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Re: Topband: Spurs

To: topband@contesting.com
Subject: Re: Topband: Spurs
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 21 Apr 2011 08:54:16 -0700
List-post: <topband@contesting.com">mailto:topband@contesting.com>
On 4/20/2011 10:37 AM, Roger D Johnson wrote:
> I had always assumed that the 1820 spur was the second harmonic
> of a 910 kHz BC station.

What you are observing is simple intermodulation distortion. It can be 
created anywhere that the two RF signals are present at a level in a 
non-linear circuit.  You're hearing a mix between the fundamental of one 
of the signals and the second harmonic of the other.  Or the mix between 
the second harmonic of one and the third harmonic of the other.  One 
place this commonly occurs is in the output stage of one of the 
transmitters when something has "broken" in the output circuitry. It's 
also common for mixes to occur in wiring far from the transmitter, even 
in wiring near your receiver.

The link below is a simple Excel spreadsheet that I put together about 
seven years ago for computing the frequencies at which intermod products 
will be produced by two or three signals. It was originally intended for 
use with wireless mics, but the math is the same for signals of any 
frequency. No, I haven't done a spreadsheet to work the equation 
backwards. :)

http://audiosystemsgroup.com/intermod.xls

To use the spreadsheet, enter the suspect broadcast frequencies in the 
bold row under A, B, C, D, E, and F across the top.  Resulting intermod 
frequencies will show up in column B.

There's also a Quattro Pro version of this spreadsheet. Simply change 
.xls to .wb2 in the link.

73, Jim K9YC
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK

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