Integrated seismic instrumentation system


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ISIS 4

INTEGRATED SEISMIC INSTRUMENTATION SYSTEM

TELEMETRY SYSTEM

VLF DESIGNS

1621 Bella Vista Dr.

Jackson, Mo. 63755
PH:573-204-1286

vlf@cablerocket.com

TABLE OF CONTENTS

ISIS 4 OVER VIEW 3
ISIS 4 SPECIFICATIONS 4
FUNCTIONAL BLOCK DESCRIPTIONS
ISIS CANISTER I/O CONNECTIONS 5
BASEPLATE 6
SEISMIC PREAMPLIFIER 7-8

VOLTAGE CONTROLLED OSCILLATOR (VCO) 8-10

POWER SUPPLY, REFERENCE OSCILLATOR 10-11
INPUT, MULTIPLEXER BOARD 12-13
SYSTEM ADJUSTMENTS 14
ALGEBRAIC SYSTEM RESPONSE 15
POLES AND ZEROES RESPONSE 16
ISIS 4 OVERVIEW
The ISIS 4 hardware represents a redesign of the field hardware for triaxial analog short period monitoring. The circuit boards have been reduced in size while retaining the crystal stabilized VCO design along with digital carrier signal generation and a higher efficiency isolated DC power supply.

All capacitors are either mylar, ceramic or high temperature low leakage electrolytic in order to maximize longevity of the circuitry. All pc board connections utilize gold plated connectors to prevent interconnection problems due to oxidation.

A lower noise preamplifier with 30 sec low frequency cutoff insures that the system will work well with 1 Hz seismometers even in seismically quiet locations.

The inclusion of a high efficiency isolated regulated power supply allows the system to remain fully functional with input supply voltages as low as 11.5 VDC, and as high as 14.8 VDC. Power consumption has been reduced to less than 170mW from a 12volt supply. Galvanic isolation of signal ground from power ground minimizes the creation of ground loops and further protects the connected sensors from damaging transients.

All power and output signal are made via a single five pin circular connector. A single seven pin input connector is used for all three signal inputs

The entire package is housed in a polished stainless steel cylinder capped with PVC end caps and o-ring seals for watertight operation to a depth of 10 ft.

______________________________________________________________________________

Copyright disclaimer:

This manual may be copied in full or in part by the equipment owner or user without written permission from the manufacturer. VLF Designs retains the right to make improvements or changes in the equipment design as needed. If any additional information is required by the equipment user, please contact VLF Designs by phone or email.

ISIS 4 SPECIFICATIONS
PREAMPLIFIER

INPUT IMPEDANCE: 18.2KΩ (seismometer damping resistance)

INPUT NOISE: <0.4µv r.m.s.

OUTPUT VOLTAGE: 10Vp-p

GAIN ADJUSTMENT: 6Db steps, 54,60,66,72,78 dB

OPTIONAL GAIN RANGING: -30Db ONE STEP WITH 2 SEC POST TRIGGER DELAY

HIGH PASS FILTER: 1 POLE 0.0328 HZ

LOW PASS FILTER: 6 POLE, 25 HZ STANDARD, (50 OR 100 HZ OPTIONAL)

LOW PASS FILTER TYPE: BUTTERWORTH STANDARD, (BESSEL OR OTHER OPTIONAL)
VCO

TYPE: CRYSTAL STABILIZED, HIGH LINEARITY

FREQUENCIES: 750 HZ TO 4500 HZ (375 HZ SPACING) STANDARD,

FREQUENCY DRIFT: < 0.1 HZ

DEVIATION: ±140 HZ STANDARD, OTHER DEVIATIONS AVAILABLE

FREQUENCY JITTER: 0.01 Hz OR LESS

MODULATION LINEARITY: 1 % OR BETTER

RESIDUAL AM: < 1%

BANDWIDTH: .02 to 200 HZ

CARRIER DISTORTION: <0.05% THD OR LESS

OUTPUT AMPLITUDE: 1Vp-p, FIXED
POWER SUPPLY

OPERATING VOLTAGE RANGE: 11.5 TO 14.9 VDC

OPERATING CURRENT: < 16mA.

REGULATION: 1 mV or better

PROTECTION: RF SUPPRESSION, 2 STAGE TRANSIENT SUPPRESSION
INTERNAL MULTIPLEXER

TYPE: 3 CHANNEL, INDIVIDUALLY ADJUSTABLE (±3Db), TRANSFORMER COUPLED

GAIN RANGE SIGNAL: +100% AMPLITUDE OF CENTER CARRIER

OUTPUT LEVEL: 0.5 to 6Vp-p, ADJUSTABLE

OUTPUT IMPEDANCE:50Ω, (600Ω AVAILABLE)

DISTORTION: 0.05% OR LESS

TRANSIENT SUPPRESSION: 2 STAGE
PHYSICAL SPECIFICATIONS

DIMENSIONS: 3” DIAMETER X 13” LONG

CASE: TYPE 304L POLISHED STAINLESS STEEL WITH IMPACT RESISTANT PVC END CAPS

OPERATING TEMPERATURE: -20˚ TO +60˚C

HUMIDITY: 0-100% RH

WATERPROOF: TESTED TO 10FT IMMERSION, SINGLE O-RING SEALED,

CONNECTORS

INPUT MATING CONNECTOR: MS3106A-16S-1P

OUTPUT MATING CONNECTOR:MS3106A-14S-5P




ISIS4 Canister



The ISIS 4 canister consists of an alloy 304 stainless steel cylinder with PVC end caps. O-ring seals are used to make the canister waterproof to at least 10 feet. The o- rings should be coated with a thin film of grease to allow them to easily slip into the cylinder.

The canister may be entered by unscrewing the 2 machine screws on the output end of the case and then gently pulling the top cap upward. The signal cable may then be unplugged from its connector on the input multiplexer circuit board. The cylinder may be removed by gently pressing down on the two spacers and withdrawing the board stack from the bottom of the cylinder. Whenever the system is opened for servicing, the o rings should be cleaned and re-greased. Four small desiccant packs in the cylinder should also be replaced at this time as well.

Short wires routed behind the preamplifier pc boards connect the sensor input signals and shield to the baseplate.
I/O CONNECTIONS
Input output connections are shown in the drawings below. The shield connections are the same as case ground. Case ground is however galvanically isolated from power common.


ISIS 4 Baseplate
The baseplate couples signals from the external sensors to the preamplifiers as well as delivering power to all the boards in the ISIS 4 system. The baseplate also couples various I/O signals from the power supply, preamplifiers, and VCOs. There are no active components on the baseplate.






ISIS 4 PREAMPLIFIER




The preamplifiers amplify the input signal from the external sensors to a level high enough to overcome the noise in the telemetry system channel. Normally this noise is about 66dB below clipping level or about 5mV p-p. The amplifier also band limits the signal as part of the anti aliasing system and blocks amplifier induced dc offsets.

Two diodes placed in opposing directions across the input provide transient suppression for input signals greater than ± 500mV, thus protecting the amplifier input stages from overvoltage damage that could occur during large seismic events. The minimum gain of the amplifier is sufficient to insure that these diodes do not conduct under normal conditions.

The preamplifier, U301, is a non-inverting design utilizing an OPA277 low noise op amp in the front end gain stage. A four position switch, S301, provides adjustment of the preamplifier gain in 6 dB steps from 24 to 48 dB. Only one switch is normally closed at a time to select a specified gain value. If none of the switches are closed, the default maximum gain of 48 dB is selected. The total system gain will be the value selected plus 30dB.

Gain switch settings

1 closed 54dB

S2 closed 60dB

S3 closed 66dB

S4 closed 72dB

All open 78dB

Following the front end gain stage is a unity gain 6th order 25 Hz Butterworth (maximally flat) low pass filter, U302. Other cutoff frequencies and filter shapes are available as an option when the equipment is ordered. This filter provides the high frequency band limiting to the signal and thus is used as a portion of the anti-aliasing filter when the data is later digitized. Full response information for this filter is provided in the section of the manual for telemetry system frequency response.

The output of the low pass filter is passed through a single stage high pass filter to block DC offsets and used as the input to the final 30dB amplifier. An offset current nulling adjustment is provided to zero the offset of the final amplifier stage.





ISIS 4 Voltage Controlled Oscillator (VCO)




The VCO is a linear frequency modulator used to convey the baseband seismic information for each component of the seismic signal. The VCO’s are similar to those used in the previous version of the ISIS hardware although these units have a lower corner frequency so as to produce negligible distortion with 1Hz seismometers.

The output signal from the seismic preamplifier is routed through a removable jumper to a variable input attenuator R 402. This attenuator is made variable so that the exact conversion sensitivity of the VCO can be set. In the standard ISIS system a ±5.000V signal causes a deviation of ±140.0 Hz. The jumper is only removed when the VCO is being calibrated while installed in a canister containing preamplifiers.

The signal from the variable attenuator is summed with the error correction signal and applied to the frequency control input of the VCO. There are 2 components to the error signal, feedback from the phase comparator/ integrator and a static signal derived from trimpot R405 that serves to set the center frequency of the VCO to the desired value while maximizing the bidirectional correction range of the feedback signal. A positive voltage at the output of summing amplifier U401 causes an increase in the frequency of the VCO and vice versa. Several of the resistors in amplifier U401 are made variable so that the effect of the trimpots and error correction voltage is constant regardless of VCO operating frequency.

The VCO consisting of U402 and U403 and their associated components actually operates at 10 times the desired output frequency. R414 provides linearity correction to the VCO by correcting the error caused by the frequency compensation capacitor inside amplifier U403. C401 is a high stability silver mica capacitor to minimize temperature drift and component aging.

The output of U403 is approximately a 50% duty cycle square wave. It drives divide by 10 counter U405 whose outputs are summed by a resistor network and U406A to produce a low distortion stepped approximation to a sine wave. Higher order harmonics of the square to sine conversion process are further reduced by low pass filter U406B. The corner frequency of this filter is set at 1.68 times the highest operating frequency of the VCO so that no appreciable amplitude modulation of the carrier occurs during routine frequency modulation of the carrier. After filtering, this output signal from the VCO is sent to be multiplexed on the input multiplexer board with carriers from the other VCOs in the system.

The output of U403 also drives a dual counter U404 in the feedback loop. The input signal is divided by 4 in the first stage of the counter. The second stage is programmable and is a divide by N, where N is an integer between 2 and 12. A table on the schematic lists the output frequency as a function of N. The effect of this division is to produce a constant (without seismic modulation) output frequency from the counter of 937.5 Hz. The output signal from the counter is compared with a 937.5 Hz signal derived from a crystal oscillator on the power supply board by a phase comparator, also inside U404.

If the frequency of the VCO is lower than the reference frequency, the phase comparator in U404 outputs a string of positive pulses. These pulses are integrated by long period integrator U 401. Because U401 is naturally inverting, its output will slowly ramp in a negative direction in response to the positive pulses. This signal is fed into the inverting input of summing amp U401, which causes the output to slowly ramp in a positive direction thus raising the frequency of the VCO. This action will continue until the frequency of the VCO after division just equals the frequency of the crystal reference oscillator, at which time the output of the phase comparator will cease putting out any pulses and transition into a high impedance state. Had the frequency of the VCO been higher than the reference, the phase comparator would begin putting out a string of negative pulses in order to drive the circuitry in the opposite direction to attain balance.

Integrator U401 has a long time constant (.02Hz) compared to the baseband seismic signals that modulate the VCO normally , so that when the VCO carrier frequency changes in response to the baseband seismic signal, (which has an average value of zero) the error voltage at the output of the integrator does not appreciably change and the VCO operates around its normal center frequency. However, signals with a slow rate of change, such as temperature induced drift and component aging will cause the error voltage to change in order to maintain the desired center frequency.

The VCO board is symmetrical and care should be taken to install it correctly in the telemetry system. The “TOP” designator written on the PC card indicates that this end of the VCO card should face the top cap of the telemetry system package.

ISIS 4 Power Supply, Reference Oscillator

The power supply portion of this board performs two functions, regulating the input voltage and synthesizing an analog ground. In addition, this board contains the crystal oscillator used to stabilize the VCO carrier frequencies.

The power supply contains two low drop out style series regulators. The first regulator, regulates the incoming voltage to 11.000VDC. It will maintain full regulation with input voltages as low as 11.5 VDC. A portion of the output voltage is selected by trimpot R106 and compared to precision reference U103. The output of error amplifier U101A directly controls the conduction of P channel mosfet Q101 to set the output voltage.




R104 selects precisely one half of this voltage to generate an analog ground used by the circuitry inside the sensor canister. It is important to note that analog ground is 5.500V above input voltage (battery) ground. Analog ground test points on the various PC boards are denoted with green sleeves on the test points. Battery ground test points are denoted with black sleeves. Accidental shorts between the two potentials will not damage the system, but will cause the system to stop functioning.

The reference oscillator is configured around a CMOS oscillator/binary counter chip U102. The oscillator frequency of 60 KHz is determined by X101, a miniature tuning fork crystal operating at 60.000Khz. The 60Khz is then divided down by 64 by binary ripple counters to an output frequency of 937.5 Hz. This frequency is the reference frequency used by the phase comparators in the VCO’s for frequency stabilization.

This PC board is symmetrical (like the VCO boards) and can be installed upside down. The board is correctly installed when TOP (printed on the board) faces the top of the isis3 canister. No damage to the board or system will occur if the board is inserted incorrectly, however the input power will be shorted out, causing the protective polyfuse in the radio canister to open.



ISIS 4 Input Multiplexer
The input multiplexer circuit board contains the circuitry to frequency division multiplex the output signals from the individual VCO’s into a composite tone bundle. This board also has an isolated dc power supply and transient suppression for all I/O signals.



Power and audio output signals are coupled from a five pin connector to the input multiplexer board via a short umbilical cable. The plus power connection to the canister can come in via a separate pin on the input connector or via the center tap of the audio coupling transformer when used in a balanced line with dc power configuration. As supplied from the factory, the pc board will be configured for balanced line operation. If this configuration is not desired it will be necessary to cut a short jumper wire (CT) adjacent to the coupling transformer.

Varistors are installed on both audio input lines relative to power common. An additional varistor is connected between power common and analog ground to protect the dc to dc converter from excessive common mode voltage. This varistor conducts at approximately ±200 VDC. A tranzorb rated at 15 VDC is connected across the input terminals of the isolated dc to dc converter. A small capacitor is connected between power common and analog ground to reduce common mode noise arising in the dc to dc converter module.

The audio output signals from the individual VCO’s are coupled through individual trimpots to the input of a summing amplifier gain block. The trimpots allow for up to ± 3 dB of adjustment in the relative amplitudes of the carriers in order to allow equalization of the signals when they are transmitted through radio channels. An additional trimpot in the feedback loop of the summing block allows overall composite level changes without affecting equalization concerns. The range of this pot is ± 6dB.

Following the summing block is a unity gain inverting amplifier which allows the primary of the coupling transformer to be driven in true differential form. Clamp diodes on the outputs of both amplifiers prevent damage to the op amps from any transients that make it past the input varistors.

Normally the output impedance of the multiplexer is approximately 50 ohms. If the telemetry system is used to directly drive a phone line, R511 can be replaced with a 536Ω resistor so that the output impedance is 600Ω.


ISIS 4 System Adjustments

Perform the adjustments in the order given to maximize the accuracy of the calibration procedure. The power supply board must always be adjusted first because many of the subsequent adjustments use the power supply voltages as references.

Power Supply Adjustment

1.) Apply input voltage to the power supply board in the range of 11.5 to 13 VDC. The input power may be applied between TP503 (+12, red) and TP504 (com,blk) on the input multiplexer board. The input voltage value is not critical.

2.) Adjust R106 to produce 11.000 ±.002VDC between TP101 (red) and TP102 (blk).

3.) Adjust R104 to produce 5.500 ±.002VDC between TP 103 (grn) and TP102 (blk).

4.) Verify that the crystal reference oscillator is outputting a 937.50 ± .01 Hz output on pin 4 of U102.

This completes the adjustment of the power supply board.
Preamplifier Output Offset Adjustment

1.) Short Preamplifier input by installing a jumper clip lead across D302.

2.) Adjust R320 for 0.000 ±.005VDC between TP302 (wht) and TP301 (grn).

This completes adjustment of the preamplifier.
VCO Adjustment

1.) Remove shunt H401 installed on header J401.

2.) Place an oscilloscope probe between TP402 (yel) and TP405 (grn).

3.) Place a jumper clip lead between TP403 (dk brw) and TP 405 (grn).

4.) Adjust R405 for a slowly time varying signal with the absence of either positive or negative going pulses. This adjustment is not critical.

5.) Remove the jumper clip installed in step 2. Pulses may return to the signal on the oscilloscope, however, the signal should settle to a flat line near ground level after a minute or so. Remove the oscilloscope probes after this condition has been verified

6.) Connect a signal generator to TP401 (wht) and TP405 (grn). Set the output of the signal generator to produce a sine wave at 58.220 ±.001Hz with an amplitude of 10.00Vp-p.

7.) Connect an audio spectrum analyzer (or scope with FFT function) between TP404 (blu) and TP405 (grn).

8.) Adjust R402 to null out the center frequency carrier component in the spectrum display.

This completes the adjustment of the VCO board. There are no further adjustments in the ISIS 3 canister.
Multiplexer adjustment

1.) Adjust the individual VCO trimpots, R501, R502, R503 to the their center position. The individual trimpots can be adjusted slightly so that all VCO carriers are equal in amplitude. Adjust the composite level trimpot R508 to produce the desired output level. For a radio link, this would be 2.5Vp-p or 550mVrms. The multiplexer output level can be observed on the yellow test points (TP501 and TP502).

Individual carrier amplitudes can be further adjusted so that the levels observed at the receiver end of a radio link are approximately equal. Once this has been done, the composite level can be readjusted to produce the required output level.
ISIS 4 RESPONSE

These are the response curves for the ISIS 4 field hardware. In order to get a total system response, they should be convolved with the discriminator and recorder responses.





ISIS 4 POLES AND ZEROES


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