[Amps] Measuring IMD

[Carl]

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 at audiosystemsgroup.com>
To: <amps at 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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