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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. |
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