[Amps] Checking for IMD

[Tony King - W4ZT]

Just swapping sidebands will not take you out where you need to be to 
look for the IM products. You need to be looking out about 4.5 KHz.

Rich, Tom and others discussed the measurement of IMD in a previous 
thread. I summarized some of their comments and included a method that 
Rich suggested for measuring AVERAGE IMD with another receiver and a 
calibrated step attenuator at <http://gs35b.com/imd.html>. Of course 
this method can't be used to quote specific numbers for the 3rd and 5th 
order products but produces an overall look at your system performance.

73, Tony W4ZT

Gary Schafer wrote:
> Simplest is to talk into the mike and use a second receiver and flip to the
> opposite side band and compare levels. An attenuator of known calibration
> ahead of the second receiver will get you in the ball park for overall IMD.
> 
> If using two tones, use the second receiver as your spectrum analyzer. Tune
> it to where the 3rd and 5th order frequencies should fall using the cw
> filter. That will separate the IM products from the fundamentals. Then read
> the difference in levels with the calibrated attenuator. This will give you
> just as good readings as a spectrum analyzer will.
> You must take care to not overload the receiver and make sure the signal
> only arrives to the receiver via the attenuator and not stray pickup.
> It is best to check the attenuation of the attenuator / receiver setup at
> levels that you will be measuring.
> 
> An HP selective level meter can be used to do the same thing. They can be
> picked up for around $150.00. Works very well. It gives you a direct readout
> in db of level difference.
> 
> 73
> Gary  K4FMX
> 
> 
>> -----Original Message-----
>> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
>> Behalf Of Bill Turner
>> Sent: Friday, August 11, 2006 9:24 AM
>> To: amps at contesting.com
>> Subject: [Amps] Checking for IMD
>>
>> Is there a simple procedure one can use for checking the IMD of an
>> amplifier? Frankly, it's something I have never tested myself, instead
>> just keeping the amp tuned correctly and underdriving when using SSB.
>> I've never gotten any complaints of splatter so I think it's under
>> control. Since I do mostly RTTY and some CW it has not been an issue
>> but I would like to be able to check it.
>>
>> Any recommended reading on the issue?
>>
>> Bill, W6WRT
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> 
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