See below.
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Tony King - W4ZT
> Sent: Monday, August 14, 2006 9:43 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Checking for IMD
>
> Gary Schafer wrote:
> > See below:
> <snip>
> >
> > So from the above the IM products from 300 and 3000 Hz tones are:
> > 3rd = 5.7 KHz and -2.4 KHz.
> > 5th = 8.4 KHz and -5.1 KHz.
> > 7th = 11.1 KHz and -7.8 KHz.
> >
> > All of the negative products represent the products that fall on the
> > opposite side band.
> >
> > So we can see that to receive all the positive 3rd order products the
> > receiver would need to be tuned to the very next channel. Switching side
> > bands does that exactly the same as if you tuned the receiver to the
> next
> > channel by turning the dial the amount equal to the filter bandwidth.
> But
> > instead of receiving the positive products you receive the negative
> > products. Same result.
> > <snip>
>
> Gary, I think if you re-examine this, you will find that the products
> you computed fall on both sides of the carrier frequency. Switching
> sidebands does not move you to the very next channel. Instead, it puts
> you on the other side of the carrier frequency on the same channel where
> the IM products are offset by the same amount in the opposite direction.
> That places the majority of those products outside the passband of your
> sideband filter on BOTH sidebands. You would have to TUNE off frequency
> on either side to get to them. That is what I stated before. Simply
> switching sidebands doesn't do the intended move out to the products you
> are interested in.
Hi Tony,
Switching side bands IS the same as tuning your receiver the width of your
filter. If you have a 3 KHz wide filter and listening to the upper side band
and you tune down 3 KHz with the tuning dial, you have moved the receiver
pass band the same amount as when you switch to lower side band and leave
the receiver dial alone. Forget about which side of the pass band the
carrier is on, it doesn't matter. What matters is where the pass band of the
filter lies with respect to the spectrum being received.
The two 3rd order IM products generated by a transmitter with two tones are
exactly the same strength for the upper and lower products. It is just like
an AM signal. With the receiver set for upper side band you hear what I show
as positive products. With the receiver set to lower side band you hear what
I show as negative products. The reason for the difference in frequency is
the offset the carrier provides so one side will contain a frequency
spectrum slightly different from the other because of it. If you selected
the opposite side band on the transmitter the positive and negative numbers
would be reversed.
But the difference doesn't matter because with a voice signal we are not
dealing with just two individual tones. We have a wide spectrum of
frequencies that have the opportunity to cause IM products. Many many
different IM frequency products will fall into the pass band close in even
5th and 7th order products as I showed before.
We can say that this is not a valid method of checking because we are
relying on different audio frequencies to generate the IM products and I
agree that it is not an absolute. But we are trying to use voice that has
varying frequencies and varying levels instead of fixed tones at fixed
levels.
What I am saying is that the opposite side band will give you as good a
sampling of unwanted products as will tuning the receiver to some other
frequency while trying to use voice as the modulation method. With either
scheme you will not see all the resultant products at one time but I think
you will get a good enough sampling to get a rough idea of how bad things
are. That is about as close as you are going to get to any quantifiable data
with voice.
73
Gary K4FMX
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