[TowerTalk] AM Filter options

[n4kg@juno.com]

The interstation beat note on AM is equal to the channel spacing,
10 KHz (not 5).  A good 6 pole (or more) filter with 8 to 10 KHz
bandwidth (+/- 4 to 5 KHz) will eliminate interference from an
adjacent carrier and typical speech modulation.  

A DSP brickwall low pass audio filter set to 4 to 5 KHz will further
reduce interference from adjacent channel modulation.  BUT,
remember that to prevent desensitization from the adjacent 
channel (due to the undesired channel station capturing the AGC)
it is necessary to have a good IF filter in place.

A receiving loop (with a sharp null) is a excellent idea as others
have already suggested.

de  Tom  N4KG  (receiver design engineer)

# # # # # # # # 

On Sun, 30 Apr 2000 20:02:42 -0500 "Brad Rehm" <bradrehm@texas.net>
writes:
> 
> 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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