[Amps] Checking for IMD

[Gary Schafer]

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Tom W8JI
> Sent: Monday, August 14, 2006 12:09 PM
> To: amps at contesting.com
> Subject: Re: [Amps] Checking for IMD
> 
> > Think about that again. If you are listening to upper side
> > band with a 3 KHz
> > filter and you tune down 3 KHz in frequency with the dial
> > you have moved the
> > band pass the same amount lower in frequency as you would
> > if you switched
> > side bands with the side band switch. The only difference
> > is that the
> > carrier will be on the other side of the filter.
> 
> 1.) The passband of the filter is 3000Hz.
> 
> 2.) The offset is 300Hz from the carrier.
> 
> 3.) The sideband switch now moves the start of the receiver
> filter 600Hz lower than the start of the transmitter filter.

Yes you are right Tom, my apologies. I forgot to add the offsets in when
tuning. An equivalent to changing side bands would be to tune down 3.6 KHz
rather than 3 KHz.

> We miss the low frequency tone mixes that are often
> problematic, and we also miss the high frequency tone mixes
> that are close spaced because we never checked above the TX
> signal.

The transmitter doesn't really care what frequencies mix to cause the IM
products. A particular frequency IM product can come from many combinations
of fundamentals. Yes some transmitter's audio may be emphasized at certain
frequencies where the products may fall outside of where we are looking.

But again, any of this is not meant to be precise measurements.

> 
> 4.) If I moved the receiver down 3000Hz and kept the same
> sideband, that would not happen. The upper edge of the
> receiver filter would hit the lower edge of the TX filter.
> 
> I don't see the big deal about moving the dial. Someone
> could check by moving up the SSB filter BW and down. .
> 
> I'm not saying the test is useless, just that anyone
> proposing or using a test should know what the test is
> measuring and what it isn't measuring. It's always a good
> idea to think about a new test procedure and understanding
> how it all works and explaining limitations and what it
> actually measures before going public.This is a pretty poor
> method for over the air measurements because of noise
> bandwidth problems.

It is no poorer than tuning the receiver to another frequency to listen for
IM products. The same kind of noise and interference will exist on other
frequencies too. 

My contention of using the other side band is that you are always moving the
receiver pass band the same amount every time you switch side bands as
opposed to tuning around. You can quickly switch back and forth to compare
levels of wanted and unwanted signals. It give you pretty much the same
information as you get by tuning but it is very repeatable when checking
various signals on the band. You can get a feel for what the signal levels
should be on the opposite side band with some practice.

73
Gary  K4FMX

> 
> 73 Tom
> 
> 
> 
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