GB2144287A - Analog-to-digital converters for seismometers - Google Patents

Abstract

The analog output signal of a seismometer SM is compared in a comparator COMP with the output of a ramp signal generator RG to produce pulses of width proportional to the instantaneous amplitude of the output signal as determined by pulses from a clock CK via a gate G. The number of clock pulses, as counted by a multi-bit counter CT, provides a digital output signal. The variable-width pulse from comparator COMP is also applied to a converter PC to switch positive and negative precision voltage sources (PVS1) and (PVS2) (Fig. 3) into a low-pass filter which thereby provides an analogue feedback signal to the seismometer SM. <IMAGE>

Description

SPECIFICATION Improvements in or relating to seismometers and to analog-to-digital converters suitable for use therewith This invention relates to seismometers and to analog-to-digital converters suitable for use with low frequency analog signals produced byseismometers.

This output signal of a seisometer is an electrical signal ofamplitudevarying with time. The more sensitive seisometers of the feedback type, e.g. the Ouraip device, UK patent application 2017303 A, have an output signal amplitude range with a theoretical maximum of some 140 dB. The frequency range of the amplitude variations is from d.c. to some 1 OHz or more. It is necessary to record the signal for off-line study. At present the signal has to be recorded, using a tape-recorder of special form, in several frequency bands as it is not possible to record over the whole band-width with a single-band apparatus.One limitation is that conventional recorderamplifiers do not have sufficient dynamic range for the low frequency signals, say periods of some 20 seconds. This multipie band recording technique is inconvenient and wasteful.

It is one object of the invention to provide a seismometer arrangernentwhich avoids the need for multiple band recording.

According to the invention there is provided an arrangement for a seismometer of the type including a feedback loop including in thefeedbackloop means responsive to the seismometer output signal to producefrom time to time an electrical pulse of a width proportional to the value ofthe amplitude of the output signal atthe time, means to produce regularly electrical impulses and means to determine, and if required indicate,the width of the pulse in terms of a countofthe numberofthe regularimpulses,thereby providing frorn timeto time a numerical value of the amplitude of the ou.pu signai atthetime as a single signal for the frequency range ofthe output signal.

Conveniently the pulse of width proportional to amplitude value is produced by comparison of the amplitude with a stable ramp signal.

Preferably the feedback loop includes means to convert the pulse width to a feedback signal which does not depend on the amplitude ofthe pulse.

In one form the means to convert pulse width includes switching means controlling a pair of precision voltage sources and responsive to the pulse amplitudeto supply one or other precision voltage as the amplitude of the feedback signal and a low-pass filter to remove hig h4requency components from the feedback signal.

The countofthe outputsignal amplitude may bea multi-bit binary of say 20 bits, to represent the instantaneous value ofthe amplitude. The arrangement may include means to average the value of the count.Theelectrical pulse maybe producedata rate of about 20 times a second.

Also according to the present invention an analogto-digital convertersuitable for use with a low frequency analog signal produced by a seismometer comprises a stable repetitive ramp signal generator, meansforcomparing the amplitude ofthe analog signal with that of the ramp signal and detecting the instant when the two signals are of equal amplitude, means for producing regular electrical impulses, and meansfordetermining the amplitude ofthe analog signal at each instantof detection asthe numberof regular electrical impulses occurring between the start of each ramp signal and the corresponding instant of detection.

Embodiments ofthe invention will now be described with reference to the accompanying drawings in which: Figure lisa block schematic diagram of a seismometer embodying the invention, Figure 2 is a partly schematic circuit diagram of a ramp generatorfor use in the circuit of Figure 1 and Figure 3 is a partly schematic circuit diagram of a precIsion cornparatorfor use in the circuit of Figure 1.

In Figure 1, a seismometerSM,which may be of the type disclosed in UK published patentapplication 2 017 303 A, is arranged to provide en output OP of a signal having a waveform whose frequency and amplitude represents seismic activity. Such a waveform typically has a frequency range from d.c. to about 10 Hz, or even up to 20 Hzf and 2 dynamic amplitude range of unto 140 dB above the noise level.

A ramp signal generator RG is arranged to generate a voltage ramp to have the same dynamic range, and absolute level, in this instance symmetrical about zero, as the expected analog waveforms representing seismic activity. The voltage ramp is arranged to have a repetition rate of about Hz, i.e. above the highest expected or required frequency in the waveform. The ramp generator RG is described in more detail below.

The seismometer output OP and the ramp voltage RV are compared in a comparator COMP The comparator produces one output signal of a pulse signal DO which starts with the beginning ofthe ramp and ends when the ramp voltage, atthat instant, equals the waveform amplitude, at that instant. The individual pulses in DO will thus vary in length. The ramp generator RG produces an output signal STA when the ramp begins and the comparator a further output signal STO when the ramp equals the waveform amplitude and the pulse is to end. A gate G is responsive to the signals STAand STO and to a clock pulse signal from a clock CKto supply a train of clock pulsesthroughouttheinterval between signals STA and STO. The clock rate is 16 MHz in this example. The number of clock pulses is counted in a 24-bit counter CTto provide a digital output signal, DOP, of a digitally represented binary number which thus represents the amplitude of the seismic waveform at an instant in time. The amplitude is conveniently represented by a 20-bit value to give sufficient resolution for a 120 do dynamic range with a 16 MHz clock. Higher bit-count values areto be expected at a higher clock rate. The output signal DOP is repeated at the ramp repetition rate.

Returning to the comparator COMPthe output pulse DO is applied to a precision convertor PC including a limiter and low pass filter. This converts the output pulse signal to provide a feedback signal to be applied tothefeodbacktransducer FBT of the seismometer SM. This stabilises the seismometer and permits an analog signal reflecting the action of the feedback loop to be available at AOP. The converter at AOP. The converter PC is described below in more detail.

If required, the digital value DOP can be averaged in an averagerAVovera number of values, say 16, to provide an averaged signal ADOP. This will reduce the noise level for long-period seismic application, where a lower conversion rate is acceptable. The averager AVcan boa microprocessor.

In one p, rticular embodiment the overall response of the saismometer with a 20-bit digital output is some 0.05 Hzto 10 Hz providing a selected 120 dB range of amplitude. The exact values wili depend on the clock rate and sampling rate selected and the type of averagerAV used. Noise problems can arise if an amplitude range of 140 dB is used. A resolution of 1 in 2 x 1 0s is obtainable with the arrangement described.

Figure 2 shows a suitable ramp generator, RG in Figure 1, in more detail. The ramp generated must have good linearity and low noise level to ensure reliable and accurate analogidigital conversion of the seismometer signal.

The ramp generator in the illustrated embodiment is based on a Wilson current source. This source has a very high output impedance astransistort2 provides negative feedback and raises the output resistance.

The source isthereby also less sensitiveto transistor "beta" asfirst-order cancellation of the base currents is achieved. A reference currentflows in resistor R.

The current source controls the change of charge in capacitor Cto produce atthe emitter of transistorT3 the required voltage ramp. The starting ofthe ramp is controlled by a switch, of integrated circuit IC1 of J-f.e.t. type, driven by an oscillator OSC of any suitable form to give the required 20 Hz repetition rate for the ramp RV, as discussed above. The voltage ramp atthe emitter of T3 is buffered by an operational amplifier, IC2, ofthe f.e.t. type, to produce the ramp signal RV at a suitable impedanceforthe comparator COMP in Figure 1.

Figure 3 shows a suitable precision comparator PC for the feedback signal applied to transducer FBT of seismometerSM (see UK patentsspecification 2 017 303 A). The feedback signal in this embodiment is converted from digital to analog form, actually a slowly varying d.c. potential, using the transitions of the digital signal DO to control the switching of positive and negative precision voltage sources into a low-pass filter.

In Figure 3 one precision voltage source (PVS1) is shown in detail,the other (PVS2) is similar. The precision voltage has a value of 10 volts d.c. and is determined by a voltage reference device type LM399.

This device include a zener diode and an on-chip temperature controller (a heating element) as shown schematically;n the box outlined in the drawing. The 6.9 volts ofthe zener diode are converted into an exact 10 volt level bythe associated circuit elements. The operational amplifier IC3 isolates the source from the effect of changes of load and produces a very low output impedance. The output also energises the zener diode in the voltage reference device through the 5K resistor in the loop around IC3. In this way the zener diode current is held very nearly constant as the supply potential is the precision output voltage which is a constant 3 volts d.c. potential above the zener cathode potential.The source can achieve a noise performance of 10 microvolts peak-to-peak over the important part of the seismic spectrum.

The +1 Ovolts d.c. source PVS1 and the similar -10 volts d.c. source PVS2 are connected to integrated circuit IC4which is formed of J-f.e.t. devices and arranged as a switch to permit the connection of one or other ofthe sources to a low-pass filter LPF and thence to the feedback transducer FBT. The control for the switching operation is the signal DO, ie source PVS1 is connected during the successive intervals of DO from STAto STO and source PVS2 is connected during its successive intervals from STO to STA.

In this way the pulse width of the ramp-derived signal DO is converted to d.c. for the feedback path with the required low noise level to stabilise the seismometer.

By converting the seismometer signal to a digital form the problems of analog recording of wide dynamic-range low-frequency signals are avoided.

Instead of having a record the signals in separate frequency and amplitude bands the whole signal can be in a single record which opens up the possibility of much better processing of the signal to reveal details of the seismic record.

Claims (9)

1. A seismometer ofthe type including a feedback loop including in the feedback loop means responsive to the seismometer output signal to produce from time to time an electrical pulse of a width proportional to the value of the amplitude of the output signal atthe time, means to produce regularly electrical impulses and means to determine, and if required indicate, the width of the pulse in terms of a count of the number of regular impulses, thereby providing from time to time a numerical value of the amplitude of the output signal atthetime as a single signal forthefrequency range of the output signal.

2. Aseismometeras claimed in claim 1 wherein the pulse of width proportional to amplitude value is produced by comparison of the amplitude with a stable ramp signal.

3. A seismometer as claimed in claim 1 orclaim 2 including means to convert the pulse width to a feedback signal which does not depend on the amplitude ofthe pulse.

4. A seismometer as claimed in clause 3 wherin the means to convert the pulse width includes switching means controlling a pair of precision voltage sources and responsive to the pulse amplitude to supply one or other precision voltage as the amplitude of the feedback signal and a low-pass filter to remove high-frequency components from the feedback signal.

5. A seismometer as claimed in any preceding claim including means providing said count ofthe amplitude of the output signal as a multi-bit binary numberto represent the instantaneous value of the amplitude.

6. A seismometer as claimed in claim 5 including means for averaging the value ofthe count.

7. An analog-to-digital converter suitable for use with a low frequency analog signal produced by a seismometer comprising a stable repetitive ramp signal generator, meansforcomparing the amplitude ofthe analog signal with that of the ramp signal and detecting the instant when the two signals are of equal amplitude, means for producing regular electrical impulses, and meansfor determining the amplitude of the analog signal at each instant as the number of regular electrical impulses occurring between the start of each ramp signal and the corresponding instant of detection.

8. A seismometer substantially as hereinbefore decribed with reference to the accompanying drawings.

9. An analog-to-digital converter suitable for use with a low frequency analog signal produced by a seismometer and substantially as hereinbefore described with reference to the accompanying drawings.