BFO freq how far down SSB filter skirt?
Tom Holden
the skirt of the SSB filter. Pete, KE9OA, says -24dB.
1. What is the reasoning behind these choices?
2. How critical is it that the BFO be so positioned?
3. Wouldn't the shape factor have some bearing on where it should be?
4. Is linear interpolation between the -6 and -60dB points accurate enough
to determine the BFO freq?
TIA,
Tom
Rick Karlquist N6RK
suppression by means of the filter. Most balanced
modulators are not able to get good enough
carrier suppression when operated at low enough
levels to have good IMD. This is main reason
why phasing rigs went out of fashion.
Rick N6RK
"Tom Holden" <holden...@sympatico.ca> wrote in message
news:6CDUb.7045$ZN1.5...@news20.bellglobal.com...
Gary Schafer
20 db down is where they are usually set. It is not too critical
though. It does provide that much carrier suppression in addition.
The best way is to set it where it sounds the best. Too far down and
you get better opposite side band suppression but it also cuts the low
frequency response of the wanted side band.
Yes it will depend on the shape factor of the filter as to where the
best point on the slope is.
You can easily measure 20 db down or how far you want it by just
measuring the output voltage level from what you get at the center of
the filter. Measuring is much easier than interpolating the correct
point as only a few tenths of a cycle change on the slope will make a
large change in db down.
73
Gary K4FMX
Wes Stewart
<holden...@sympatico.ca> wrote:
Perhaps an example is the best way to explain this. Assume that you
have a nominal 9 MHz crystal filter, with a 2.0 KHz BW at the -6 dB
points.
The -6 dB frequencies are thus 8.999 and 9.001 MHz. Forgetting the
bandwidth restrictions on the incoming signal, let's say you would
like the recovered audio signal to be 6 dB down at 500 and 2500 Hz
respectively.
For an upper sideband signal, the BFO should be 500 Hz below the lower
cutoff frequency, i.e. 8.9985 MHz. For lower sideband, the BFO should
be 500 Hz above the upper cutoff frequency, 9.0015 MHz.
Where these two frequencies land on the skirts is dependent on the
shape factor. These "rules" that say the BFO should be so many dB
down are really "rules of thumb" that work with typical shape factors.
Wes Stewart N7WS
Steve Nosko
>
> |ARRL Handbook says that the BFO frequency should be at the -20dB point
down
> |the skirt of the SSB filter. Pete, KE9OA, says -24dB.
> |1. What is the reasoning behind these choices?
> |2. How critical is it that the BFO be so positioned?
> |3. Wouldn't the shape factor have some bearing on where it should be?
I think you understand. The filter is placed in relation to the AUDIO or
sideband that it has to pass. The filter shape factor then determines where
on the skirt the BFO happend to fall. On the type of filter (commonly)
used, it appears to be the -20dB point.
Someof the other reaponses are saying the same thing in various ways.
> |4. Is linear interpolation between the -6 and -60dB points accurate
enough
> |to determine the BFO freq?
That's probablu not to bad, but the answer above says that this is going
at it the wrong way. Place the filter pass band where it needs to be and
the BFO goes on the carrier freq, wherever it is. If you want the filter to
cut off some of the highs, then put the BFO further down the skirt -
visa-versa.
If I actually answer your question and consider a "typical" (in my mind
anyway) filter shape, linear interpolation will put the BFO a little too
close (less than -20dB down -- as in like -18dB down) to the bandpass.
--
Steve N, K,9;d, c. i My email has no u's.
Tom Holden
Thanks for all the useful replies! I neglected to explain that this question
is related to upgrading a particular receiver and another design target is
that the BFO frequency must be fixed at +/-1500Hz from the filter and IF
centre frequency (1 side for USB, the other for LSB) due to the way this
double conversion receiver tunes and displays frequency. So it is more a
question of determining what filter bandwidth and shape would be most
suitable. By targetting -20 to -24dB attenuation with this offset, I
computed that example filters suited to that offset would have -6dB/-60dB
bandwidths and corresponding audio passbands of:
1) 2kHz/5.4kHz ----- 500-2500Hz
2) 2.3kHz/4.7kHz --- 350-2650Hz
3) 2.5kHz/4.2kHz --- 250-2750Hz
Do these calculations seem reasonable?
Examples 2 and 3 seem to be acceptable for communications speech but the
steeper the skirts the greater the risk that filter tolerances will place
the BFO somewhere else on the skirts well removed from the -20dB target. Are
there any economical 455kHz filters with the characteristics of examples 2
or 3 with symmetrical skirts?
TIA ,
Tom
Pete KE9OA
You can purchase the "low cost" Mechanical Filters from Rockwell
Filter Products Division, in Costa Mesa, Ca. These would be the 526-8695-XXX
series. Price, including shipping is around 86USD. The Z in/out is 2k, and
no resonating capacitors are needed, as long as your strays are below 30pF.
These are the mechanical filters that are used in the AOR3030, AOR7030, and
the Palstar R30, to name a few.
The -24dB spec is something I remember from my old HT-46 transmitter, so it
doesn't need to be taken as gospel. A good thing is to listen to how the
transmitted signal sounds.
Pete
"Tom Holden" <holden...@sympatico.ca> wrote in message
news:9kZUb.39883$9U5.1...@news20.bellglobal.com...
Tom Holden
> Hi Tom,
> You can purchase the "low cost" Mechanical
> Filters from Rockwell Filter Products Division, in Costa
> Mesa, Ca. These would be the 526-8695-XXX series. Price,
> including shipping is around 86USD. The Z in/out is 2k,
> and no resonating capacitors are needed, as long as your
> strays are below 30pF. These are the mechanical filters
> that are used in the AOR3030, AOR7030, and the Palstar
> R30, to name a few.
> The -24dB spec is something I remember from my old HT-46
> transmitter, so it doesn't need to be taken as gospel. A
> good thing is to listen to how the transmitted signal
> sounds.
>
> Pete
Pete, that's about as much as I paid for the receiver I'm trying to improve!
A Radio Shack DX-394. I'm hoping to find something for a lot less. Probably
an unrealistic (no pun intended) expectation.
Tom
Fred McKenzie
A Radio Shack DX-394 >>
Tom-
I missed your original posting. What does the DX-394 currently use as an IF
filter? If it happens to be made by Murata (or maybe Panasonic), a replacement
with better specifications may available to fit in the same PCB holes. Check
your filter's part number with the company's web site, and you may find a
solution there.
However, if it uses the same filter for AM reception, a tighter filter could
reduce sound quality of music.
73, Fred, K4DII
Tom Holden
Fred, thanks for your follow-up. The DX-394 has two 5-element Murata ceramic
filters, one 9kHz or wider for AM and one 6 kHz or wider for SSB/CW with a
BFO offset of +/-3.5kHz from 455kHz. That's why a narrower filter will need
a BFO mod. There does not appear to be a much narrower filter in the same
package and from what I understand Murata is out of the ceramic filter
business. So I guess I will have to keep an eye out for a surplus filter or
shell out $85 or so for a new 'economical' Collins!
Tom
Ken Scharf
(or less) the cost of a new one. Fair Radio used to have 2.0 and 4.0
khz filters from the famous R390 receivers at about $25 or so.
Pete KE9OA
Murata did get out of the high performance filter
business..................about the only things that they have now are the
4-element CFU series, and the 6-element CFWS (now CFLW) series.
Pete
"Tom Holden" <holden...@sympatico.ca> wrote in message
news:RBwVb.508$sO4.1...@news20.bellglobal.com...
Pete KE9OA
526-8695 series is that they have an input/output Z of 2K, so you can
substitute these filters for the ceramic filters. Another good thing is that
these new Torsional Mode filters don't require any tuning caps at the I/O
pins.
Pete
"Ken Scharf" <wa2mze...@bellsouth.net> wrote in message
news:SOxVb.513$8W....@bignews6.bellsouth.net...