Recording of Time Marker Signals

Description of Decoder

A useful option to the seismometer system is the addition of highly accurate time-marker ticks to the seismic record. This is especially important if you are interested in correlating your data with other seismic stations for locating epicenters using triangulation methods. Unfortunately, the time-base of the best high-end computer is still completely inadequate to provide true, time-of-arrival data. The most expensive Pentium IV, 3-Ghz computers in 2007 are still being shipped with cheap, 29-cent, internal time-of-day clocks that simply cannot keep accurate time to better than 1 or 2 minutes per month. Something better is needed; so the following system was developed, which provides for unambiguous, always-accurate time-marks for our seismic recordings: The NIST operates an easy-to-receive short-wave time-signal service, broadcast by radio station WWV, in Fort Collins, Colorado. The transmitted time-signals contain various tones that can be used to automatically insert markers onto the seismogram, as it is being recorded. Any short-wave radio, such as those available at Radio Shack, that can tune to either 2.5, 5.0, 10.0, 15.0 or 20.0 MHz, can be used to receive WWV. Tune the radio to the frequency that gives the best reception. The audio output of the receiver's recorder or earphone jack is used to feed the following circuitry: The first stage of the time-marker decoder is a simple op-amp that allows us to establish a 200-mV p.p. audio signal for the decoding circuits. This gain of this amplifier stage is adjustable to compensate for the differences in audio levels available from different short-wave receivers. The output from the op-amp is then fed, simultaneously, to three NE-567 tone-decoder integrated circuits. The first decoder is tuned to 440-Hz (to provide for the 45-second-long marker beginning at 2-minutes past each hour, except the first hour). The second IC is tuned to decode the 1.5-kHz tone during the first second of each hour; and the third IC decodes the 1.0-kHz tone during the first second of each minute other than the first minute. The ICs are amazingly immune to falsing from noise on the received signal. In general, if you can hear the tones from the radio at all, the decoder ICs will properly respond to them. Remember, the transmitted tones are based on Coordinated Universal Time (UTC) -- formerly known as Greenwich Mean Time. The 440-Hz tone, normally sent during the second minute of each hour, is not sent during the first hour of each day -- thus indicating (by its absence) the beginning of the next day, UTC. All three decoded outputs are connected in parallel (a logical OR circuit) and to an amplitude adjustment potentiometer to provide a slight offset pulse whenever any of the special tones are transmitted. The offset pulses are then connected to the output amplifier of the seismic preamplifier at the attachment point. Usually, the amplitude of the time pulses is adjusted to appear as about 3% of the full-scale seismic signal.

Schematic - WWV Time Marker Decoder