> RefCD > Testing a Seismograph Amplifier

 

Related Products

RefCD

 

sponsored by
RefCD
SIGNAL GENERATOR AND
AUDIO REFERENCE CD

Testing a Seismograph Amplifier

   Seismograph amplifiers are designed to operate at very low frequencies, typically from 10Hz down to direct current.  When analyzing the data recorded by a seismograph, it is desirable to know the frequency response of the amplifier, to insure that it is relatively flat.  Otherwise some signals could be emphasised over others, leading to an erroneous interpretation of the seismogram.  At the very least, the operator needs to know that the amplifier has low drift for DC inputs, and that the nominal gain is correctly set.  How does one go about testing such an amplifier?

   Ideally, one can apply a very low frequency (VLF) input signal to the amplifier and observe the output on an oscilloscope.  A voltmeter could also be used, provided it is fast enough to track the changes in the output without delays in the reading.  Care should be taken to avoid using the AC ranges as they may not linear at such low frequencies.  The problem then is how to obtain the necessary low frequency input.  Very low frequency signal generators are commercially available, but they are relatively rare and expensive.  It is also possible to
build a circuit using a function generator IC (see "Signal Generators and their uses").

   An attractive alternative is to produce a relatively easily generated audio frequency tone and modulate it at the appropriate low frequency.  This is most easily done on a PC with the
appropriate software.  Several shareware programs available on the World Wide Web will allow
the user to generate a very precise, modulated sine wave.  The sound is then played back through the sound card and demodulated to produce the required VLF signal.  A simple demodulation circuit is as follows:

   R and C are chosen to be able to filter out the carrier frequency, yet still respond quickly to the modulation. R=100Kohm and C=0.1micorFarad are good choices.  Just about any diode will do, like the popular 1N914.  Note that the amplitude of the carrier frequency must remain greater than the forward voltage drop of the diode, typically 0.6V, in order for the circuit to
work.  What one adjusts is the % modulation of the carrier.  For example, a 1V peak-peak carrier signal of 1Khz frequency, modulated by 1%, will give a 0.01V output at the modulation frequency.

RefCD contains a special track designed for just this test. It consists of a 1KHz sine wave
modulated +/- 10% by a 1Hz sine wave.  One utilizes a CD player to play back the track through the simple circuit above, thus eliminating the need for a low frequency oscillator.

For more information, check out the RefCD page.