PSN-L Email List Message

Subject: more filters
From: S-T Morrissey sean@...........
Date: Tue, 30 May 2000 02:30:15 -0500 (CDT)

Barry, Chris, and Co,

Good question about the availability of the NASA active RC filter book.
I bought my copy of the NASA book in 1978; I have no idea about
its current availability. I think it has a library of congress number:
N78 18312 is stamped on the cover. Maybe someone can wade through some web
sites and locate a source. They probably printed a NASA size quantity
of this rather droll book, so there might be crates of them somewhere.

Regarding other source info about equal-value, unity gain filters:
Lancaster does show the configuration for a standard two pole, unity
gain filter on page 125. Here, for the low pass design, for the first 
capacitor (the feedback to the center of the two Rs) the standard value
is multiplied by 2/d, and the second capacitor (at the non-inverting input)
is multiplied by d/2, where d is the damping. So you can use the tables 
for frequency and damping per desired rolloff response, but you have to 
calculate the Cs. The feedback to the inverting input is preferably
a resistor of value 2R to minimize offsets, although a wire will do.

I calculated values for a standard (1khz, 10k, 0.016uf) 4-pole Butterworth, 
and got C values of:
for the first section: (d = 1.848): C1 = 0.173, and C2 = 0.01478;
for the second section: (d = 0.765): C1 = 0.0418, and C2 = 0.00612

The NASA book gives 0.01723, 0.01470, 0.04159, and 0.006091 for the
standard z = 10k, fc = 1khz Butterworth filter. This book only gives
the capacitance values for Butterworth, Chebyshev (1db ripple), and 
Bessel filters through 8 poles, as well as alternate ripple (0.25db 
and 3.0db) for the Chebyshev filters.. There are no damping values.

Lancaster doesn't use the standard names, but explains them on page 72;
like "best delay" is the Bessel. There are 4 Chebychev or "slight dips"
variations. All are described through 6 poles, but use equal Cs so
the damping is controlled by the gain, which varies from 1 to less t
than 3. Tables give both the frequency and damping per pole as well
as listings of the actual values. The 1978 printing cost $15.

Chris: I hope you find one of these books for the Bessel info. If I 
should try to list all the values, there are serious chances for error.

Your comments about very low noise amplifiers reminded me of another
point about seismic amplifiers: the first one is the most important.
All the following gain amplifies the noise of the input amplifier.
And unity gain multi-pole (=multi-amplifier) filters should follow 
all gain stages so that their cumulative noise is not amplified.
So special considerations for the input amp (expense, power, etc.,) are 
much less important for later stages. I have found that an OP-27 is a 
good choice for the input for most seismometer Rs, and LM308A or similar
for filters. For a micro-power amplifier, the LM4250 works well if they
are screened for noise and the quietest used for the input; (ps if one
solders these in, their noise often goes up by an order of magnitude). 
BUT these are my preferences, and I don't want to get into a major 
discussion about others' reading of amplifier specifications.


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Larry Cochrane <cochrane@..............>