[Amps] Checking for IMD

[Gary Schafer]

We are looking for an easy method here. Swapping side bands does just that.
4.5 KHz doesn't get you out far enough to get all products for all voice
frequencies either, but again we are looking for an easy way to get a
ballpark idea. 

Look at what swapping side bands does: It is a fixed repeatable move in
frequency from the main signal and the attenuation between those two
frequencies is a minimum of 40 db. Anything that is heard on the opposite
side band is going to be pretty much distortion products.

Transmitter side band filters are generally set where the carrier is 20 db
down or more. The receiver is usually set the same. That gives a minimum of
40 db of attenuation, at the receiver, of the lowest frequency of the wanted
side band that could be transmitted. 

So anything you hear on the other side that is less than 40 db below the
wanted side band level is going to be distortion products. That is for the
lowest voice frequencies that are transmitted. As the voice frequency is
higher so is isolation on the other side band greater than 40 db.
Not a precise measurement by any means but neither is using voice very
precise. But it is easy and repeatable.

IM products get generated on both side bands. There are always 2 sets of
products. Example: 300 Hz and 3 KHz frequencies produce 3rd order products
at 5.7 KHz above the carrier and also at -2.4 KHz which is on the other side
band and within the band bass of the receiver set to the opposite side band.
Both the 5.7 KHz and the -2.4 KHz signals will be at the same levels.

Listening on the opposite side band therefore separates the wanted signal
from the IM products. Voice frequencies vary all over the place so the
chances of IM products falling within the voice bandwidth are very high for
3rd and even for 5th and 7th order products.

Still don't believe it? Listen to some of the commercial SSB broadcast
stations. The opposite side band is very clean on most. Then listen to some
of the opposite side bands on the ham bands.

Always take care not to overload the receiver.

73
Gary  K4FMX



> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Tony King - W4ZT
> Sent: Saturday, August 12, 2006 7:43 PM
> To: amps at contesting.com
> Subject: Re: [Amps] Checking for IMD
> 
> 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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