A softrock and powersdr will do the job nicely for under $20. As long as
the sound card is not clipping and you don't have stray RF, the measurements
will be very accurate.
The bigger challenge is getting a clean 2-tone source for the TX side.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Karl-Arne Markström
Sent: Saturday, November 02, 2013 3:07 AM
To: jim@audiosystemsgroup.com ; amps@contesting.com
Subject: Re: [Amps] Measuring IMD
A quite little known IMD test set is the Racal 9058 Selective Analyser.
This is a small self-contained unit consisting of a two-tone generator and a
manually tuned direct conversion
spectrum analyser covering 1 to 100 MHz.
It was designed for quick field checks of manpack and vehicle SSB sets when
carrying a complete IMD measuring
setup was considered impractical.
http://www.milcom.co.nz/items/item024.html
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från: jim@audiosystemsgroup.com
Datum: 2013-11-02 08:45
Till: <amps@contesting.com>
Ärende: 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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