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?
4. Is linear interpolation between the -6 and -60dB points accurate enough
to determine the BFO freq?

TIA,
Tom

On transmit, you need to get some of the carrier
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" wrote in message
.. .
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?
4. Is linear interpolation between the -6 and -60dB points accurate enough
to determine the BFO freq?

TIA,
Tom

On transmit, you need to get some of the carrier
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" wrote in message
.. .
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?
4. Is linear interpolation between the -6 and -60dB points accurate enough
to determine the BFO freq?

TIA,
Tom

On Thu, 5 Feb 2004 22:13:05 -0500, "Tom Holden"
wrote:

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?
4. Is linear interpolation between the -6 and -60dB points accurate enough
to determine the BFO freq?

TIA,
Tom



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

On Thu, 5 Feb 2004 22:13:05 -0500, "Tom Holden"
wrote:

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?
4. Is linear interpolation between the -6 and -60dB points accurate enough
to determine the BFO freq?

TIA,
Tom



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

On Thu, 5 Feb 2004 22:13:05 -0500, "Tom Holden"
wrote:

|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?
|4. Is linear interpolation between the -6 and -60dB points accurate enough
|to determine the BFO freq?
|

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

On Thu, 5 Feb 2004 22:13:05 -0500, "Tom Holden"
wrote:

|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?
|4. Is linear interpolation between the -6 and -60dB points accurate enough
|to determine the BFO freq?
|

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

|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.

|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 wrote:
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?
4. Is linear interpolation between the -6 and -60dB
points accurate enough to determine the BFO freq?

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