## PSN-L Email List Message

Subject: Re: coil/magnet sensors
From: Pete Rowe ptrowe@.........
Date: Thu, 19 Feb 2009 17:20:31 -0800 (PST)

```Hi Randall
Thanks for your lucid thoughts. What are your high pass and low pass freque=
ncies? The high pass function in my system is just the coupling caps in the=
amplifier chain and I'm not sure where it rolls off on the bottom end. It =
seems that I should try a real high pass and I'd like some advice on settin=
g the corner frequency.
Thanks,
Pete

--- On Thu, 2/19/09, Randall Peters  wrote:
From: Randall Peters
Subject: coil/magnet sensors
To: "psn-l@.............."
Date: Thursday, February 19, 2009, 4:42 PM

=20
=20

Larry,=20

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 The primary reason for poor
signal to noise ratio at long periods with a coil/magnet system involves th=
e
physics employed.=C2=A0 Such a sensor functions on the basis of Faraday=E2=
=80=99s
law, which says that a time changing magnetic flux gives rise to a voltage.=
=C2=A0
The key word in this process is =E2=80=98changing=E2=80=99; the output depe=
nds on
the flux rate.=C2=A0 If the boom of the seismometer is swinging sinusoidall=
y at
a frequency f, then the amplitude of the output voltage of the sensor is
proportional to f=E2=80=94which is the very nature of the time derivative o=
f the
sine function, involving the chain rule. =C2=A0Because the signal is propor=
tional
to f, the signal level decreases by the same amount as f decreases.=C2=A0
Regardless of the nature of the noise, this means there is a significant
reduction in the signal to noise ratio for accelerations having a frequency
below the natural frequency of the instrument.=C2=A0 When operating without=
the
high-pass filter with your S-G instrument, your output is not rate sensitiv=
e;
therefore it isn=E2=80=99t afflicted with this low frequency loss.=C2=A0 Th=
e =E2=80=98wandering=E2=80=99
you experience is actually representative of the various changes
occurring.=C2=A0 Although some of those changes are oftentimes the undesira=
bles
of instrument thermal coefficient or whatever, there are also significant v=
ariations
associated with the Earth itself.=C2=A0 When you high pass the output, any
earth motions having frequencies lower than the cutoff =C2=A0are suppressed=
..=C2=A0
I operate the VolksMeter with a high pass and also low pass filter of recur=
sive
type in WinSDR.=C2=A0 The primary reason is so that the helicord record is =
well
behaved for purpose of seeing earthquakes.=C2=A0 But for viewing tides, or
magnetoelastic phenomena, or earth hum, or =E2=80=A6. It is necessary to lo=
ok at
the unfiltered output, which is what I save.=C2=A0 =20

=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 The obsession with =E2=80=98velocity=E2=80=
=99
sensing is almost universal.=C2=A0 Even force balance instruments of the
highest dollar type use a network that causes the seismograph to behave jus=
t
like a magnet/coil system with a low corner frequency, usually about 30
s.=C2=A0 Thus the greatest advantage of the capacitive sensor is actually l=
ost,
for signals having frequencies lower than the design corner.=C2=A0 For sign=
al variations
having a frequency below the corner, the output from such an instrument is =
not =E2=80=98velocity=E2=80=99;
it is instead the derivative of acceleration, called the =E2=80=98jerk=E2=
=80=99.=C2=A0
Only for drive frequencies above the corner can one use the term =E2=80=98v=
elocity=E2=80=99
appropriately.=20

=C2=A0=C2=A0=C2=A0 Randall=20

=20

Hi RandallThanks for your lucid thoughts.=
What are your high pass and low pass frequencies? The high pass function i=
n my system is just the coupling caps in the amplifier chain and I'm not su=
re where it rolls off on the bottom end. It seems that I should try a real =
high pass and I'd like some advice on setting the corner frequency.Than=
ks,Pete--- On Thu, 2/19/09, Randall Peters <PETERS_RD@=
mercer.edu> wrote:From: Randall Pet=
ers <PETERS_RD@..........>Subject: coil/magnet sensorsTo: "ps=
n-l@.............." <psn-l@..............>Date: Thursday, Februar=
y 19, 2009, 4:42 PM

=20
=20

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Larry,=20

The primary reason fo=
r poor
signal to noise ratio at long periods with a coil/magnet system involves th=
e
physics employed.  Such a sensor functions on the basis of Faraday=E2=
=80=99s
law, which says that a time changing magnetic flux gives rise to a voltage.=

The key word in this process is =E2=80=98changing=E2=80=99; the output depe=
nds on
the flux rate.  If the boom of the seismometer is swinging sinusoidall=
y at
a frequency f, then the amplitude of the output voltage of the sensor is
proportional to f=E2=80=94which is the very nature of the time derivative o=
f the
sine function, involving the chain rule.  Because the signal is propor=
tional
to f, the signal level decreases by the same amount as f decreases.
Regardless of the nature of the noise, this means there is a significant
reduction in the signal to noise ratio for accelerations having a frequency
below the natural frequency of the instrument.  When operating without=
the
high-pass filter with your S-G instrument, your output is not rate sensitiv=
e;
therefore it isn=E2=80=99t afflicted with this low frequency loss.  Th=
e =E2=80=98wandering=E2=80=99
you experience is actually representative of the various changes
occurring.  Although some of those changes are oftentimes the undesira=
bles
of instrument thermal coefficient or whatever, there are also significant v=
ariations
associated with the Earth itself.  When you high pass the output, any
earth motions having frequencies lower than the cutoff  are suppressed=
..
I operate the VolksMeter with a high pass and also low pass filter of recur=
sive
type in WinSDR.  The primary reason is so that the helicord record is =
well
behaved for purpose of seeing earthquakes.  But for viewing tides, or
magnetoelastic phenomena, or earth hum, or =E2=80=A6. It is necessary to lo=
ok at
the unfiltered output, which is what I save.  =20

The obsession with =
=E2=80=98velocity=E2=80=99
sensing is almost universal.  Even force balance instruments of the
highest dollar type use a network that causes the seismograph to behave jus=
t
like a magnet/coil system with a low corner frequency, usually about 30
s.  Thus the greatest advantage of the capacitive sensor is actually l=
ost,
for signals having frequencies lower than the design corner.  For sign=
al variations
having a frequency below the corner, the output from such an instrument is =
not =E2=80=98velocity=E2=80=99;
it is instead the derivative of acceleration, called the =E2=80=98jerk=E2=
=80=99.
Only for drive frequencies above the corner can one use the term =E2=80=98v=
elocity=E2=80=99
appropriately.=20

Randall=20

=20

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