See below:
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Tom W8JI
> Sent: Sunday, August 13, 2006 3:03 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Checking for IMD
>
>
> > We are looking for an easy method here. Swapping side
> > bands does just that.
>
> It's certainly easy.
>
> > 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.
>
> Let's see.
> The frequency spread of IM is the difference of the two
> frequencies. So if we have speech (an ahhh won't work well
> to replicate real speech) we can consider the frequency
> spread of the lowest and highest frequencies.
>
> You'd get all of the third order opposite sideband products
> but only some of the 5th and higher.
To be a little more precise; the IM is the difference of the two frequencies
PLUS the high frequency and MINUS the low frequency for 3rd order products.
For 5th order it is 2 times the difference plus the high frequency and minus
the low frequency.
For 7th order it is 3 times the difference plus the high frequency and minus
the low frequency.
The actual transmitted frequencies are going to be offset by the carrier of
the transmitter.
Or if preferred it can be figured by: 3rd order= 2a-b, 2b-a.
5th order= 3a-2b, 3b-2a.
7th order= 4a-3b, 4b-3b.
Where a and b are actual transmit frequencies.
Here are the products for 300 Hz and 3000 HZ tones:
3rd order = 3000 - 300 = 2700 Hz. 3000 Hz plus 2700 Hz = 5.7 KHz
300 Hz minus 2700 Hz = -2.4 KHz
5th order = 3000 - 300 = 2700 Hz. 2700 x 2 = 5.4 KHz. 3000 Hz plus 5.4 KHz =
8.4 KHz.
300 Hz minus 5.4 KHz= -5.1 KHz.
7th order = 3000 - 300 = 2700 Hz. 2700 x 3 = 8.1 KHz. 3000 Hz plus 8.1 KHz =
11.1 KHz.
300 Hz minus 8.1 KHz = -7.8 KHz.
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.
Now as far as receiving the higher order products (5th, 7th etc.) using 300
Hz and 3000 Hz tones, you would need to tune farther out in frequency.
But with voice used rather than fixed tones, the reference frequencies can
be most any combination of tones between 300 and 3000 (assuming our
bandwidth is out to 3300 Hz).
As an example; 1000 Hz and 2000 Hz tones will produce 5th order products at
4000 Hz and -1000 Hz.
7th order products would fall at 5000 Hz and -2000 Hz.
So with the various voice frequencies involved we can easily see many
products (3rd through 7th and higher) on the opposite side band even though
they may not all be produced by the same fundamentals.
Again with voice there is going to be a variation in levels and it is not an
exact measurement but it can give us a pretty good idea if there are
problems. We are not so much interested in what IM frequencies of each order
are generated as we are with the dynamics of the transmitter which is the
whole reason for using voice rather than fixed tones.
It can be seen that with voice, the higher orders of IM products can be seen
close in to the carrier as well as out far away that we know as splatter.
But the measurement of those higher order products that a transmitter will
produce can be measured close in as well as measuring them out to the
maximum bandwidth produced.
>
> > 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.
>
> The question os from what. The receiver or transmitter? The
> test would be with the main signal INSIDE the roofing filter
> of most radios. RECEIVER IM3 dynamic range is between the
> 40dB area to 90's on most receivers, but the useful AGC
> measurement range using the S meter would be less because
> you couldn't go to noise floor with the S meter.
>
> With a better close-spaced receiver you'd be OK, assuming
> you used it properly. Not with some radios though!
> It would be a lot better to get the roofing filter into
> action. A sideband change doesn't do that on most radios.
>
> > 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.
>
> You just have to know if they are from the receiver or the
> transmitter.
>
Yes agreed.
But I wouldn't think that a receiver that produced noticeable IM products
from a single voice signal that was 40 or 50 db above the noise would be of
much use on the air either.
73
Gary K4FMX
> 73 Tom
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