[Amps] IMD test question

[Jim W7RY]

Larry.. Why not just use one transmitter with two audio tones 2 KHz apart? 
Can you expound on this a bit?


73
Jim W7RY


-----Original Message----- 
From: Larry Benko
Sent: Wednesday, August 01, 2012 1:36 PM
To: amps at contesting.com
Subject: Re: [Amps] IMD test question

Bill,

I have made amp IMD measurements with the circuit shown at
http://www.w0qe.com/Technical_Topics/imd_testing_of_amplifiers.html
which can measure any amp currently made.

As I'm sure you are aware there are not any exciters made today with IMD
in the -60dBc range.  This is why I did the test with 2 exciters.  Also
you need to check the final input to the amp to be sure that any IMD is
due to the amp and not the driving circuitry. An amp which has an
impedance which varies throughout the RF cycle (any non class A amp) can
cause the exciter to produce IMD which may improve or reduce the IMD of
the amp.  The circuit I use can drive a 2 3CX800 amp with IMD levels
-70dBC so that the amp IMD is easy to see.  Also theory and tests I have
done say that the IMD doesn't change for carrier separations of 1kHz
thru 100kHz.  In your case you might want to use 10kHz based on the
receiver you are using.

Have fun.

73,
Larry, W0QE

On 8/1/2012 2:15 PM, Bill, W6WRT wrote:
> Is there a way for a homebrewer to accurately measure IMD without 
> expensive
> equipment? I'm thinking of doing two-tones at full power into a dummy load 
> while
> listening on a second RX nearby. How does one accurately analyze what one 
> hears?
>
> 73, Bill W6WRT
>
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