Dont pull the ARRL, and used by some manufacturers, crap of cherry picking
the audio frequencies to get the best results for reviews.
Running a wide band noise spectrum that covers the radios filter specs is
possible with digital gear and has become the current standard since it
fully exercises the transmitter. I doubt the K3 would pass scrutiny with
that method.
The Flex 5000 has a very nice analyzer.
Carl
KM1H
.
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <amps@contesting.com>
Sent: Saturday, November 02, 2013 2:45 AM
Subject: Re: [Amps] Measuring IMD
On 11/1/2013 9:31 PM, Bill Turner wrote:
Aside from buying a very expensive spectrum analyzer, is there a way for
the
average ham to do it in his shack?
Yes, with some effort and ingenuity. First, you need two clean audio
oscillators to modulate the transmitter, and a way to cleanly combine
them. You could do that with a couple of vintage HP oscillators and a
decent audio mixer as simple as a Mackie 1202. You feed that to the
transmitter audio input, making sure that you don't overload it. Vintage
HP oscillators are common at hamfest flea markets for cheap. The K3 has a
2-tone generator built in, you simply activate it from the menu.
Second, you need a spectrum analyzer. The Rigol unit (don't recall model
number, but I have one), for about $1400 will get you there, and do a lot
of other useful stuff. The DG8SAQ VNWA is a fine network analyzer, and can
also do spectrum analysis, costs about $750 shipped to the US, and runs on
a Windoze computer USB port. The dynamic range of this unit is limited in
spectrum analyzer mode, I don't remember if it's good enough for this.
There are LOTS of audio FFT analyzers that run in Windoze, take the audio
out of an RX and give you the audio spectrum. That will give you magnitude
of the two tones, the magnitude of difference frequency, and it will give
you the magnitude of the sum frequency if it is within the audio bandwidth
of the RX. You can even do this with many sound editing programs like
Audicity and WavePad, which are free or cheap. For best quality, they
should use a decent USB sound card for I/O. A Tascam USB card that sells
for about $125 is plenty good enough.
The Elecraft P3 spectrum display can be tuned to almost any IF, including
the output of a TX, and can be set for any scan width between 2 kHz and
200 kHz, so it could also be a direct detector (small piece of wire for an
antenna), or hooked up to the IF of a good RX. It's very flexible, wide
range of scales and sensitivity. The one thing it lacks is a cursor that
reads the amplitude at a frequency -- you've got to interpolate from the
vertical axis.
HP gear of various sorts also shows up on the auction sites. I paid about
$1800 for an 8590D tht has a frequency calibration issue, but other wise
works fine. Before that, I owned a tube version with modular plug-ins and
CRT display that was a real arm-stretcher, but worked fine and cost about
half as much.
Also, ask around your local ham club -- you may be surprised by what test
gear lurks that you can borrow. I'm happy to loan my stuff to locals.
So depending on what you have laying around and what you might use for
other things, IMD measurements can be made for no more than a monthly
mortgage payment!
73, Jim K9YC
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