FDNR in active filters (Frequency Dependent Negative Resistor) - Page 2 - diyAudio
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Old 9th April 2003, 08:43 AM   #11
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Default Re: analog filter humor

Quote:
Originally posted by jackinnj
Analog electronics has two distinct sides: the theory taught by academic institutions (equations of stability, phase-shift calculations, etc.), and the practical side familiar to most engineers (avoid oscillation by tweaking the gain with a capacitor, etc.).


That's why I try to ask people with personal exeperience. I can't remember how many thick books I have seen with loads of math. Noone talks about how good they (the fantastic filters) are in real life. Parts have ideal performance in their world.
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Old 9th April 2003, 09:22 AM   #12
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Quote:
I think there is some truth in "not in the signal path" becuase at low frequencys (LP-filter) the signal passes simply through. Disadvantage of the filter is the high output impedance.
Thats what I already tried to say.

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Charles
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Old 30th September 2003, 01:50 PM   #13
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Has anyone here used FDNR filters in some application?

Has anyone used HP-filters built by FDNR?
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Old 30th September 2003, 02:20 PM   #14
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Quote:
Originally posted by peranders
Has anyone here used FDNR filters in some application?

Has anyone used HP-filters built by FDNR?
wasn't there an article on FDNR Filters in EDN a few months ago?
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Old 2nd October 2003, 12:23 AM   #15
GaryB is offline GaryB  United States
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Default Re: FDNR in active filters

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Originally posted by peranders
Hi!

Has anyone used FDNR (frequency dependent negative resistances) in an anctive filter? I started long time ago to design such a filter but other things got in the way... still FNDR looks very cool.


Take a look at the PCM63 datasheet: http://focus.ti.com/lit/ds/symlink/pcm63.pdf

You'll see on page 10 an application using FDNR low pass filters. I modified an old Pioneer Elite CD player using this type of circuitry and thought it was one of the more transparent filters that I'd heard in a CD player. Definitely worthwhile.

---Gary
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Old 17th April 2004, 02:11 PM   #16
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here's another FDNR souce:
http://www.kemt.fei.tuke.sk/Predmety...ialy/ch029.pdf
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Old 18th April 2004, 06:17 PM   #17
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jackinnj, thank you for the pdf but it didn't tell me much I didn't know already, sorry.

First of all: I have not yet had any practical experience with FDNR filters.

Ten years ago I started to make a filter with this topology and the result of this is an Excel file. I have never seen a HP-filter using this topology so I wonder is it possible? Somewhere I read that the FDNR circuit must be grounded but I don't see why (at the moment). Maybe a simulation will tell me?

I have small plans to make a really cool cross over filter. I had in mind, 2nd, 3rd and 4th order LP sub, LP, BP and HP filter. Super universal with universal charateristics, Chebushev, Butterworth, Bessel (anykind) etc.

It seems that FDNR is a totally unexplored field. Maybe we should explore it for hifi usage?
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Old 19th April 2004, 01:40 AM   #18
GaryB is offline GaryB  United States
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Quote:
Originally posted by peranders
I have never seen a HP-filter using this topology so I wonder is it possible? Somewhere I read that the FDNR circuit must be grounded but I don't see why (at the moment).
P-A,
The FDNR type filter is achieved by doing a 1/s transformation. Normally the impedance of various elements is:

Inductor = jwL
Capacitor = 1/jwC
Resistor = R

For an FDNR filter, the transformation does the following:

1/s x Inductor = L . . . can now use a resistor instead
1/s x Resistor = R/jw . . . can now use a capacitor
1/s x Capacitor = -1/wwC . . . now need to use a FDNR (frequency dependent negative resistance) synthesized from op amps, resistors and capacitors.

One can do this transformation for any type of filter, but it works out nicely for filters where the capacitor is shunted to ground because that means that the op amps used to synthesize the FDNR element are not directly in the signal path.

An example of how this is done can be found in the following short article:

http://www.elecdesign.com/Articles/I...ArticleID=6366

There used to be a nice apps note from Burr Brown (AB-026) with the title "A Low Noise, Low Distortion Design for Anti-aliasing and Anti-imaging Filters," that went through a lot of this but it seems to have disappeared from the web now that BB is merged into TI.

Anyway, this should be enough information to make you dangerous.

---Gary
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Old 20th April 2004, 01:47 PM   #19
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From my book, by Lawrence P. Huelsman, Active and passive analog filter design - An introduction (400 pages!)
Quote:
Since FDNR is an active element, it's important from power supply considerations that one end of it always be grounded. This is equivalent to requiring that all capacitors in prototype network to which the RLC-CRD transformation is to be applied also be grounded. Such a requirement is generally satisfied by low-pass filter realizations. Thus the use of FDNR's is especially suitable to realization of such filters.
Is it possible to have a FNDR from signal source in series with output plus a resistor down to ground, like a "super highpass filter"?

I other words: is is possible to make a high-pass filter?
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Old 20th April 2004, 02:16 PM   #20
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jackinnj, the problem with all filter books except for "Active filter cookbook" is that they never jump into the real world, like noise, distortion, frequency properties, shortcomings in design and i passive and active elements etc.
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