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Re: [Amps] Checking for IMD

To: amps@contesting.com
Subject: Re: [Amps] Checking for IMD
From: Steve Thompson <g8gsq@eltac.co.uk>
Date: Fri, 11 Aug 2006 17:25:28 +0100
List-post: <mailto:amps@contesting.com>

Martin AA6E wrote:

> You could check the boxes being sold by www.cleanrf.com as described in 
> the latest CQ.  They let you look at "linearity" on a scope.  My take is 
> that you will only catch gross non-linearity (overdrive), but you 
> wouldn't get quantitative IMD numbers with this approach.
Single Sideband Principles and Circuits includes a section on 
calculating IMD from measurements of input vs output voltage. I reckon 
this will include any effects from power supply sag.

For amplifiers where the transfer characteristic is linear, then going 
into gain compression it follows through to graphs of IMD against % flat 
topping on a two tone test. You can look at this by sampling the input 
and output and comparing on a X-Y display, or make two identical 
detectors and adjust the coupling so that the voltages detected on the 
input and output are identical at low level. A meter showing the 
difference will show you when non-linearity sets in. It won't be 
automatic readout of IMD to 5 digits, but if you learn how to use it, it 
will tell you most of what you need to know with voice or test modulation.

As a very rough rule of thumb, don't go beyond the point where Po/Pi has 
dropped by 20% from its low level value. Don't be surprised if this less 
output than you think you should run.

Steve
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