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Topband: "Thinking out loud" - Something else to Think About

To: "topband@contesting.com" <topband@contesting.com>
Subject: Topband: "Thinking out loud" - Something else to Think About
From: Dave Bowker via Topband <topband@contesting.com>
Reply-to: Dave Bowker <david.bowker@yahoo.com>
Date: Fri, 23 Dec 2016 20:34:39 +0000 (UTC)
List-post: <topband@contesting.com">mailto:topband@contesting.com>
Crystal filters are nonlinear passive devices by natureof the characteristics 
of the crystal(s).  The motional characteristic ofa quartz crystal is a 
function of the level of the signal applied to it, knownas drive level 
dependence or (DLD) in engineering terms.  Crystal filters can 
contributeinter-modulation when a strong out-of-pass-band signal is present, 
whether theundesired signal is immediately adjacent to the pass band frequency 
or removedsome distance in frequency from it.

Another concern of placing a narrow bandwidth filter inthe RX input is the 
effect it has upon the receiver impulse noiseblanker.  A narrow bandwidth 
filter stretches the noise pulse bandwidth bymany orders of magnitude and can 
render the NB ineffective and contributes IMDproducts.  For this reason, in 
modern day receivers, the noise blankersampling/processing is placed ahead of 
the narrow band IF filters.  

Placing a narrow bandwidth filter atthe input of the RX will cause IMD if the 
NB is in use.
In a contest operationthe input filter will be subjected to MULTIPLE nearby 
strong signals that arenot in the pass band of the filter.  This condition is 
accentuated in amulti-multi operation.
Dave, K1FKFort Kent, ME
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