Download J322X Replacement Seismic Telemetry System Rev B 08/2014 VLF

J322X Replacement Seismic Telemetry System
Rev B
08/2014
VLF Designs
1621 Bella Vista DR
Jackson, MO.63755
573-204-1286
[email protected]
J322 SYSTEM OVERVIEW AND FEATURES
The J322x represents the next generation of the J3 series of seismic AMP/VCO systems. The
system was designed to replace the J302 in response to severe component aging issues and
obsolescence of components in the original design. Special attention was paid to minimizing
current consumption, lowering preamp noise and improving VCO stability and output distortion.
Furthermore the design was simplified from the original concept and diagnostic features that were
no longer used have been removed from the current version. The existing front panel, gain switch
and tubular case are being reused in this version to minimize cost.
The new VCO system utilizes a phase locked modulator referenced to a crystal oscillator so
that frequency drift is no longer a problem. The VCO utilizes a plug in daughter board so that any
power supply/amplifier card can be used with any VCO card without the need for recalibration of
the VCO. VCO over modulation has been completely eliminated by the use of rail to rail opamps in
the preamp stages. The VCO output has been designed to work into a 600Ω load, however
operation into higher impedance loads where possible will lead to significantly greater battery life
with no degradation in signal quality. The low pass filter in the output stage has been improved and
overall distortion is now less than 0.4%
The calibration system injects a current pulse into the seismometer main coil, and through
differential techniques removes the static voltage caused by the current pulse from the resulting
waveform so that a normal cal pulse signal is generated similar to those that would be seen if a
separate seismometer cal coil was available. Both upward and downward movements of the mass
are seen using this technique. This signal has a small adjustment range to allow for a normalized
±1V peak signal to be obtained.
System response has been designed to match the previous J302 system. Some individual
gain constants have been slightly reduced to account for higher sensitivity in the VCO modulation,
however, the net effect to the overall system response is to exactly match the existing system
response. No changes to metadata files are required.
The input impedance has been set to 10KΩ so that damping and gain setting resistors ( S
and T values) for the seismometers will remain unchanged. These resistors are now mounted to a
miniature terminal block on the pc board, which allows for easy change in the field without
soldering the parts to the board.
Input and output connections utilize the same terminal block as before and are arranged in
identical positions.
Test points have been added to the board to facilitate current measurements, nulling and
other adjustments. A high stability low dropout regulator is used after the battery to eliminate all
drifts from battery voltage change over time while fully utilizing more of the available capacity of
the battery. Low resistance shunts in series with the batteries allow for simple operating current
measurements using only a voltmeter. Self resetting polyfuses protect the batteries from
hazardous discharge currents that could be caused by component failures.
Sockets have been provided for easy replacement of chips that may be damaged by
transients.
Copyright Notice
This manual may be copied whole are in part by the system owner at any time without notification
to VLF Designs.
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J322X Replacement System Specifications
Preamplifier
Low pass filter: 2 single poles 48.2Hz and 49.8 Hz (match existing)
1st Stage gain: 48.9dB (280)
2nd stage gain: 42.1dB (128) (48dB attenuation step unused as in previous version)
Attenuator: 0-48dB 6 dB steps (match existing)
Input impedance: 10KΩ or greater
S and T resistors: resistors attached to euro style terminal block on PC board
Input: Single ended, transient suppressed, RF filtered
Input voltage clip level: maximum 0.5Vp-p (transient suppression)
Input Noise: 1.1uV p-p, Max.
Adjustments: Gain, 1st stage null, 2nd stage null
Calibrator
Calibration: bipolar pulse type, amplitude adjustable, automatically scaled to attenuation setting,
injected into seismometer main coil,
Pulse duration: 8 Second(other values may be specified)
Ouput Pulse Amplitude ± 1V (nominally adjustable ±0.5 to ±2V)
Cal repeat: fixed once per day
Cal amplitude: ± 2V Peak at monitor test Point(other values may be specified)
Cal reset: momentary PB switch, initiates cal sequence and resets cal clock
Adjustment: Cal reset, cal amp, cal null
VCO
Output frequencies 680-3060Hz (standard IRIG)
VCO type: PLL stabilized 50sec loop
Reference oscillator: 25.6Khz/ 3400Hz
Frequency stability: 0.1Hz or better (-10 to 50˚C) (no temperature compensation required)
Initial settling time after power up: 5 minutes or less, (fast settle switch for more rapid settling)
Deviation: ±3V/±105Hz (fully hard limited)(compensates reduced gain in preamp)
Residual FM noise: .05Hz or less
Output amplitude: 0-4 Vp-p (unloaded) 0-2Vp-p 600 ohm load
Output impedance: 600 ohm
Output distortion: 0.4% or less
Output waveform: synthesized, low distortion sine wave
Output style: transformer coupled, floating secondary
Residual AM: 0.1% or less
VCO Mounting: plugs into main board, any VCO may be used with any base system without
Recalibration
Transient suppression: single stage, common mode, 12Vp-p to ground, 6Vp-p differential
Adjustments: PLL center, deviation, output, fast settle switch
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Power Supply
Power supply: ±3.65VDC
Regulator: ±3.000 VDC ±.002V
Power source: “D” lithium battery: TL2300T ( 16.5AH) or TL5930T (19.0AH)
Battery current draw: 650uA typical or less(dependent on output level and loading)
Expected battery life time: TL2300T 25,000 Hour (2yr 10mo approx)
TL5930T 29,000 Hour (3yr 5mo approx)
Power switching: plug in shunt on board
Current measurement: 100 ohm shunt resistor in each battery line (10mV/ 100uA)
Battery protection: 100mA self resetting fuse (PTC polyfuse)
Mechanical
Case: existing J302 Anodized Aluminum Case 2-7/8” Dia. X 12” long
Input / Output connection: existing terminal strip
All connectors and sockets, except I/O terminal block, gold plated
Construction: Single PC board for: preamp, calibrator, power supply, battery attachment
Single VCO daughter board
Test points: color coded loop type for all major adjustment test points
Sockets: provided for all I/O chips
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J322X Theory of Operation
Power Supply
Two 3.65V lithium batteries supply power to the system. Headers J1 and J2 remove power
from the circuitry during storage and shipment. Poly fuses F1 and F2 (100mA.) prevent
catastrophic failure of the batteries should a short develop in the system. R40 and R41 are in series
with the batteries prior to the regulator and provide a convenient location to measure the current
consumed from each of the batteries independantly. The resistor values are scaled to produce a
1mV signal for every 10 uA of current flowing through them.
D5 is a precision 2.5V reference and is used to stabilize the output of the regulators. It
operates between the plus battery output and ground. The positive regulator U3b is a non inverting
series regulator with a gain of 1.2 and produces the +3.000V output. This output is inverted by U3a
to produce the -3.000 V supply, and will track the value of the + supply. Trim pot adjustments are
provided on both regulators to set the output voltages to exact values.
Reference oscillator
U4 is used as a crystal oscillator operating at a frequency of 25.6Khz utilizing a miniature
quartz tuning fork crystal. The oscillator is connected across only the negative portion of the power
supply in order to conserve power and balance out the current consumed by reference diode D5 in
the power supply. U4 also contains a binary divider string which divides the crystal frequency by
256 to produce an output of 100Hz. This signal is level shifted by Q1 to ±3V levels in order to
operate the remainder of the digital circuits.
The 100Hz signal drives the cal clock circuits U8,U9 and U10 which produce a one pulse per
day signal used to activate the daily calibration signals. The 100Hz signal also provides a reference
to a PLL consisting of U5,U6 and U7. This PLL translates the 100Hz to 3.4KHZ which is used as a
reference by the VCO circuitry. Test points are provided to verify the presence of the 6Vp-p 3.4Khz
signal (yellow TP) and the PLL error voltage (orange TP). The error voltage should be less than ± 1v
(The exact value is not critical and represents thermal drift in the PLL).
A test point has also been provided where the internal 1Hz clock signal can be overridden
with a 6Vp-p 4320Hz in order to verify the remaining clock circuits are working correctly.
The cal pulse is generated by U11b. The duration of this calibration signal can be changed by
varying the value of R57. The formula for the time is T=1.1* R*C, where C= 10uF. The cal pulse is
used to turn on analog switch U2. The amplitude of the cal pulse is controlled by R21. R4 is used to
vary the amplitude of the signal applied to the – input of the amplifier and thus used to null the DC
component of the cal pulse applied to the + input of the amplifier.
U11a generates a 1mS. reset pulse once per day, or any time switch S2 is pressed. A cal
pulse is produced when this switch is pushed and automatically every 24 hours thereafter.
Preamplifier
The preamp is very similar to that found in the J302. The main difference is that the relay
switching network at the input of the amplifier has been removed and the amplifier is single ended.
The original amplifier was (pseudo) differential, however its common mode rejection was heavily
compromised by the non symmetrical T network at its input. This amplifier processes the
calibration signal differentially to remove its DC component. The filtering effect of the 1st amplifier
represents a single pole low pass filter at 48.2 Hz. The output of this stage passes through a high
pass filter prior to the 6dB/ step attenuator. Placing the high pass filter at this location means that
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offsets in the first amplifier stage are not passed to the attenuator, thus minimizing asymmetrical
load currents from the battery.
The second stage is a fixed gain (42dB) amplifier containing a single pole low pass filter at
49.8Hz. The second stage amplifier is followed by a high pass filter, which couples the preamp
output to the VCO input. Both high pass filters are built from mylar type capacitors so that long
term drift is minimized. A jumper J3 can decouple the preamp from the VCO so that test signals
may be fed into the VCO without the preceding amplifier loading the signal.
Because the amplifiers do not have nulling pins, external nulling networks are used for each
stage. The amplifiers are very high input impedance devices with excellent offset specifications.
Lower noise preamp chips are available, however their quiescent supply current is too high for use
with battery powered equipment.
VCO Subsystem
The VCO system uses a phase locked modulator design. Very low frequency signals ( < .02
Hz., mainly temperature and component drift) are removed from the base band seismic signal used
to FM modulate the VCO carrier. The VCO, U3, operates at 10 times the output frequency so that
The output can be converted to a low distortion stepped sine approximation to a sine wave by
decade counter U5. The output of the sine converter is applied to a three pole linear phase low
pass filter to remove the remaining high frequency artifacts from the square to sine conversion
process. R10 varies the amplitude of the VCO carrier signal. The output transformer is capacitively
coupled to minimize distortion of the signal by DC currents in the primary winding of T1. The
output is configured to approximate an impedance of 600Ω by means of R25, R26 and the
resistance of the secondary of T1. D1, provides common mode protection to the output of the VCO
from transients. Additional tranzorb diodes (D3 and D4 on the main pc board) provide line to
ground protection as well.
The output of U3 is also coupled to a divide by N counter U4. The value of “N” is
programmed into the VCO by four jumpers. U4 divides the VCO carrier frequency by N to produce a
3.4 Khz signal which can then be compared to the 3.4Khz signal from the calibrator circuit. Both of
these signals are then applied to phase detector 2 inside U3. The output of the phase detector is
applied through an attenuator network to the input of a long time constant inverting integrator
U1b. A test point (yellow) has been provided to allow viewing the error signal from the phase
detector and is useful during the initial VCO setup. The output of U1b is applied to the – input of
the VCO input amplifier to counter the effects of temperature or component drift in the VCO.
Switch S1 can be used to short out the integrator capacitor to speed the settling of the VCO during
the alignment process. The switch is of limited usefulness in the field because it removes the error
signal generated by temperature drift in the VCO. The white test point (TP FB) is used to short the
input to the VCO and remove all error and modulation signals during the setup of the VCO system.
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J302X Operational Adjustments
The J302X should not require much adjustment in the field once a specific L-4 has been
selected and the necessary S and T resistors are installed. Note that the calculation of the S and T
resistor values is identical to the existing J302 system.
POWER
The system is normally shipped with the switch power jumpers in the off position. Before
any systems setup can be performed, move jumpers J1 and J2 from the “off” position to the “on”
position as indicated on the pc board.
PREAMP NULLING
Once S and T resistors have been installed on the terminal block, rotate the seismometer
from its normal operating position so as to block the coil, thus minimizing the seismometer output
during the nulling adjustments.
The first stage may be nulled, using the NULL 1 adjustment pot, R31. The output of the first
stage is on the white test point, (TP1). Ground is on the green test point (COM).
Once the desired gain setting has been set, the second stage amplifier should be nulled
using the NULL 2 adjustment pot, R33. The second stage output is available on the blue test point,
(TP2). Ground is on the green test point (COM).
Exact nulling is required only to maximize dynamic range. It will have a minimal effect on
battery life.
CALIBRATION PULSE ADJUSTMENT
Connect an oscilloscope and voltmeter between TP1 and COM. Depress the cal enable
switch. Adjust R21 (CAL AMP) to produce a cal signal of the desired amplitude(typically
500mVpeak). Adjust the CAL NULL pot R4 (R21)for minimum offset (0.0V) of the signal at TP1, while
the calibration step signal is present. This adjustment will be required anytime the system is used
with a different seismometer or when the values of the S and T resistor are changed. Any time the
cal enable switch is pushed, the internal 24 Hour clock will be reset and the next cal pulse will occur
24 hours later.
VCO output Level
VOC output levels are set based on what type of telemetry equipment is connected to the
system. For a 600Ω multiplexer, connect an AC true rms voltmeter across the output of the VCO
along with a 600 ohm load. Adjust R10 (Output Level) for the desired output level (typically -10dBm
or 245mVrms). Note voltage levels will be different if working into a different impedance load.
POWER CONSUMPTION
When all set up adjustments have been performed, the operating current of the system can
be obtained by measuring the voltage between the BLK and BRW test points adjacent to the
battery. This is the current from the (-) supply battery. In a similar fashion, Measuring the voltage
between the ORG and RED test points gives the current drain from the (+) supply battery.
The scaling of this measurement is such that 1mV=10uA, thus a reading of 65 mV would
indicate the system is drawing approximately 650uA of current. The currents should be
approximately equal in amplitude. Differences greater than 5uA should be investigated as they are
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indicative of a possible component failure. Current consumption is nominally 650uA with a -10dBm
VCO output level into a 600 ohm load, however higher frequency VCO’s draw slightly more current
than the low frequency ones and vice versa. The current consumption is highly dependent on the
amplitude of the audio carrier from the VCO and the loading of the VCO output. Significant power
can be saved by using as high impedance a load as possible with the desired output amplitude
from the system.
FULL SYSTEM CALIBRATION
Power supply
The power supply adjustments (or check) should be performed in the following order
before any other adjustments.
1. Move switch headers J1 and J2 to the on position to provide power to the system.
2. Adjust R36 (+3v ADJ) for 3.000 VDC between the yellow and green test points.
3. Adjust R42 (-3v ADJ) for -3.000 VDC between the violet and green test points.
Preamp
All preamp adjustments are done during the field setup after a seismometer and its
associated damping resistances have been installed on the terminal block.
Gain is most easily measured by using a calibrated function generator set to produce a 4Vpp sine wave at around 5 Hz. This signal is coupled to a precision step attenuator set to be the
inverse of the gain selected. The output of the amplifier should under these conditions
approximately equal the unattenuated (attenuator switch setting 0 dB) signal amplitude of the
signal generator. With no attenuation selected the maximum gain of the system is approximately
91dB.
Reference Oscillator
The reference oscillator sets the repeat time of the cal pulse and the base frequency of the
VCO. The oscillator is not temperature compensated and slight frequency changes have a minimal
effect on system operation.
1. Adjust C23 (Osc Trim) for 1.00000 Hz between the gray test point (1Hz) and green test
point (COM).
2. Verify that the 3.4000KHz is present between the yellow test point (3.4Khz) and green
test point (COM).
3. Verify that the voltage at the orange test point (Synth) is less than plus or minus 1 VDC
relative to common.
4. If it is desired to test the calibrator repeat sequence timing; connect a function generator
set for a square wave, 6Vp-p amplitude and a frequency of 4320 Hz to the grey test point (1Hz). The
cal lamp should light after about 10 seconds, stay lit for 10 seconds, go dark, then relight 10
seconds later.
VCO
The J322X can utilize any calibrated VCO board with any base board without recalibration.
This is made possible by the use of a highly regulated standardized power supply voltage. Because
this VCO is of a frequency stabilized design, routine adjustment of center frequency is not required.
The control loop for the VCO has a very long time constant (50Seconds) so that low frequency
seismic signals are not distorted by the correction process. The VCO will automatically adjust to the
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desired center frequency, however this process may require 5-10 minutes when the system is
initially powered up(especially if the site temperature is greatly different from the temperature of
thesite where the setup was performed. If desired, this process can be speeded up by depressing
the PLL reset push button on the VCO card. This will speed up the synchronization process to less
than 10 seconds.
Initial VCO Adjustment
1. Place a jumper lead between the white test point (FB) and the green test point (Com).
2. Adjust R16 (PLL Zero) to set the VCO to approximately the desired center frequency. If a
frequency counter is not available, center frequency may be determined by observing the pulses at
yellow test point (PC Out). R 16 is adjusted to the point where the polarity of the pulses at the
orange test point change to the opposite polarity, or cease completely.
3. Remove the jumper, press the reset switch. The VCO should adjust to the desired center
frequency in less than 10 seconds.
4. Deviation adjustment can be done using two different techniques. The most accurate
adjustment utilizes the Bessel Null technique, but requires an oscilloscope with FFT capabilities or
an audio frequency spectrum analyzer.
VCO Bessel null technique
The Bessel null technique uses the disappearance of the center frequency component in
the spectral display of the VCO output signal at full deviation and a specific modulating frequency.
The VCO modulating frequency is determined by dividing the desired deviation (typically 105 Hz) by
a value of 2.4048 to obtain the frequency that will modulate the VCO (typically 43.66Hz). The
amplitude of the modulating signal is set to the value that produces full scale deviation, (typically
6Vp-p).
1. Set the signal generator for a sine wave with an amplitude of 6Vp-p and a frequency of
43.6626Hz.
2. Remove jumper J3 from the main PC board (this disconnects the preamp out from the VCO
input).
3. Connect the signal generator between the blue test point (Mod in) and the green test point
(Com) on the VCO board.
4. Connect an FFT display device across the VCO output lines (top ends of R25 and R26).
5. Adjust R2 (Deviation)on the VCO board until the center frequency spectral line in the display
disappears.
Standard Technique (less accurate)
1. Remove jumper J3 from the main PC board (this disconnects the preamp out from the
VCO input).
2. Connect a frequency counter across the vco output lines (top ends of R25 and R26). The
VCO should be indicating its nominal center frequency.
3. Using the yellow test point (+3.000 V) on the main pc board, quickly connect it to the blue
(Mod In) test point on the VCO board. Read the deviation and adjust R2 for a frequency increase of
105 Hz above the carrier frequency. The connection and measurement must be made quickly,
otherwise the VCO correction loop will begin to correct for the step offset in frequency as drift and
start attempting to remove it. A further check of VCO operation may be done by now connecting a
jumper lead from the violet test point (-3.000V) on the main. The counter should now read
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approximately 105 Hz below the carrier center frequency. Exact deviation values of this
measurement are unlikely due to slight non-linearity in the 4046 VCO chip.
VCO Output amplitude
1. Connect an AC true rms voltmeter across the output of the VCO along with a 600 ohm
load.
2. Adjust R10 (Output Level) for the desired output level (typically -10dBm or 245mVrms).
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J322X Base Board Schematic
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VCOSchematic
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Component Location Diagrams
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