Boonton 75D User manual

DIRECT CAPACITANCE BRIDGE, MODEL 75D (1 MHz)
INSTRUCTION MANUAL
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BOONTON
ELECTRONICS
GOfRROfR/ATI O' Ni
TEL. [phone]
TWX: 710-986-8241
ROUTE 287 AT SMITH ROAD
PARSIPPANY. N J — 07054

TABLE OF CONTENTS
CHAPTER PAGE
I GENERAL DESCRIPTION 1
1 .1 General 1
1 .2 Equipment Description 1
1 .3 Optional Accessories 2
1 A Technical Characteristics 3
1 .5 Theory of Operation 7
II INSTALLATION 14
2 0 1 Unpacking and Inspect! on 14
2.1.1 Unpacking 14
2.1.2 Inspection 14
2.2 Packing for Reshipment 14
2.3 Power Connections 14
2.4 Rack Installation 15
III OPERATION 17
3.1 General 17
PART 1 . Preliminary Operating Instructions
3.2 Controls and Indicators 17
3.3 Starting Procedure 21
3.4 Initial Switch and Control Settings 22
3.5 Zero-Balancing the Bridge 22
3.5.1 Test Level Control 23
3.5.2 Gain Control 23
3.5.3 Bias Arrangements 23
PART 2. Amplitude-Mode Measurements
3.6 General 24
3.6.1 C, G, Rp, Q and D Measurements 24
3.6.2 L, G, Rp, Q and D Measurements 25
PART 3. Phase-Mode Measurements
3.7 General 29
3.7.1 Capacitance Measurements Only 29
3.7.2 Inductance Measurements Only 30
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PART 4. Single-Limit Testing
3.8 General 32
3.8.1 Single-Limit Capacitance Test 32
3.8.2 Single-Limit Inductance Test 33
PART 5. Differential Measurements
3.9 General 35
3.9.1 Differential Capacitance in Amplitude Mode 35
3.9.2 Capacitor Temperature Coefficient, Amplitude Mode 36
3.9.3 Differential Inductance in Amplitude Mode 37
3.9.4 Differential Capacitance in Phase Mode 38
3.9.5 Capacitor Temperature Coefficient in Phase Mode 39
3.9.6 Differential Inductance in Phase Mode 41
IV MAINTENANCE
4.1 General 44
4.2 Preventive Maintenance 44
4.3 Corrective Maintenance 44
PART 1 . Equipment Performance Tests
4.4 Using the Equipment Performance Checklist 44
4.5 Test Equipment and Accessories Required 45
PART 2. Troubleshooting
4.6 General 62
PART 3. Alignment and Calibration
4.7 General 62
4.8 Null Indicator Alignment 62
4.9 Oscillator-Detector Alignment 63
4.10 Bridge Capacitance Alignment and Calibration 65
4.11 Bridge Conductance Calibration Check 70
PART 4. Disassembly and Reassembly
4.12 Cabinet 73
4.13 Bridge 73
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LIST OF ILLUSTRATIONS
FIGURE PAGE
1
Direct Capacitance Bridge, Model 75D
A
2
Basic Bridge Configuration
8
3
Model 75D Block Diagram
10
4
Simplified Schematic, Phase Detector
11
5.
Basic Bridge Equations
13
6
Controls and Indicators
16
7
Conductance Correction Curve, MULTIPLY C XI Range
26
8.
Conductance Correction Curve, DIVIDE G/MULTIPLY R XI
27
9.
Capacitor Nomograph
43
10
Voltage-Resistance Test Chart
82
11
Oscillator-Detector Assembly
83
12
Bridge Assembly (With Cover Removed)
84
TABLES
TABLE NO.
PAGE
1 Optional Accessories 2
2 Technical Characteristics 3
3 Controls and Indicators 17
4 Initial Switch and Control Settings 22
5 Equipment Performance Checklist 45
6 Troubleshooting Chart 74
TABLE OF REPLACEABLE PARTS
Following Page 84
Schematics
Oscillator-Detector Test Points and Waveforms
Endfold
Endfold

CONDUCTANCE
CAPACITANCE pr
DIRECT CAPACITANCE BRIDGE
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Figure 1 „ Direct Capacitance Bridge, Model 75D, (1 MHz)
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CHAPTER I
GENERAL DESCRIPTION
1 .1 GENERAL. The Direct Capacitance Bridge, Model 75D, (1 MHz) is a self-
contained test instrument ( Figure 1 ) designed for use as a precision laboratory bridge
to measure values of capacitors, inductors, and semiconductor devices, or to perform
single-limit sorting tests of capacitive or inductive specimens,,
The instrument provides direct (3-terminal ) readings of parallel capacitance (C),
effective parallel conductance (G), and equivalent parallel resistance (Rp). Parallel
inductance (L) is measured in terms of equivalent capacitance, and dissipation (D) or
Q are readily determined by a simple computation.
The Model 75D is largely insensitive to capacitdnce from either side of the specimen
to ground and provides accurate readings even in the presence of relatively large stray
capacitance,, Thus, the direct capacitance of specimens is precisely measured, and a
remote test jig or coaxial cables may be used to connect specimens without introducing
errors due to jig capacitance or cable capacitance. Test specimens may be biased
directly by the internal dc supply, or an external bias voltage may be applied through
terminals on the front panel. External capacitance standards also may be connected at
the front panel to check calibration or to perform comparison or differential measurements.
Other typical uses of the Model 75D are as follows:
Measurement of the capacitance of semiconductors
Measurement of the capacitance of varactors
Test or measurement of capacitance elements of microcircuit chips
Measurement of the temperature coefficient of capacitors.
Measurement of stray circuit capacitance.
Measurement of stray circuit capacitance
In addition, the Model 75D is used for single-limit and dual-limit testing and sorting
operations of capacitors, inductors, and semiconductor devices during production testing,
incoming inspection, quality assurance, and similar applications.
1 .2 EQUIPMENT DESCRIPTION „ The M odel 75D contains, in one cabinet, abridge,
signal source, null detector, power supply and bias supply. All operating controls and
indicators are mounted on the front panel. If required, the Model 75D may be removed
from the cabinet and installed in a standard 19-inch rack.
A bracket is installed on the rear panel to secure the instrument's ac power cable,
and a terminal shield is provided for the TEST binding posts. This serves to shield the
binding posts from hand-capacitance or stray-capacitance effects when the REMOTE
coaxial terminals are used to connect specimens of standards.
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1.3 OPTIONAL ACCESSORIES. The accessories described in the following table are
available for use with the Model 75D to increase its versatility. For additional
information, call on either your nearest sales engineering representative or the
Boonton Electronics Corporation.
Table I. Optional Accessories
ACCESSORY
FUNCTION
Capacitance Standards, Model CS
Range Extender, Model 77-2A
Range Divider, Model 77-4A
Ground Plane Adapter, Model 75-6A
Dust Covers
Transfer capacitance standards calibrated
against NBS- certified reference standards.
Used to calibrate or check the accuracy of
3-terminal capacitance bridges and to
provide high-resolution comparison
measurements. Available in any value
from 0.01 to 1000-pF at 0.1% accuracy.
Designed for operation from 1 kHz to
1 MHz.
Extends the capacitance range of the
Model 75D to 0.1 pF, conductance range
to 1 mho, resistance range to 1 ohm, and
inductance range to 0.25 pH.
Divides Model 75D capacitance readings
by a fixed factor of 10 and permits
measurements down to O.OO0O1 pF.
Can be used with the Model 75D to perform
(if required) grounded (2-terminal)
measurements or tests. When installed on
the TEST binding posts, the adapter
provides a ground plane that enables the
(3-terminal) Model 75D to simulate
2-terminal (grounded) measurements or
tests.
Perforated metallic covers. Replace
cabinet when the Model 75D is rack¬
mounted.
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1 A TECHNICAL CHARACTERISTICS,, The technical characteristics of the Model 75D
are shown in the following table:
TABLE 2
Technical Characteristics
a. Capacitance Measurement
Capacitance Range: 0 to 1000 pF
Capacitance Resolution and Accuracy:
Resolution
Accuracy
MULTIPLY C
Setting
Capacitance
Range (pF)
Phase
Mode (pF)
Amplitude
Mode(pF)
Phase Mode
Amplitude Mode
1 .0
0 to 1000
0.2 0.2 +(0.25 + 1°-)% + 0.5pF*
Q
+0.25%+(1000/R D +0.5)pF
— P
0.1 0 to 100
0.02 0.02
+(0.25 +-L)% + 0.05pF
w
+0.25%+(1000/Rp+0.05)pF
0.01 0 to 10
0.002 0.002
+(0.25 +-jL)% + 0.005pF
+ 0.25%+(l 000/R p +0.005)pF
0.001 0 to 1
0.0002
0.0005
+(0.25+—)%+0.001pF
Q
±0.25%+(l 000/Rp+0.001 )pF
0.0001 0 to 0.1 0.00005
0.0005 +(2+-J°)% +0.00005pF
±2% +(1000/Rp+0.0005)pF
Where Rp is the equivalent parallel resistance of the specimen in ohms.
* Q must be 10 or greater.
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Table 2. Technical Characteristics (cont'd.)
bo Inductance Measurement
Inductance Range: 25 fjH to 25 mH.
Inductance Accuracy:
multiply c
Setting
Accuracy
Inductance Range
Phase Mode
Amplitude Mode
1.0 25 pH to 250 M H
+ (0.25+-!^ + 2L)%*
+ (0.25 + 2L + 4 ° 00L ) %
R P
0.1
250 pH to 2.5mH + (0.25+-^-+-^% ±(0.25+ L + 4000L )%
10 R p
0.01
2.5mH to 25mH
±(0 - 2 5+ l + -S0- )%
+ (0.25+ L + 4000L )%
50 R p
Where L is the inductance of the specimen in millihenries.
Rp is the equivalent parallel resistance of the specimen in ohms.
c. Conductance Measurement
Conductance Range (amplitude mode only): 0 to 1000 pmhos
Conductance Resolution and Accuracy:
DIVIDE G
Setting
Conductance Range
Resolution
Accuracy
1
0 to 1000 pmhos
5 pjnhos
+ (10 + -^^)% + 10 pmhos
10
0 to 100 pmhos
0.5 pmho
±( 5 + ^)%+ 1 Mmh °
100
0 to 10 pmhos
0.05 pmho
±( 5 + 36o )% + o j Mmho
1000
0 to 1 pmho
0.01 pmho
+ (5+—)% + 0.01 pmho
500
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Table 2„ Technical Characteristics (cont'd.)
d. Resistance Measurement
Resistance Range (amplitude mode only) 1 kQ to 100 MQ
Resistance Resolution and Accuracy:
MULTIPLY R
Setting
Resistance Range
(ohms) Resolution
Accuracy
1
1 K to 100 K
(Rp/10 3 )%
+ (10+ Q/500 +Rp/10 3 )%
10
10 K to 1 M (Rp/l0 4 )%
± (5 + Q/500 + Rp/l 04)%
100
100 K to 10 M
(Rp/1 0 5) %
± (5 + Q/500 + R^l 0 5 )%
1000
1 M to 100 M
(Rp/10 6 )%
±(5+ Q/500 + R p / 10 6 )%
Where R is the equivalent parallel resistance of the specimen in ohms.
r
e„ Test Signal
Frequency: 1 MHz ( +100 Hz) crystal controlled
Level: Continuously adjustable from 250 mV to 1 mV
(250 mV to 15 mV for phase mode)
f. D C Bias
Internal:
Continuously adjustable from -6 V to +150 V.
External, 2 Terminal: Up to +400 V at 100 mA from HI post to LO post.
External, 3 Terminal: Up to ±400 V at 100 mA from HI post to ground, or
from LO to ground (total potential from HI and LO
posts to ground not to exceed 400 V at 100 mA.)
g. Power Requirements : 105 - 1 25 V, 50/60 Hz or 210 - 250 V, 50/60 Hz.
h. Power Consumption: 50 watts
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Table 2. Technical Characteristics (cont'd.)
i „ Dimensions :
Installed in Cabinet: 11-1/4" h x 19-1/2" w x 12-3/4" d
Cabinet Removed: 10-1/2" h x 19" w x 10-1/8" d
j. Weigh t:
Installed in Cabinet: 35 lbs.
Without Cabinet: 25 lbs.
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1 .5 THEORY OF OPERATION. The 75D Capacitance Bridge consists of three basic units:
a signal source, a bridge, and an amplifier/detector with null indicator.
1 .5.1 Signal Source,, The signal source is a stable, crystal-control led oscillator with
a 6AU6 vacuum tube (V203) in a modified Colpitts circuit. This oscillator
operates at a frequency of 1 MHz + 100 Hz.
Two outputs are supplied by the oscillator. One, of 100 volts peak-to-peak,
is used to provide the reference voltage for the phase detector; the second,
of between 1 and 750 millivolts (variable), furnishes the signal source for the
bridge itself.
1 .5.2 Bridge . The bridge network used in the 75D is a modified Young circuit.
Figure 2 shows the basic configuration of the bridge in simplified form.
The 1 MHz oscillator voltage is injected into the bridge by transformer T501 .
The secondary windings of this transformer are accurately balanced and tightly
coupled, ensuring that the signal voltages appearing in both sides of the bridge
are identical in amplitude and opposite in phase. The tight coupling between
halves of the secondary, and the low impedance level, also serve to minimize
the effects of external capacitance unbalance. Any unbalance appearing on
one side (as, for instance, a connecting cable) is transferred equally to the
other side. Thus the 75D is effectively immune to extraneous capacitance
effects.
Referring to Figure 2, the two LO posts (A and B) are the signal-injection
corners of the bridge, and the HI post (D) and ground (C) are the detection
corners. When the bridge is balanced for capacitance and conductance (C
and G), the impedances appearing between each of the LO posts and the HI
post are equal. The voltages that are summed at the HI post are equal and
opposite in phase; under this condition, the net voltage developed at the
detector corner is zero and the null indicator will show a balance.
The Capacitance dial, calibrated in pF, controls a variable differential air
capacitor of high quality and maximum stability. The dial mechanism is
carefully designed to hold backlash to negligible proportions, allowing full
advantage to be taken of the high resolution capabilities of the instrument.
Capacitance range multiplication is accomplished by switching multiplier
capacitors in series with the rotor of the main capacitor. The multiplier
capacitors are precision air variables, carefully adjusted at the factory.
The Conductance control is an infinite-resolution potentiometer connected
between the Standard and the Test arms of the bridge, with its rotor connected
to the HI post. The range of the Conductance control is varied by switching
multiplier resistors in series with the rotor arm of the potentiometer.
1 .5.3 Amplifier/Detector . Although Figure 2 indicates just a detector, the 75D is
actually fitted with two detectors. When both capacitance and conductance
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DIVIDE G/
MULTIPLY R
Figure 2. Basic Bridge Configuration

of a specimen are to be measured, the indicator meter is switched to the
amplitude detector. The indicator then operates with zero at the left end of
the scale. When only capacitance is to be measured and conductance is to be
ignored, the indicator is switched to the phase detector. In this mode, the
indicator operates zero center. The mode to be used is selected by the switch
just beneath the meter: ZERO LEFT for both capacitance and conductance;
ZERO CENTER for capacitance alone.
Figure 3 shows the 75D in simplified block form.
The unbalance signal from the bridge is fed to the pre-amplifier stages VI01
and VI02. Output from the pre-amplifier is then coupled to V201 in the
Oscillator-Detector section, where the signal is split into two paths. One
signal path goes to the phase detector chain, while the other goes to the
amplitude detector.
The amplitude detector is basically a straight amplifier circuit (V202) and a
diode rectifier (CR204). The dc output of this rectifier is applied to thenull
indicator meter. To prevent errors and difficulties resulting from the presence
of stray rf fields, oscillator harmonics and other extraneous signals, the
signal input to V202 is passed through a highly-selective quartz crystal filter
(Y201 - Y202). This two-section filter exhibits a passband of between 50 and
100 Hz, with very steep skirts.
Figure 4 shows, in a simplified schematic, the phase detector circuits. The
1 MHz signal resulting from a bridge unbalance is amplified and fed to the
phase detector at the junction of C204, L203 and C209.
The parallel-resonant tuned circuit formed by L203, C204 and C205 causes
an equal and opposite voltage to appear at the junction of C205, L203 and C210.
These two voltages are applied to diodes CR202 and CR203, while the reference
voltage from the oscillator is applied to the junction of these two diodes.
When the PHASE control (Cl07) is correctly adjusted, the input voltages to the
phase detector resulting from a capacitive unbalance are set so that one is in
phase with the reference voltage and the other is 180° out of phase with it.
The input voltages resulting from a conductance unbalance, on the other hand,
are such that one leads the reference voltage by 90° and the other lags by 90°.
The input voltages are combined vectorially with the reference voltage in the
phase detector. For the condition of a capacitive unbalance in the bridge, one
input (in phase wih the reference voltage) adds to the reference vector, while
the other input ( 180° out of phase with the reference voltage) subtracts from
the reference vector. These voltages are combined and rectified by CR202
and CR203. The resultant dc outputs are such that one becomes less negative,
while the other becomes more negative. These dc voltages are applied to the
grids of the dual-triode dc amplifier V301 . As the dc grid potentials under
these conditions are not equal, there is a resultant change in the anode current
which deflects the null indicator accordingly.
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Figure 3. Model 75D Block Diagram
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Figure 4. Model 75D Phase Detector (Simplified Schematic)
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When an unbalance exists in conductance, the vector addition results in
both of the rectified voltages becoming slightly less negative, and by the
same amount. The resultant dc voltages applied to the grids of V301 are
therefore of equal value, and no change occurs in the anode circuit as a
result. Thus, using the phase detector (ZERO CENTER), the bridge can
be balanced for capacitance regardless of differences in conductance.
Figure 5 illustrates the basic bridge equations.
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CHAPTER II
INSTALLATION
2.1 UNPACKING AND INSPECTION
2.1 . 1 Unpacking. The instrument is supported in the carton by the notches in the
four corner cylinders ot plastic foam material. After opening the top of the carton, take
hold of the two front panel handles through the plastic covering and slowly pull the instru¬
ment up out of the carton. Remove the plastic covering from the cabinet and strip off the
plastic sleeves from the handles. See that any accessories ordered have been included in
the shipment. It is strongly recommended that the carton and all packing material be
carefully saved for possible reshipment in the future.
2.1 . 2 Inspection. Once the instrument is out of its carton, it should be inspected
for any signs ot physical damage resulting from shipment. If any damage is found, be
sure to notify the carrier immediately. When the instrument has been found to be in good
order mechanically, its electrical performance should be checked. This should be done by
following the Equipment Performance Checklist, Table 5, in Chapter IV. Should the bridge
show evidence of a malfunction during this test, notify your local Boonton representative or
the Boonton Electronics Corporation directly.
2.2 PACKING FOR RESHIPMENT
If, for anyreason, it becomes necessary to return the instrument to the factory, the
company should be notified before shipping. Please follow these special packing instructions:
a. If possible, use the original shipping carton and packing material.
b. Secure the ac power cable around its bracket on the rear panel.
c. Include any accessories.
d. Tie a tag or envelope to a front-panel handle with the following data:
1 „ Name and address of purchaser.
2. Model and serial number.
3. Full description of damage or defect.
2.3 POWER CONNECTIONS
Make certain that the available power source agrees with the voltage and line
frequency specified on the identification plate, and plug the cable into a power
receptacle. The instrument is furnished for either 105-125 volts, 50-60 Hz, or 210-250
volts, 50-60 Hz.
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2.4 RACK INSTALLATION
a. Remove the four screws at the sides of the front panel and take off the two
edge strips.
b„ Grasp the two front-panel handles and carefully slide the unit out of the
cabinet; feed the power cable through the cutout of the rear panel, and
remove the cabinet.
Co Installation of dust covers (Part Nos. 600228 and 600229) is strongly
recommended. These are mounted as follows:
1 . Feed the power cable through the cutout in the lower dust cover.
2 0 Fasten the lower cover to the chassis bottom with six No. 6 self¬
tapping screws 1/4" long.
3. Use eight No. 6 self-tapping 1/4" screws to fasten the upper dust
cover.
4. Unscrew the nameplate from the rear of the cabinet and fasten it
to the rear of the upper dust cover.
d. Install the Model 75D in the rack and secure with panel screws.
e. Connect the power cable to an ac power receptacle.
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