PSN-L Email List Message

Subject: Re: Wielandt's "inverse filter" or integrator (long)
From: "David H. Youden" dyouden@.............
Date: Thu, 16 Dec 2004 17:17:29 -0500


That's an impressive piece of work that you have done there. I have 
downloaded and printed the files of interest and will start reading them 
tonight. Thanks so much for your help.

Yes, Chris is still around and says that he will try to get the Wielandt 
paper on noise.

I have read the Stuttgart papers and find them very helpful.

Thanks again for your help.

Brett Nordgren wrote:

> Dave,
> 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.
> Brett
> 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
>> Dave Youden
> __________________________________________________________
> Public Seismic Network Mailing List (PSN-L)
> To leave this list email PSN-L-REQUEST@.............. with the body of 
> the message (first line only): unsubscribe
> See for more information.

Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]