> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Joe Subich, W4TV
> Sent: Monday, August 14, 2006 2:55 PM
> To: garyschafer@comcast.net; amps@contesting.com
> Subject: Re: [Amps] Checking for IMD
>
>
>
>
> Gary writes:
>
> > 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.
>
> Gary, there is one problem with your analysis. Most modern rigs
> have an 8 to 15 KHz wide filter in the front end (first IF).
> This filter is symmetric around the "carrier."
>
> When switching to the opposite sideband the fundamental tones
> are still inside that filter. However if one stays in USB
> and tunes down by 3 to 5 KHz, the fundamental frequencies
> are down on the slope (or outside) the front-end (roofing)
> filter and much less likely to generate additional IMD in the
> receiver. By putting the fundamental tones outside the front
> end filter, the receiver is also more sensitive to the IMD
> and can potentially be used without AGC allowing more accurate
> measurement.
>
> 73,
>
> ... Joe, W4TV
But if we tune outside the range of the roofing filter it is said that we
will miss some of the lower order close in IM products. :>)
I have some receivers with no roofing filters and they do fine as long as
you keep the signal levels down (attenuator) so as not to overload the
receiver.
Some receivers don't even allow you to turn off the agc.
If receiver IM performance is that great a problem how do we even manage to
hear anything intelligible on the bands. Seems there are some mighty strong
signals out there that our receivers have to deal with.
Even if there is some receiver IM generated it only needs to be a few db
below what we are looking for to be masked.
By the way most spectrum analyzers have filters with a rather broad shape
factor compared to our receivers and seem to do just fine.
73
Gary K4FMX
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