Why not directly detect the output RF with a linear rectifier / filter?
Also, why not use closely spaced tones (20-30 Hz) to make things easier
on the sound card?
This is basically what the PSKmeter
(http://www.ssiserver.com/info/pskmeter/) does. It's very cheap. The
question is whether the IMD numbers it delivers are useful or
believable. Probably you need a higher quality detector if you want to
go down to < -30 dB reliably.
Agreed that what you are measuring is the exciter plus amp, but at least
you get an upper limit. And, what we care about is what gets radiated,
not just what the amp does.
73 Martin AA6E
Bill Fuqua wrote:
> An oscillator, good mixer and a sound card running FFT should work.
> You want to mix the RF down to around 10 kHz ( center of the sound
> card frequency range) and observe sidebands around it.
>
> You can use a sound card to also produce the two tone test signal to
> insert into the SSB exciter.
>
> 73
> Bill wa4lav
>
>
>
> At 10:43 AM 8/11/2006 -0400, Martin AA6E wrote:
>> Bill,
>>
>> I've wondered about this, too. The lab method is to use a high-priced
>> spectrum analyzer, but couldn't you do something useful with a soundcard
>> based spectrum analyzer? You would need to have a linear detector and a
>> two-tone audio source. This would be similar to the IMD measurement you
>> get with a PSK31 monitor. The trouble is you need pretty good dynamic
>> range to measure a healthy system.
>>
>> 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.
>>
>> 73 Martin AA6E
>>
>> Bill Turner wrote:
>> > Is there a simple procedure one can use for checking the IMD of an
>> > amplifier? Frankly, it's something I have never tested myself, instead
>> > just keeping the amp tuned correctly and underdriving when using SSB.
>> > I've never gotten any complaints of splatter so I think it's under
>> > control. Since I do mostly RTTY and some CW it has not been an issue
>> > but I would like to be able to check it.
>> >
>> > Any recommended reading on the issue?
>> >
>> > Bill, W6WRT
>> > _______________________________________________
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>> >
>>
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