On 11/9/2012 7:31 AM, GMVoeth GM wrote:
> On 11/9/2012 6:30 AM, Bob Smither wrote:
>>
>> On Fri, 2012-11-09 at 06:10 -0800, GMVoeth GM wrote:
>>> Hello PSN,
>>>
>>> I know with a velocity sensor the voltage increases with frequency 
>>> after
>>> F0 if given the same
>>> vertical motion amplitude. This seems to be because as the frequency
>>> increases
>>> so does the velocity and thus, the voltage output.
>>>
>>> This means to me the energy also increases as the square of the 
>>> velocity.
>>> So like, double the velocity, quadruple the energy.
>>>
>>> What i would like to do is: filter at f0 (1.0Hz) so that given
>>> a steady shake amplitude (sine wave) but incresing frequency
>>> that the voltage will remain constant.
>>>
>>> Now I understand this may not be possible.
>>
>> I think it is.  A simple RC low pass filter (R from signal to C, C to
>> ground, output at junction of R and C) gives:
>>
>>    T(S) = 1 / (R*C*S + 1)
>>
>> Above w = 1/(R*C) this is
>>
>>    T(S) ~ 1 / (R*C*S)
>>
>> exactly what you need to reduce gain proportional to frequency.
>>
>> For your example, w = 2*pi*1Hz = 1/RC so C = 1ufd, R = 160K should work.
>>
>
> I find this interesting.
>
> I have eliminated filters except for a single HPF n=1 at like 100seconds
> and am looking at pure noise.
>
> At 8AM MST today I get occasional over driving (jamming) of my
> preamplifier with no filtering.
>
> I need this level of amplification to see the smaller seismic signals.
>
> If I apply n=2 LPF at 1.0Hz using winquake I get the best flattening
> of the noise baseline above 1.0Hz. Noise is human vehicular related.
>
> It is an absolute must for me to apply n=2 at 1.0 Hz to prevent
> jamming by vehicular and other noise.
>
> Does this mean there is a problem with my geophone
> or is vehicular noise increasing with frequency like n=1
> to add to the velocity profile of the geophone ?
>
> None of this is speaking of Alizing problems.
>
> this is an extremely noisy town for vehicular activity.
> (Apache Junction AZ USA)
>
> Would it be best to stop recording during the day
> or best to heavily filter to keep the system from being
> jammed. Heavy filtering terribly affects the resulting
> waveforms ?
>
> Regards,
> geoff
>
>
>
>
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>
Using the formulas on wikipedia i have decided to try the following values.
The lowpass filter is a Sallen-Key
N=2
K=1.0
R1=R2=112539 ohms
C1 = 2uf
C2 = 1uf
f0 = 1.00Hz
I will build this today and
add it to my circuit.

Regards,
geoff (GVA)
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