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.
Tom's points are well taken regarding the roofing filters and
suitability or non-suitability of some receivers for any of this.
As I said once before, use of the "S" meter was to compare signals only
and not to gain absolute values or determine peak or average readings. I
think the confusion about peak versus average came from the discussion
of "determining average IMD using another receiver and 'aaaah'". Agreed,
it may not be average, but it might give us a quantity to compare which
can be measured (using the step attenuator). NONE of this should be done
on the air because of the effects other things like QRM, QRN, QSB, etc.
73, Tony W4ZT
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