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

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

"Tom Holden" wrote in message
...
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

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

"Tom Holden" wrote in message
...
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

Pete KE9OA wrote:
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

Pete KE9OA wrote:
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

Pete, that's about as much as I paid for the receiver I'm trying to improve!
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

Pete, that's about as much as I paid for the receiver I'm trying to improve!
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

Fred McKenzie wrote:
Pete, that's about as much as I paid for the receiver
I'm trying to improve! 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

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

Fred McKenzie wrote:
Pete, that's about as much as I paid for the receiver
I'm trying to improve! 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

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

Tom Holden wrote:
Fred McKenzie wrote:

Pete, that's about as much as I paid for the receiver
I'm trying to improve! 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


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


You can probably find a surplus collins filter on ebay for about half
(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.