PSN-L Email List Message
Subject: Re: Wielandt's "inverse filter" or integrator (long)
From: Brett Nordgren Brett3kg@.............
Date: Wed, 15 Dec 2004 17:54:23 -0500
Sorry, I don't have a copy of the "Noise" paper. If Chris Chapman is still
monitoring the list, he might be able to help you find it. I know he'd
been working to acquire many of the old journal articles.
Enlightenment is good. Feedback systems are some of the more non-intuitive
things you're likely to come across. To tell the truth, when I was first
doing feedback design, I think it was a couple of years before I could
honestly claim that I had a "feel" for what was going on.
One general comment on why feedback seismos exist.... Starting with a
spring-mass which has a position sensor and some means for applying
force-feedback, the feedback is used to *reduce* and shape the frequency
response to predictably match the shape you want. There is a fairly
standard set of seismo. frequency-response curves that seem to be in common
use by seismologists for different purposes. In most cases, around here,
the Velocity Broad Band response seems to be the most common. The purpose
of having feedback is to tightly control the instrument response (at the
expense of sensitivity). However in electronic seismo. systems, high
sensitivity is quite easy to get. The more fundamental issue will usually
be instrument noise.
The "inverse filter" you refer to was one of many terms coined in the early
days of feedback seismometers as designers were first putting together and
publishing the concept. A more accurate name might be "one of those things
you need in order to make the frequency response what you want", though I
admit, it is a little lacking in elegance and brevity. His choice for the
term probably comes from the fact that when *any* element is put in the
feedback path, the overall instrument response that results will tend to be
the inverse of that element's frequency response. That fact is fundamental
to *all* feedback designs.
If you haven't spent time with them, I would very much recommend Wielandt's
articles "Seismic Sensors and their Calibration " online at
and "Seismometry", located at
In particular, the sections on force balance seismometers should be
helpful. These are much more understandable and technically refined than
the early journal articles, which often were not easy to follow.
And you can try my Web site downloads index at
The files "feedback", "complex" and particularly "loop3" might be of
interest. Also, today, I put a draft copy of "loop4" on the site, but
haven't put it in the index yet. You can access it directly at
It is considerably improved from "loop3" (and somewhat larger). These
relate to an analysis of the feedback loop of S-T Morrissey's STM-8 leaf
spring vertical. Its loop design is very similar to those of the
commercial VBB instruments. If you can follow the math, great, but there's
still a lot there that should be helpful even neglecting the math details.
Hope these help.
At 07:11 AM 12/13/2004 -0500, you wrote:
>Good morning all,
>I am looking for some enlightenment on the subject circuit. I know just
>enough about it to be dangerous and not enough to understand what exactly
>it does and how to select the corner frequency of the high pass filter. I
>have not been able to obtain a copy of Wielandt's paper "Noise in
>Electronic Seismograph Systems", so if anyone has an electronic copy (in
>english) I would appreciate a copy. Short of that, can anyone provide an
>explanation of how this thing works, and what the alternatives to it might be.
>I understand that the output of the high pass filter is fed back to the
>input amplifier and subtracted from the input signal, but doesn't the
>phase shift in the filter prevent this from working as Allan Coleman
>describes in his papers?
>Thanks for any help you may provide
Public Seismic Network Mailing List (PSN-L)
[ Top ]
[ Back ]
[ Home Page ]