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 Peterswrote: 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 Randall
Thanks 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 sensors
To: "ps= n-l@.............." <psn-l@..............>
Date: Thursday, Februar= y 19, 2009, 4:42 PM=20 =20=20Larry,
=20The 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.
=20The 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.
=20Randall
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