One of the major shortcomings of a home-made seismometer is the minuscule size of the seismic signal available at the output of the magnetic pickup coil. In older, professional-grade instruments, massive pickup coils, together with large, powerful magnets, provide a seismic signal sufficient for even the most insensitive chart or drum recorder. In addition, the coils often generate enough excess signal to allow for fully-adjustable self-damping of the seismic mechanism. The small, disassembled relay coils used in our instruments, however, provide less than 1% of the signal available from professional seismometers; and as a consequence, such weak signals must be amplified, electronically, to a level sufficient to drive a chart recorder or computer interface device.We have found that it is simpler, easier and far less expensive to construct an electronic preamplifier than it is to construct precision, massive, coils. There seems to be little difference in the results (a clean, accurate seismogram) using either method. This paper describes the preamplifier and its construction details. The electrical seismic signal is generated in the pickup coil, which is located within 1-cm of the magnet attached to the seismometer's pendulum. Seismic disturbances cause the seismometer's frame and base to move with respect to the pendulum and magnet (and, hence, with respect to the pickup coil which is attached to the frame). The motion is quite small and very slow -- not visually perceptible -- and the amplitude of the resultant signal is in the low microvolt region. Such a weak signal is quite vulnerable to electrical interference and noise; it must be amplified and filtered before it can used to drive a chart recorder or other instrument. If the preamplifier must be located more than 30-cm from the seismometer, it is also necessary to use shielded wire for the connection to the preamplifier's input to minimize such noise pickup. Unless the seismometer is located within 1-meter of a power transformer or electric motor, it is usually not necessary to provide for magnetic shielding of the pickup coil. The signal at the preamplifier input connection is applied to the inverting input of a commutating auto-zero (CAZ) instrumentation operational amplifier, a Maxim MAX420CPA. The CAZ amplifier is unique in its ability to sense its own internal offset voltage and automatically correct for it. Long-term output voltage drift, due to thermal changes and 1/f noise, is completely eliminated, making the device ideally suitable for the amplification of the long-period seismic signals. The amplifier is configured as a single-ended (unbalanced) inverting amplifier -- the non-inverting input is connected to ground potential. A 10-megohm feedback resistor sets the stage-gain to 1000 (30-dB). In parallel with the feedback resistor, a 0.01-µFd capacitor is incorporated to filter-out 60-Hz noise which may have been introduced via magnetic coupling to the pickup coil. The CAZ amplifier IC is relatively expensive -- the cost in unit quantities, $7.31, makes it, by far, the most costly of the electronic components used in the preamplifier; but it is indispensable to the excellent overall performance of the seismometer. Standard CAZ support circuitry is included, consisting of two, 0.1 µFd capacitors.
In geographical areas of nearby seismic activity or where only very large,
distant earthquakes need to be recorded, the overall gain of the seismometer
may be reduced. The original gain is sufficient to show average background
microseisms and, in our instruments, produced small but clean records of
m-4.5 earthquakes at a distance of 2,000-km. It is difficult to predict
how a given instrument will respond, but we found that a m-6.2 (<35-km depth)
earthquake at the same distance produced the maximum recordable signal before
degradation of the waveform due to amplifier saturation.
The gain of the seismometer may be reduced by changing the value
of the 10-megohm feeback resistor of the CAZ op-amp. The gain is
proportional to the value of the resistance, and is computed by
the formula:
The CAZ amplifier is somewhat sensitive to the circuitry that follows it -- requiring a load impedance greater than 10-kilohms. Therefor, a second op-amp (any general-purpose amp, such as the LM-741, TL-081, etc.) is used, in part, to isolate the first stage from the load. Because there is no necessity for further amplification of the signal, the second-stage is configured as a 2nd-order low-pass filter with a Butterworth response characteristic. The filter is tuned to a cutoff frequency of 0.31-Hz to reduce the response of the seismic output signal for events with a periodicity of less than 3.2-seconds, such as would be caused by students walking past the seismometer, nearby automobile traffic and remaining electrical-type interference caused by computers, motors, fluorescent lamps, etc. This degree of filtering is quite heavy; so if you live in an area of local seismic disturbances, you may wish to incorporate substantially less filtering. The cutoff period for the filter can be adjusted by changing the value of the 5.1 megohm resistors. For example, using 2.2 meg resistors, the cutoff period becomes 1.4-seconds; 1-meg resistors provide for a 0.63-second cutoff period. The cutoff period of a 2nd-order filter is not sharp -- it rolls off at a 12-dB per octave rate. Click here for a more comprehensive tutorial of seismic filter considerations. The inverting input of the second stage is also used to introduce an offset bias if necessary when a particular chart recorder or data acquisition system requires a signal voltage that never drops below zero volts; i.e., one that requires a zero-signal setpoint at some specified DC voltage level. Normally, the offset adjustment is set to produce zero volts with no seismic signal; but the adjustment is included to provide for a +/- 5-volt offset capability. A third connection to the inverting input of the second stage is used to accept the time-marker pulses from the optional WWV receiver/decoder circuitry.
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With no heatsinks installed on the ICs, this supply should be adequate
for up to four preamp/filter circuit cards.
