A New Instrument for Analog Telemetry

The McVCO is a microcontroller based replacement for the VCO instrument used in the analog telemetry of seismic signals. Installation of the instrument in the Pacific Northwest Seismic Network began in August 1994. The US Geological Survey's Cas cade Volcano Observatory uses it on Mt Saint Helens and the USGS Volcanic Disaster Assistance Program adopted it after suc cessfully deploying it at the Monserrat volcano in the summer of 1995.

The general operation of McVCO is presented in the Block Diagram.. The design uses digital data acquisition and direct frequency synthesis to produce an audio output modulated with the seismometer input signal. Switching is provided for selecting seismic signal gain and the audio carrier center frequency.

The microcontroller conducts the operation of the instrument via the serial interface bus. Common clock and data lines connect the microcontroller to the 12 bit analog to digital converter, the programmmable sinewave generator, and to the two 12 bit digital to analog converters which are parts of the offset and gain control circuits. Separate lines from the microcontroller provide chip selection.

The signal from the seismometer is amplified and filtered by the preamplifier. As shown in the block diagram, the preamplifier has three stages. The first stage is an instrumentation amplifier which takes the differential low level signal from the seis mometer and changes it to a ground referenced signal amplified by 40dB. The signal is conditioned by a 30Hz, 4 pole, low pass Butterworth filter in the second stage. The third stage is a variable gain amplifier whose gain is set by the microcontroller from the gain DIP switches after an instrument reset.

The measurement cycle begins with a request from the microcon troller to the ADC for a conversion of the preamplifier output. The microcontroller uses this measurement and the center fre quency selected by the DIP switch to calculate the instantaneous output frequency required by the conversion. The microcontroller codes the calculation and sends it to the programmable sinewave generator which synthesizes the corresponding frequency. The measurement cycle begins again and repeats 660 times a second.

The average value of a seismic signal is zero and the micro controller uses this fact to regulate the preamplifier output offset. It periodically adjusts the count in the offset control DAC to remove offset and keep the output frequency centered in the telemetry channel band. The initial offset DAC count is set by an auto-zero process when the instrument is reset.

A wide range of options are possible because the microcontroller can be programmed to change the way the instrument operates. There is a real time clock in the controller and periodic test routines can be triggered. A second ADC channel is supported with two uncommitted op-amps and provision for a second sine wave generator so that a dual gain instrument can be easily constructed.

The instrument can be readily converted for serial data output by removing the sinewave generator, making some minor circuit board changes, and reprogramming the microcontroller. When digital telemetry arrives, this instrument can adjust to the change.

The following further information is available:

1. Specification Sheet
2. Circuit Description for the schematic diagrams
   
3. Schematic Diagrams at 144 dpi
   1. Sheet 1, POWIO.GIF (41K)
   2. Sheet 2, PREAMP.GIF (30K)
   3. Sheet 3, UCNTRL.GIF (33K)

   The same files are available at 300dpi for ftp downloading:(use 'gzip -d' or 'gunzip'; PC versions are available)

   1. Sheet 1, powio-2.gif.gz (128K)
   2. Sheet 2, preamp-2.gif.gz (87K)
   3. Sheet 3, ucntrl.gif.gz (90K)

Download this software to decompress gzip (gz) files on a Win95 or Win3 PC. There is also a MacGzip available elsewhere. Do a search. Read the text. Use the Web to its greatest.....