[Kenwood] SSB Filters: Need To Learn More

[Alan Wolke]

Hello Bob,

I'm not sure of any filter-primer websites, but I can give you a couple of
basics here...

The first concept you need to understand is that any modulated RF signal
occupies a range of frequencies, not just one.   A SSB phone signal occupies a
range of frequencies that is equal to the audio frequencies transmitted.  For
example, if a given station's transmitted audio response goes from 200Hz (bass)
up to 2700Hz (treble), and he is transmitting on USB at 14.200MHz, the actual
frequency range that is transmitted is from 14.200.30 MHz up to 14.202.70 MHz.
The "width" of the signal in this example is 2400Hz.  Typical SSB phone signal
bandwidths are 2100-3000Hz.

The filter in the receiver is a circuit that controls what frequency range is
passed through the receiver, and what frequency range is blocked.  Kind of like
a coffee filter - which passes the brewed coffee but blocks the coffee
grounds.  A filter in your rx is used to pass the desired frequencies and block
the undesired ones. Of course, your rx has no way of knowing what you want to
pass through, so the selection is left to you.

Another way to think about the filter is this: They permit the receiver to be
more or less selective about what signal to pass through. The filter is what
allows you to hear the station you're tuned to, and not hear the stations that
are above and below (higher or lower in frequency) the station that you are
tuned to.

The larger the number on the filter (i.e. 12K vs 2.7K), the wider the filter
is.  In other words, the more frequencies it will pass.  In audio terms, a
12KHz filter will pass up to 12KHz (very high pitched signal), while a 2.7KHz
filter will pass up to 2700Hz (high pitched whistle).  Wider filters will
permit you to hear signals with more fidelity, such as the trebly details of
S's and T's, because these details are in the upper audio frequency range.  The
downside to using the wide filters is that you may also begin to hear
noise/interference from adjacent signals because the wider filter will "see"
the edges of those signals too.

The narrower filters will improve the selectivity of the receiver by blocking
more of the adjacent signals.  It may also tend to begin to filter some of the
desired signal. You have noticed this as a change in tone - narrow filters make
it sound like you turned the treble down because the highest audio frequencies
of the signal are getting blocked.

Your TS-850 has two banks of selectable filters.  These filters (from your
standpoint) appear in series.  The 8.83MHZ IF filters appear first, then the
455KHz IF filters are next.  In general, simply pick the combination of filters
that give you the most pleasing response for the given conditions.  When the
band is very crowded, especially with strong adjacent signals, you'll find the
best results by selecting a narrower filter in the 8.83MHz IF - so that you
block the strong adjacent signals before they can cause distortion/noise/AGC
action in the subsequent circuits.   If you select the 2.7KHz filter in both
the 8.83 and 455 banks, the overall response will be closer to 2400Hz, which
will be the most selective combination with the stock filters.

The TS-850 has another very nice feature that you can use in conjuction with
your filter selection.  This is the Slope Tune feature.  This feature allows
you to individually "slide" the upper and lower cutoff frequencies of the
filter(s) up or down.  This allows you to tailor the position and width of the
filter to "fine-tune" its characteristics.  For example, if the 2.7KHz filter
isn't quite narrow enough to remove some interference at the high end (treble),
then the slope tune can be used to pull the upper cutoff frequency down a
little lower to block the offending interference.

I hope this gives you a little more understanding on the filters for SSB use.
There's a LOT more to this subject, so don't be afraid to ask!

73,
Alan W2AEW

Bob Snelgrove wrote:

> Hi Gang
>
> I'm still confused about what filters do and how they work.  Sometimes they
> seem to make or break whether I can hear and other times seem like nothing
> more than a fine "tone control" Any websites that have a nice simple primer
> on how filters work?
>
> I'm mostly listening to 40 and 20 meters, SSB.  My TS-850 has the 8.83
> switch: (off, 6K, 2.7K.) 455 switch: (12k, 6K, and 2.7K) ( no off)
>
> 73's
>
> bob
>
> - - -
>
> Your moderator for this list is:
> Larry Wilson KE1HZ kenwood-owner@mailman.qth.net
>
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--
Alan Wolke, W2AEW
"i usta cuDn't speL enjinere, noW i ar wuN"