Larry,
You are 100% right if an absolute accurate measurement is needed. But if
you simply want to know if you are in the 20's or the 30's, then the method
I describe is fully adequate to the task.
I just completed protos of a crystal modified Wentzel design with some
tweaks added by Bill Carver that is for high DR3 range receiver testing. We
are talking about IMD levels for the entire system of sub -110dbc. Right on
all counts - LPFs needed - ferrites needed - lots of added return loss via
pads sprinkled. I'm fully on board your claim here. But just not for Joe's
basic ham shack needs. For the basic "is my tx/amp combo generating crap"
kind of test, the 10dbc better rule of thumb for the sources is good enough
and the modern sound card without being overdriven is fully up to the task.
The problem as I view it is that guys are not measuring at all. Or if they
are it's with something like an SB610 or whatever. To my thinking, better
to have some minimal indication even if it's "gross" in resolution than none
at all. This solution provides that - it's better than the SB610 at least.
And given the number of "I've never built anything before and don't really
know what I'm doing" guys comments we see on the boards, this has got to be
a step in the right direction. Even if it's filled with significant
shortcomings.
73/jeff/ac0c
--------------------------------------------------
From: "Larry" <larry@w7iuv.com>
Sent: Sunday, May 23, 2010 1:51 PM
To: "Jeff Blaine AC0C" <keepwalking188@yahoo.com>; "AMPS"
<amps@contesting.com>
Subject: Re: [Amps] How to read the 3-500Z spec sheet?
>
> On 23-May-10 10:45 AM, Jeff Blaine AC0C wrote:
>>
>> Of course, a real IMD test requires a 2 -tone source and you can download
>> those applications on the web free of charge.
>
> Wrong answer for amplifier testing. While you can approximate proper
> "two-tone" testing with a sound card tone source when evaluating a
> transceiver, a LOT more is required for accurate and technically correct
> amplifier testing.
>
> To do accurate IMD (and more correctly OIP3) testing on an amp, you NEED
> two separate clean signal sources. This means two exciters that are known
> to be very clean and free from harmonics (much greater than 60 dB down)
> plus a high power combiner and usually a set of bandpass filters at each
> of the tone frequencies. Plus, just to keep everything happy, a bunch of
> attenuators sprinkled around the bench.
>
> Anything less and you are only testing the source(s) and not the amp. Some
> time back, W8JI detail the correct test setup for amplifier testing and I
> think it's on his web site someplace. Anything less and you are just
> fooling your self.
>
> 73,
>
> Larry - W7IUV
> DN07dg - central WA
> http://w7iuv.com
>
>
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