[AMPS] Two-tone audio generator for testing IMD?

[Jerry Flanders]

At 07:33 PM 12/16/2001 -0500, Tom Rauch wrote:


>Date sent:              Sun, 16 Dec 2001 17:09:24 -0500
>To:                     amps@contesting.com
>From:                   Jerry Flanders <jflanders2@home.com>
>Subject:                [AMPS] Two-tone audio generator for testing IMD?
>
> >
> > Is software available for the PC sound card to generate two audio
> > tones for doing some IMD tests on this 5CX3000A amp that is slowly
> > taking shape here?
>
>You can not, unless you have an exceptional exciter, dump two
>audio tones in the audio and measure the amplifier.
>
>If your exciter is 60 or more dB down for all IMD and spurious on
>two-tone tests, then you can use it (provided the amp isn't
>expected to be anywhere near that good).

If I can measure the baseline (the exciter) first, the additional IMD 
generated in the amp shows up as an increase from that baseline. Subtract 
the baseline and I have a number that represents the amps distortion. I 
only need relative nrs anyway as I change some operating parameters on the 
final. If the increase in IMD is not enough to show in the (detected audio) 
spectrum, it is probably low enough for me to ignore - test completed 
successfully.

The above procedure is widely used in analytical labs to subtract out 
interfering spectra. It works there.

I don't need numbers rigorous enough to submit to FCC on an application for 
approval.

>You certainly can't measure the IMD on anything but a good
>selective level meter or spectrum analyzer...looking at the audio of
>a receiver is a total waste of time.

Analyzer2000 - DSP spectral anaysis on the PC soundcard. Looks pretty good. 
Seen it?


>Keep in mind most two-tone tests can't show most IMD caused by
>PS or grid voltage regulation problems.
>

Understood. Thanks for your comments.

Jerry W4UK


>
>73, Tom W8JI
>W8JI@contesting.com


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