[TowerTalk] AM Filter options

[Brad Rehm]

David,

We're going to need to know what kind of receiver you have in order to make
any definitive suggestions about filtering.  Some receivers have provisions
for adding crystal and other kinds of filters and others don't.  If yours
accepts plug-in filters, you can almost certainly buy them from ham radio
stores or from stores which specialize in short wave receivers and carry the
brand you've got.

Filtering will largely remove the annoying 5 kHz heterodyne you hear when
the signal from a distant AM station is nearly as strong as that of a local
station.  This makes listening to either station difficult.  Filtering will
remove the whistle, but it will also make the audio sound muffled, because
high-frequency sounds are suppressed along with the heterodyne.

If your receiver doesn't accept optional filters, your choices boil down to
two: add an external DSP (Digital Signal Processor) or buy a new radio.  DSP
units are available from companies such as Timewave Technologies.  A few
others have offered these products, but their ads seem to have disappeared
from the ham radio magazines.  This is probably due to the radio
manufacturers including DSP capabilites in the new generation of radios.

External DSP units cost $300-$400 and connect to the speaker terminals of
the receiver.  They digitize and process the audio, leaving out the unwanted
noises.  The results of the process can be amazing.

You can contact Timewave at http://www.timewave.com

73,
Brad KV5V

> I was wondering what options would be available to filter out one adjacent
> AM broadcast radio station from another.  In this case reception from one
> about 60 miles away is desired, but a second one on a close spacing is
> causing some overload of the receiver.  Frequencies are both around 1200
> KHz.  Thanks for any ideas.
>



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