Tom Rauch wrote:
> High order products have little or nothing to do with low order products.
> Some systems go out "forever" in IMD bandwidth while others, with the same
> or even higher close-spaced IM levels, roll off quickly with high order
> products.
>
> I see this happen all the time.
>
> Another issue, doing a opposite sideband test, would be filter skirts.
> Filter skirts (or shape factor) are often not good enough just to do a "flip
> sideband" test.
>
> I also hear (and measure) rigs consistently that have non-symmetrical in IM
> distribution between upper and lower frequency products. This is
> particularly true with voice.
>
> 73 Tom
>
>
Hi Tom,
I agree that higher order products may have little to do with low order
products. My point is that in most transmitters if there are severe or
harmful high order products present there are almost always lower order
products present also. The low order products being easy to detect on
the opposite side band.
Filter slope can have some effect on measurement (as I had earlier
acknowledged) too. But most SSB filters are set with the carrier at
-20db or so down the skirt. Most filters now days do not have terrible
shape factors either. If in doubt the pass band tuning can be adjusted
to set the filter down a given amount on the skirt by just listening to
a carrier and adjusting.
We are not talking about precise measurements here but getting in the
ball park as to how bad a signal is by an easy method.
Any method that uses voice as a signal source is not going to be very
precise anyway.
Signals that have non-symmetric distribution of distortion products will
not usually be totally void of products on the other side. They may be
only down a few db at best from the other side I would guess.
In Stoners SSB handbook, he makes a statement that says something like
"side band suppression is directly proportional to linearity in the
amplifier". "So it would seem that amplifier linearity can be adjusted
by observing side band suppression".
"A quick check of a signals side band suppression will usually disclose
the presence of distortion products".
73
Gary K4FMX
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