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#1
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Cascading Cheap Ceramic IF Filters?
I found some old postings that did not go into the how-to of cascading cheap
ceramic filters in order to improve selectivity. I have a few 4-element and 6-element 455kHz filters with which I am experimenting in an IF downconverter for sound card DSP, specifically for DRM decoding which requires a nice flat 10kHz bandwidth. I'm wondering if it is not a good idea to DC couple them directly back-to-back. Should they be AC coupled? Should there be a terminating resistor to ground or a T-pad at the junction to provide a more uniform load/source impedance? Or is it best to make up for the losses and provide the proper terms by putting an active stage between them than having the gain makeup before or after? TIA, Tom |
#2
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See my article at: http://www.karlquist.com/FCS96.pdf
Toward the end I show some cascaded ceramic filters. You can't necessarily just cascade them directly. Of course it would be ideal to have buffer amplifiers in between stages, complete with impedance transforming stages, since the filters usually don't work with 50 ohm terminations. However, as you can see in the article, there are shortcuts. Rick N6RK www.karlquist.com www.n6rk.com "Tom Holden" wrote in message . .. I found some old postings that did not go into the how-to of cascading cheap ceramic filters in order to improve selectivity. I have a few 4-element and 6-element 455kHz filters with which I am experimenting in an IF downconverter for sound card DSP, specifically for DRM decoding which requires a nice flat 10kHz bandwidth. I'm wondering if it is not a good idea to DC couple them directly back-to-back. Should they be AC coupled? Should there be a terminating resistor to ground or a T-pad at the junction to provide a more uniform load/source impedance? Or is it best to make up for the losses and provide the proper terms by putting an active stage between them than having the gain makeup before or after? TIA, Tom |
#3
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"Rick Karlquist N6RK" wrote in message news:gdnJc.93979$Oq2.67677@attbi_s52...
See my article at: http://www.karlquist.com/FCS96.pdf Toward the end I show some cascaded ceramic filters. You can't necessarily just cascade them directly. Of course it would be ideal to have buffer amplifiers in between stages, complete with impedance transforming stages, since the filters usually don't work with 50 ohm terminations. However, as you can see in the article, there are shortcuts. Rick N6RK www.karlquist.com www.n6rk.com How old are these filters - there was a design here in 1971 (The Deltahet, using the Wadley Loop priciple) based around Murata 455KHz filters - quite impressive specs, variable bandwidth, etc. Trouble was the filters went hydroscopic fairly quickly, and I note that there are very few (at least here) ceramic IF filters now available. 73 de VK3BFA Andrew |
#4
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The filters in my article were purchased in 1995.
Note that they are 10.7 MHz, not 455 kHz. Ceramic filters are alive and well and still available at Digikey and Mouser. Rick N6RK "Andrew VK3BFA" wrote in message om... "Rick Karlquist N6RK" wrote in message news:gdnJc.93979$Oq2.67677@attbi_s52... See my article at: http://www.karlquist.com/FCS96.pdf Toward the end I show some cascaded ceramic filters. You can't necessarily just cascade them directly. Of course it would be ideal to have buffer amplifiers in between stages, complete with impedance transforming stages, since the filters usually don't work with 50 ohm terminations. However, as you can see in the article, there are shortcuts. Rick N6RK www.karlquist.com www.n6rk.com How old are these filters - there was a design here in 1971 (The Deltahet, using the Wadley Loop priciple) based around Murata 455KHz filters - quite impressive specs, variable bandwidth, etc. Trouble was the filters went hydroscopic fairly quickly, and I note that there are very few (at least here) ceramic IF filters now available. 73 de VK3BFA Andrew |
#5
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On Thu, 15 Jul 2004 15:26:54 GMT, "Rick Karlquist N6RK"
wrote: The filters in my article were purchased in 1995. Note that they are 10.7 MHz, not 455 kHz. Ceramic filters are alive and well and still available at Digikey and Mouser. Rick N6RK Believe he mentioned a problem that they could go "hydroscopic" in Australia? "Andrew VK3BFA" wrote in message . com... "Rick Karlquist N6RK" wrote in message news:gdnJc.93979$Oq2.67677@attbi_s52... See my article at: http://www.karlquist.com/FCS96.pdf Toward the end I show some cascaded ceramic filters. You can't necessarily just cascade them directly. Of course it would be ideal to have buffer amplifiers in between stages, complete with impedance transforming stages, since the filters usually don't work with 50 ohm terminations. However, as you can see in the article, there are shortcuts. Rick N6RK www.karlquist.com www.n6rk.com How old are these filters - there was a design here in 1971 (The Deltahet, using the Wadley Loop priciple) based around Murata 455KHz filters - quite impressive specs, variable bandwidth, etc. Trouble was the filters went hydroscopic fairly quickly, and I note that there are very few (at least here) ceramic IF filters now available. 73 de VK3BFA Andrew --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
#6
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Behold, J M Noeding signalled from keyed 4-1000A filament:
On Thu, 15 Jul 2004 15:26:54 GMT, "Rick Karlquist N6RK" wrote: The filters in my article were purchased in 1995. Note that they are 10.7 MHz, not 455 kHz. Ceramic filters are alive and well and still available at Digikey and Mouser. Rick N6RK Believe he mentioned a problem that they could go "hydroscopic" in Australia? Then he lives in Adelaide - same weather as Vancouver, RAIN! -- Gregg *It's probably useful, even if it can't be SPICE'd* http://geek.scorpiorising.ca |
#7
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Thanks, Rick. Looks like a fascinating article - I've just read the part on
the filters. As a starting point, I'll try two 6-element 455kHz filters with 1.5k desired source and load resistance and couple them with a cap scaled from your 22pF value. At 10.7MHz, I calc its X=680 or 2.06x the desired source/load of 330 Ohms. So 2.06 x 1.5k is 3090, leading to a coupling cap of ~100pF for 455kHz. I did find this in an old posting by VE2BVW in the thread 'Cheap 455kHz SSB filters?': "In an old article in 73 (December 1977) about the Yaesu FRG-7, reference is made to an "by Ron Risher VK3OM in the March 1977 issue of "Amateur Radio". He describes... an alternate filter, consisting of four cascaded SFD-455-B solid state filters linked by small coupling capacitors". In the same thread, from W7ZFB: "Using the simple 5 pin 2 element SFD455 in series makes a pretty good filter if the element coupling caps are around 10pF. Two in series is better than some of the multi element filters and four in series gets quite good. More gets proportionally better." His coupling capacitor seems awfully small for this frequency with an X of ~35k versus typical 1k5 to 2k specified source/load impedance. I would expect the attenuation through the filter to be very high. Regards, Tom "Rick Karlquist N6RK" wrote in message news:gdnJc.93979$Oq2.67677@attbi_s52... See my article at: http://www.karlquist.com/FCS96.pdf Toward the end I show some cascaded ceramic filters. You can't necessarily just cascade them directly. Of course it would be ideal to have buffer amplifiers in between stages, complete with impedance transforming stages, since the filters usually don't work with 50 ohm terminations. However, as you can see in the article, there are shortcuts. Rick N6RK www.karlquist.com www.n6rk.com "Tom Holden" wrote in message . .. I found some old postings that did not go into the how-to of cascading cheap ceramic filters in order to improve selectivity. I have a few 4-element and 6-element 455kHz filters with which I am experimenting in an IF downconverter for sound card DSP, specifically for DRM decoding which requires a nice flat 10kHz bandwidth. I'm wondering if it is not a good idea to DC couple them directly back-to-back. Should they be AC coupled? Should there be a terminating resistor to ground or a T-pad at the junction to provide a more uniform load/source impedance? Or is it best to make up for the losses and provide the proper terms by putting an active stage between them than having the gain makeup before or after? TIA, Tom |
#8
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Gregg wrote in message news:gjBJc.58285$eO.20454@edtnps89...
Behold, J M Noeding signalled from keyed 4-1000A filament: On Thu, 15 Jul 2004 15:26:54 GMT, "Rick Karlquist N6RK" wrote: The filters in my article were purchased in 1995. Note that they are 10.7 MHz, not 455 kHz. Ceramic filters are alive and well and still available at Digikey and Mouser. Rick N6RK Believe he mentioned a problem that they could go "hydroscopic" in Australia? Then he lives in Adelaide - same weather as Vancouver, RAIN! Nope, live in Melbourne - Adelaide is very dry! - Melbourne is miserably damp in winter - but we have had a drought for 10 years so its not!. Rick amended my statement, my experience was with 455Khz ones - hopefully the 10.7Mhz ones are better and no doubt manufacturing methods have improved in the last 30 years. Well worth a try - although there has been so much published on using computer xtals to do the same job is it worth the effort - depends on whats available I guess! 73 de VK3BFA Andrew |
#10
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Am I just crying wolf, or would cheap ceramic filters have terrible phase
linearity vs frequency? Don't most AM broadcast radios contain ceramic filter(s). |
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