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TEACHING EARTH SCIENCES ● Volume 30 ● Number 1, 2005 A high performance seismic amplifier BERND ULMANN The purpose of this article is to describe a simple but powerful amplifier with remarkably low drift to be used in conjunction with electromagnetic seismometers. Due to its reliability and ease of construction the amplifier is especially well suited for educational purposes. Figure 1 A simple Lehman type seismometer One of the first seismometers nearly every amateur will build is one which is frequently used in schools, universities, etc., for geophysics seismology instruction. It is a device that consists of a horizontal pendulum, like the Lehman type seismometer shown in figure 1 (cf. [2 ]). Typically these devices use a magnet moving in a coil as the pickup mechanism. Whenever the pendulum detects a seismic wave its tip will move relative to the coil mounted on the pendulum base plate. Thus the magnet will move inside the coil and induce a voltage proportional to the relative speed of the moving magnet with respect to the coil: U ind ~ x (x denotes the position of the magnet). Since the induced voltage U ind is proportional to the speed of the magnet it tends to become minuscle for very low frequency waves, as they are the result of teleseismic events. So it is necessary to employ a very high gain amplifier to amplify U ind to values which can be post-processed with a computer based AD-converter system or a simple x/t-recorder. Since it is possible and quite common to build horizontal seismometers with eigenperiods of as low as 15 seconds and less, the amplifier should be a DC coupled device, as high pass filters with a roll off of 15 or 20 seconds tend to be very bulky due to the necessary large capacities. As high gain is necessary it is important to use an amplifier design with very low drift. This is because changes in ambient temperature and component aging will drive the amplifier into saturation, making accurate measurements impossible. Another point worth consideration is amplifier noise – the self noise of the seismometer is negligible since, as a purely passive device, such noise is completely of ther- mal origin, meaning that the noise generated by the pickup system does not increase with lower frequencies. The amplifier as an active system does indeed generate increasing noise at lower frequencies, therefore it is crucial to use very low noise components in its design. The author has used an amplifier such as the one described below, and found it to function properly for the past two years, in spite of its being in an adverse environment with wide temperature variations. Only once during this period has an offset adjustment been required. Even with a selected amplification of 70000 the drift has been negligible. The cost to build this amplifier is well below 50 USD. The labour involved is approximately six to eight hours, depending on one’s expertise with a soldering iron. 2 The amplifier The amplifier is composed of four building blocks, which are described in detail in the following sections. 2.1 The input stage The first stage the signal to be amplified reaches is the input stage, which is shown in figure 2. The large 10M resistor ensures that the high impedance input of the operational amplifier is not saturated by stray currents if the input is left open. The switch S1 can short circuit the input of the amplifier to ease the offset adjustment procedure while the two antiparallel silicon diodes 1N4148 provide – in conjunction with the 47k resistor of the input network – some additional over voltage protection. The operational amplifier is connected in a noninverting configuration with an overall amplification of approximately 100. Since the amplifier employs a completely DC coupled design, it is crucial that drift is kept at a minimum at the input stage requiring the use of a chopper stabilized amplifier like the MAX430 (cf. [1 ]) shown in figure 2. This integrated circuit has a remarkably high stability but at the cost of some very low frequency noise being generated. This noise is inversely proportional to the chopping frequency of the operational amplifier and thus lies in the range of some 10 -2 to10 -3 Hertz. This is negligible considering the eigenfrequency of the seismometer itself. If necessary this very low frequency noise may be filtered out digitally in a later processing stage. 2.2 The low pass filter After passing the input stage with its amplification fac- www.esta-uk.org 18
TEACHING EARTH SCIENCES ● Volume 30 ● Number 1, 2005 Figure 2 The input stage Figure 3 The four pole Butterworth low pass filter tor of about 100, the signal is filtered by a Butterworth low pass filter of order four with a cut off frequency of 10Hz. This is necessary to eliminate noise from power lines and other sources. Such noise is unavoidable when using electrodynamic sensors. This low pass filter is shown in figure 3 – essentially consisting of two double pole filters connected in series but with slightly different passive component dimensions. 2.3 The main amplifier After being amplified by a factor of 100 and low pass filtered the signal reaches the main amplifier shown in figure 4. Amplification may be selected by means of a twelve position rotary switch, from factors of one through 700. This results in a total amplification range of from 100 to 70,000. Since the signal fed into this stage is very drift free and has already been amplified by a factor of 100 it is not necessary to use an expensive MAX430. Here again. experimentation has revealed that a cheap TL061 performs very well at this stage of signal processing. When the amplifier is switched on it should be allowed to adapt to its environment for at least half an hour before the necessary offset adjustment is performed. This is especially important if the device is to be used for something more than simple demonstrations. To accomplish this input is shorted by switching on S1. Then the output is adjusted to as close to 0V as possible via the offset potentiometer shown in figure 4. This adjustment should be made with the maximum amplification selected. The output stage consists of a single operational amplifier used as an impedance converter, which is protected by a 33 Ohm series resistor. 2.4 The power supply Although the power supply shown in figure 5 is simple, it is important to adequately satisfy the requirements of the amplifier. Since the circuit needs dual supply voltages of +5V and -5V the power supply consists of two monolithic voltage regulators 7805 resp. 7905. These regulators are protected by reversed diodes, which allow no back currents in case of shorting the inputs. It is crucial to feed the seismic amplifier from a stabilized voltage source! Using a simple transformer with Figure 4 The main amplifier 19 www.esta-uk.org
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