[Kenwood] SSB Filters: Need To Learn More

2003年11月17日 07:23:25 -0800

Thanks again, Alan!
You really explained it well enough for my old brain to comprehend :)
Now if you only had a way to teach it code. hi hi. But that's another
topic :)
thx
bob

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]
>> >>
>> >> _______________________________________________
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>> >
>> >--
>> >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
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