Spectrum Lab will do all you want and more.
You can find it at www.qsl.net/dl4yhf
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of Jim Barber
[audioguy@q.com]
Sent: Saturday, November 02, 2013 3:50 PM
To: amps@contesting.com
Subject: Re: [Amps] Measuring IMD
Generating a two-tone test WAV file isn't especially difficult.
If you have a PC with a semi-pro or pro external sound device, you can
get 114dB or so SNR and very low distortion. The run-of-the-mill PC
sound chip as implemented on the motherboard is usually good for 80dB or
so real SNR, which should be still usable.
I'm up to my ears in work at the moment, but if anyone has any thoughts
on what the file should "look" like (technically) I'll try to get around
to generating a few with different bit depths and sample rates. Reply
here or email as convenient.
73,
Jim N7CXI
On 11/2/2013 12:43 PM, Jeff Blaine wrote:
> 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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