[Kenwood] SSB Filters: Need To Learn More

2003年11月16日 21:41:45 -0800

Alan,
Thanks, very well put. So, when you listen to SSB, your are listening to a
range of frequencies around the one you are tuned to? That is why you hear
"bleed over" from nearby frequencies because their audio range is
overlapping "bleeding" into yours?
The trade off is loss of "fidelity" when using a narrower filter, but
separation of who you want to hear from who you don't?
So what filter would I want next for SSB work and which I.F slot would it
go in?
thx!
bob
At 09:56 PM 11/16/2003 -0500, you wrote:
>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 [email protected]
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>>--
>Alan Wolke, W2AEW
>"i usta cuDn't speL enjinere, noW i ar wuN"
>>>