biased below the 
knee, but might still have some rectification effects (although reverse 
biased diodes pretty much act as constant current sources). the 1n4678 is 
about 1.8V.  I don't know what their reverse capacitance characteristics 
are, other than strongly nonlinear.  I have heard of people using zeners 
(and power rectifiers for that matter) as varactors for low frequencies.




>  I
>figure that much voltage at a 50-ohm receiver input terminal to be
>something like S9+80dB.
>
>Thanks for your advice.
>
>73, Pete N4ZR
>The World HF Contest Station Database
>was updated 16 March 2005
>2925 contest stations at
>www.pvrc.org/WCSD/WCSDsearch.htm


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