[Kenwood] SSB Filters: Need To Learn More
Bob Snelgrove
[email protected]
2003年11月17日 18:53:43 -0800
Hi Alan,
Well, I guess I'm confused, then :) Maybe I need to see a picture of a
filter/curve/slope/skirt?
I can picture the filter sliding up or down in frequency (shifting?),
retaining its shape and width, and I can picture the filter narrrowing or
widening as you have described earlier, but I still don't see what the low
cut and hi cut do? I assume they work independently from each other and
that is their advantage of an IF shift only adjustment?
On a separate note, could you explain why there is a 8.83 and a 455 filter?
Do most radios have 2 sets like the 850?
Very intersting stuff. Thanks again,
bob
ps: enjoyed your web site
pss: If this is off topic, I can take it privately
At 08:26 PM 11/17/2003 -0500, you wrote:
>Hi Bob,
>>The slope tune does not change the curve of the filter, or slope of the
skirt. It
>simply slides the skirt higher or lower in frequency, without changing its
slope.
>The end result is to widen or narrow the filter, and/or slide its "center"
>frequency up or down.
>>(This is actually done by changing the local oscillator frequencies used
in the IF
>mixers, but that ANOTHER story...)
>>So, the slope tune is essentially a way to adjust your filters a little.
>>And yes, certainly, if you narrow the filter too much, you'll eliminate
enough of
>the desired signal to the point where it isn't understandable either. I
don't
>know if the slope tune on the 850 will allow you to take it to that
extreme. I'm
>sure an 850 owner can chime in and tell you that for sure. Based on your
comment,
>it sounds like you can essentially narrow your effective passband to nil.
>>Bottom line is, pick the combination of filter selection and slope tune
adjustment
>that gives you the most pleasant and understandable result for the
conditions at
>hand. Once you are comfortable with what each of the selections and
adjustments
>will do for you, you'll be able to easily adjust and compensate for most
QRM/N
>situations as they arise.
>>Bob Snelgrove wrote:
>>> Hi Alan,
>>>> A few more questions?
>>>> My slope filters "steepen" the curve of the filter, making it more of a
>> "brick wall" filter than a "bell curve"?
>>>> If I cut the highs and lows completely off with the slope filters, I hear
>> nothing. What have I really done at that point?
>>>> At 10:07 AM 11/17/2003 -0500, you wrote:
>> >Hello Bob,
>> >
>> >Exactly right. When you are listening to SSB, the filter determines which
>> >range of frequencies are passed through. This is called the band-pass, or
>> >passband of the filter. Outside of this range is called the stopband
of the
>> >filter.
>> >
>> >Say you are tuned to 14.200.00MHz, listening on USB with a 2.7kHz filter.
>> With
>> >this configuration, you're rx is basically passing the energy contained in
>> the
>> >frequency range of 14.200.10 to 14.202.80 MHz or so. Notice that the
filter
>> >low-freq cutoff generally is not at the carrier (I've illustrated it above
>> as
>> >100Hz from the carrier). Some rigs will allow you to shift the upper and
>> lower
>> >cutoff frequencies (like yours with the slope tuning, others with IF
>> shift) to
>> >move the "position" of the filter with respect to the carrier. So,
take my
>> >example above as just an example, and not gospel.
>> >
>> >The "bleed over" can be caused by one of two things. First, the adjacent
>> >signal has frequency components that land inside your passband and you
hear
>> >them. Second, the adjacent signal doesn't have energy specifically inside
>> your
>> >passband, but is very strong and your filter isn't good enough to reject
>> it.
>> >Note that filters aren't perfect "brick wall" devices. A "brick-wall"
>> device
>> >would perfectly pass signals in its passband, and perfectly reject signals
>> >outside of the passband. This ideal situation doesn't happen. Real
filters
>> >will pass signals in their passband, and attenuate signals that are
>> outside the
>> >passband. The transition is somewhat gradual. Signals that are just
>> outside
>> >the passband aren't attenuated as much as signals that are far outside the
>> >passband. Using the example above, energy at 14.203.00 will be
>> attenuated, but
>> >not attenuated as much as energy at 14.210.00MHz. The "slope" of the
>> >attenuation at the edges of the filter is something that some of the more
>> >expensive filters improve on. The "edges" of the filter are often called
>> the
>> >skirts of the filter.
>> >
>> >Your understanding of the trade-off between fidelity and selectivity is
>> exactly
>> >right.
>> >
>> >I'm not an expert on the 850, so someone else here may be able to give
you a
>> >recommendation on an auxiliary filter to put in the 850. However, before
>> you
>> >spend the money on that, be sure to get comfortable with the filters you
>> have,
>> >and especially comfortable with the Slope Tune control to move the skirts
>> >around. You may find that a narrower SSB filter might not be necessary.
>> >
>> >I know that InRad makes some very nice filters. Their filters generally
>> have
>> >better stop-band characteristics, and steeper skirts.
>> >
>> >Hope this helps!
>> >
>> >73, Alan W2AEW
>> >
>> >Quoting Bob Snelgrove <[email protected]>:
>> >
>> >> 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]
>> >> >>
>> >> >> _______________________________________________
>> >> >> Kenwood mailing list
>> >> >> [email protected]
>> >> >> http://mailman.qth.net/mailman/listinfo/kenwood
>> >> >
>> >> >--
>> >> >Alan Wolke, W2AEW
>> >> >"i usta cuDn't speL enjinere, noW i ar wuN"
>> >> >
>> >> >
>> >> >
>> >>
>> >>
>> >> - - -
>> >>
>> >> Your moderator for this list is:
>> >> Larry Wilson KE1HZ [email protected]
>> >>
>> >> _______________________________________________
>> >> Kenwood mailing list
>> >> [email protected]
>> >> http://mailman.qth.net/mailman/listinfo/kenwood
>> >>
>> >
>> >
>> >
>> >
>> >-------------------------------------------------
>> >This mail sent through the BlastNet Webmail
>> >
>>--
>Alan Wolke, W2AEW
>"i usta cuDn't speL enjinere, noW i ar wuN"
>>>