Download ISIS 4 INTEGRATED SEISMIC INSTRUMENTATION SYSTEM

ISIS 4
INTEGRATED SEISMIC INSTRUMENTATION SYSTEM
TELEMETRY SYSTEM
VLF DESIGNS
1621 Bella Vista Dr.
Jackson, Mo. 63755
PH:573-204-1286
[email protected]
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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
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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.
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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
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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 orings 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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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ISIS 4 POLES AND ZEROES
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