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In Reply to: RE: Any plans for alignment? (nt) posted by BFitz on August 15, 2011 at 19:00:50
Thanks for sticking with me on this. My filter theory is very rusty.
I have two basic "questions"
1. Has it been established that the 10B IF section is indeed a series of cascaded, "non-resonant" bandpass filters providing an overall Butterworth response as the 10B promotional lit seems to imply? The schematic is drawn such that it looks like a series of buffered passive filters. Or is it just a fancy conventional resonant topology? Something else?
2. If this is indeed a series of cascaded bandpass filters, to achieve max flatness within the passband (as advertised) won't the individual filter pole frequencies and Qs be staggered in some manner to achieve the desired overall response characteristic. If this is the case, it's the frequency staggering of each section that I'd be trying to reverse engineer knowing that the overall filter characteristic was Butterworth of X order with a center frequency of 10.7 MHz and a bandwidth of say 200KHz. By knowing the frequencies for each section, one could then align each section individually to produce the desired overall response characteristic.
I don't own a network analyzer and if I did I wouldn't know how to use it in an FM IF alignment procedure. Is there info on this process? I'm not planning on getting an analyzer but I'm curious about the principles behind it.
Follow Ups:
The 10B has 6 modules labeled TF2B, but no component values. So there's no way to simulate it, without measuring the individual components to get their values. This usually involves dis-assembly unless all components are marked clearly with values.
Yes, it's what I would call a buffered passive capacitive coupled filter design. All filters are resonant, so not sure where the "non-resonant" came from, perhaps marketing people made that one up. I think you/they are thinking of comparing the 10B capacitive coupled IF filters to common inductive (transformer) coupled IF filters, as seen in almost all tube tuners.
You design the filter modules with a known input and output impedance, and make sure it sees that impedance when installed in the circuit. This sort of capacitive coupled filter is seen frequently in other solid state tuners that came later, i.e. those that used LC IF filters.
If you want to read more on this, I would suggest RF Circuit Design, by Chris Bowick. A google search shows older versions of this book may be available for download as a pdf - you want to breeze through chapter 3, Resonant Circuits.
As for alignment, I already spoke to this in the above post - Dishall method, or visual sweep with an analyzer, or both. Sure, the filter has multiple poles and zeros at different frequencies, but you can only look at the whole section by non-intrusive probing of the input and output of the entire filter section (i.e. the block of components between the two active devices, tubes in the case of the 10B). Read up on the Dishall method, I think this is what you are looking for.
Bob
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