NBS

Publications

iviRcii^ 78-871

NATL INST. OF STAND & TECH

NBS RF VOLTAGE COMPARATOR

L. D. Driver

F. X. Ries

G. Rebuldela

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

December 1978

Supported in part by

Army Metrology and Calibration Center

Redstone Arsenal

Huntsville, Alabama

-QC

100

.U56 1 78-871 I

laffonaT Borean of Standards APR IV 1979

NBSIR 78-871

NBS RF VOLTAGE COMPARATOR

L. D. Driver

F. X. Ries

G. Rebuldela

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

December 1978

Supported in part by

Army Metrology and Calibration Center

Redstone Arsenal

Huntsville, Alabama

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

I. INTRODUCTION 1

II. SPECIFICATIONS 1

A. Low Level Channel (3 V max) 1

B. High Level Channel (20 V max) 1

C. General Comparator Package 1

D. Power Supply 1

E. Required Equipment for Operation 2

III. PRINCIPLES OF OPERATION AND DESCRIPTION 2

IV. OPERATING INSTRUCTIONS 6

A. General 6

B. Measurement of rf Voltages Between 0.01 and 0.1 Volt 6

C. RF Signal Generator Calibration 7

1. RF input voltages between 0.01 and 0.1 volts rms 7 2. RF input voltages between 0.01 and 3 volts rms 9 3. RF input voltages between 3 and 20 volts rms 9

D. RF Voltmeter Calibration (Feed Through Tee Types) 11

1. RF input voltages between 0.01 and 0.1 volts rms 11

2. RF input voltages between 0.1 and 3 volts rms 12

3. RF input voltages between 3 and 20 volts rms 12

V. TROUBLESHOOTING AND MAINTENANCE 13

A. Troubleshooting Chart 13

B. Maintenance 14

VI, PARTS LIST AND MANUFACTURER LIST 18

VII. LIST OF ILLUSTRATIONS 22

iii

NBS RF VOLTAGE COMPARATOR

L. D. Driver, F. X. Ries and G. Rebuldela

This manual describes a wideband rf voltage comparator which covers the frequency

range of 100 kHz to 1 GHz from 10 mV to 20 volts. This device uses a pair of

matched Schottky-barrier diodes in each independent channel of a 2 dual-channel

configuration. The coaxial line sections are impedance compensated to assure a VSWR less than 1.03 up to 1 GHz. Applications covered are calibration of signal

generators and rf voltmeters. A troubleshooting and maintenance section is also

included as well as illustrations and circuit diagrams to facilitate repair.

Key words: Comparator; rf voltage; rf voltmeters.

I. INTRODUCTION

The NBS Voltage Comparator is basically a differential voltmeter. It enables comparison

of an unknown rf CW voltage to an accurately known ac voltage (1-100 kHz). Both voltages are detected by diode detectors and algebraically summed giving an output proportional to the

difference in the two signals. The advantages of the device are (1) high resolution,

(2) high accuracy, (3) wide voltage range (10 mV-20 V), (4) broad frequency range

(0.1-1000 MHz), (5) simplicity of operation, (6) insensitivity to ambient conditions, and

(7) low construction cost. This device can be used for calibration or qualification of

both rf signal generators and rf voltmeters.

II. SPECIFICATIONS

A. Low Level Channel (3 V max)

Frequency range: 100 kHz - 1 GHz

Voltage range: 10 millivolts - 3 volts rms - Characteristic impedance: 50 ohms

Input VSWR (terminated in 50 : <1.03 to 1 GHz

Reference plane: 0.159 cm (1/16") from end of type N female plugged into comparator

Accuracy: 10 mV to 100 mV 100 kHz to 100 MHz <3%

10 mV to 100 mV 100 MHz to 1 GHz <5%

100 mV to 3 V 100 kHz to 100 MHz <1% ' 100 mV to 3 V 100 MHz to 1 GHz <3%

RF ref input/output connector: precision type N male

RF input/output connector: precision type N female

AC reference input connector: panel mount BNC female

Null output connector: panel mount BNC female

B. High Level Channel (20 V max)

Frequency range: 100 kHz - 10 MHz

Voltage range: 3-20 volts rms Accuracy: 100 kHz - 10 MHz < 1%

RF reference input/output connector: precision type "N" male

RF input/output connector: precision type "N" female

AC reference input connector: panel mount BNC female

Null output connector: panel mount BNC female

C. General Comparator Package

Housing dimensions: 4 1/8" L x 2 3/4" D x 1 5/8" H

D. Power Supply

Input: 105 to 125 VAC, 60 Hz, 1/4 amp

Output: ±15 V dc, ±25 ma

Housing dimensions: 4 1/8" L x 2 3/4" D x 1 5/8" H

An 8' cord connects the power supply output to the comparator housing.
E . Required Equipment for Operation

AC Reference Generator

Voltage range: 10 mV to 20 V rms Voltage accuracy: 0.1% for max. accuracies

Frequency range : 1-100 kHz

Distortion and noise: 0.05%

Load capability: 25 mA

DC Null Detector

Display: analog, zero center

Sensitivity: ±100 microvolts full scale minimum to ±10 volts full scale maximum

Input impedance : > = 10 megohms

Coaxial Termination

Impedance

50 ohms i 75 ohms j 90 ohms j 135 ohms j 600 ohms j1 percent

1 percent

1 percent

1 percent

1 percent

Frequency Range

100 kHz to 1 GHz

100 kHz to 10 MHz

100 kHz to 10 MHz

100 kHz to 10 MHz

100 kHz to 10 MHz

Connector

Type N male

Type N male

Type N male

Type N male

Type N male

Power Rating

1 watt

1 watt

1 watt

1 watt

1 watt

III. PRINCIPLES OF OPERATION AND DESCRIPTION

The rf voltage comparator is basically a differential voltmeter. It enables comparison of an unknown rf voltage to an accurately known, adjustable, ac reference voltage (1-100 kHz)

.

Both voltages are detected, their outputs are algebraically summed and a voltage proportional to their difference is displayed on a dc null indicator as illustrated in figure 1.

UNKNOWN

RF INPUT >

CHANNEL A

DETECTOR NETWORK A

SUMMING NETWORK

DETECTOR

NETWORK B

AC

REFERENCE >

INPUT

CHANNEL B

<

DC NULL > DETECTOR

.J <

Figure 1. Basic Block Diagram of RF Voltage Comparator

The advantages of this measurement scheme are (1) high resolution, (2) high accuracy, (3)

minimal sensitivity to ambient conditions, (4) simplicity of operation and design, (5) wide

voltage range (10 mV to 20 V), (6) broad frequency range (.1 to 1000 MHz), and (7) low

construction cost. Since the rf channel is a thru channel, one can use the comparator as an in-line instrument as well as a terminating measuring device.

2
MODIFIED n

I RF CONNECTOR

RF REF ASSEMBLY | INPUT/OUTPUT I @

RF

INPUT /OUTPUT

r

3 VOLT MAX.

I R3 200 —vAA/

COMPARATOR

I - C^ 0.1/i.F

I I

Ro.

1k'

C2 C4 J_

"0.1/iF O.t/iFX

v. REF INPUT

;CR2 R4 200

•R2 50

RF REF

INPUT/OUTPUT

20 VOLT MAX

MODIFIED n

RF CONNECTOR ' ASSEMBLY

COMPARATOR

REF INPUT

NULL

OUTPUT

J4

RF

NPUT/

OUTPUT

' NULL

1 M >

1 OUTPUT

^16<

1 M >

CIRCUIT BOARD NO 1 COMPARATOR BOARD

B1

Figure 2. Circuit Diagram of Detector and Summing Networks

of RF Voltage Comparator

The heart of the comparator circuit is a pair of closely-matched Schottky Barrier diodes.

These diodes detect the rf and reference signals and are oriented in the circuit such that

the detected outputs have opposite polarities. This allows the use of a simple resistive

circuit to sum the outputs. Also, with this differential arrangement, the temperature

coefficients of the diodes tend to compensate one another.

The comparator described in this manual is illustrated in the circuit diagram in figure 2. This device is more complicated than the basic comparator circuit of figure 1, as several

unique features were necessary to meet the design goals. The most significant feature is

the placement of the rf detector circuit inside a precision type "N" male connector. This

is accomplished, as shown in figure 3, through the use of miniature circuit components. The use of a pill packaged diode (CRi, CR^i), a pellet resistor (Rj, Rh), and a monolithic chip

capacitor (0^, ^\\) places the rf detecting diode within 0.16 cm of the end plane of a mated

type "N" female connector, while insuring a low comparator VSWR (< 1.03). The result is a very significant reduction in errors when measuring devices having large VSWR's.

3
C(Ci,Cii) - R(Ri. Rii)

CR(CRp CR^^)

Compensated center pin

of precision type N

mole connector.

Figure 3. RF Detector Subassembly

The instrument utilizes two complete and separate comparator circuits. They allow the

required, wide-range coverage in voltage (10 mV - 20 V) and frequency (.1 - 1000 MHz).

One comparator is designed for voltage levels between 10 mV and 3 V in the frequency range

of . 1 - 1000 MHz. The other comparator operates between 3 and 20 V at frequencies from

.1-10 MHz.

The 3 V max comparator uses special low capacitance (C. = .08 pf) Schottky Barrier

diodes. The low capacitance of the diode together with a slight undercut of the center

conductor of the rf channel results in VSWR's < 1.03 up to 1 GHz (see figure 3). Since

the diodes in this section must respond to voltage levels as low as 10 mV, it is necessary

to forward bias each diode. This is provided by the constant current section, shown in

figure 4, which utilizes a closely matched dual FET device. This device is integrated

into conventional constant-current circuits which provide the required (+) and (-) diode

biasing currents. Designed in the 3 volt max comparator is a circuit which triggers an audible alarm if either the rf level or the reference level exceeds 3 volts. This is

accomplished with a pair of FET input operational amplifiers and a miniature audio alarm. A modular ± 15 V power supply, shown in figure 5, is connected to the comparator housing

to supply power to the current source and alarm circuitry.

The 20 volt max comparator also uses Schottky Barrier diodes, but of the high reverse breakdown variety (70 V) . These diodes provide useful detection up to levels as high as

20 V. Since the maximum intended operating frequency of this comparator is 10 MHz, their

junction capacitance (- 2 pf) is insignificant; therefore, rf line compensation is un- necessary. In the 3 - 20 V range, the diodes behave as peak detectors, thus making diode

biasing unnecessary. Consequently, the accuracy of this comparator is entirely dependent

upon the tracking of the rf and reference diodes.

4
r

100 k -15V

CR^ BIAS ADJ. ^WV—Y>iX^—f—0-15V

R23 R25

1M 500k

RED +

WHITE

BLK -

AUDIO

ALARM

15 V

+15 C -15

HOOKUP CABLE N2

Figure 4. Circuit Diagram of Constant Current Source and Alarm Board

N1 Cord Assembly

115V AC MALE RECEPTACLE AND

LINE FILTER MODULE

F1

+ 15VOMODULAR

power CoSUPPLY

M1 _i5Vo

Hookup

Cable

N2

Figures. Block Diagram of Modular Power Supply (±15Volts)

5
IV. OPERATING INSTRUCTIONS

A. General

The two separate comparators in this instrument (the low level or 3 V max channel and

the high level or 20 V max channel) operate essentially the same. One exception is

that the 3 V max comparator must undergo an "initial null zero procedure" before being

used. This preliminary adjustment is not necessary with the 20 V max comparator.

Both comparators can be used to calibrate rf generators and rf voltmeters. These

devices must operate within the limits of the voltage comparator; i.e., at frequencies

between 100 kHz and 1 GHz and at voltage levels between 10 mV and 20 V rms. Also, rf voltmeters can be either the terminating type or the feed thru tee type.

Always check section II (Specifications) to determine the appropriate comparator

section to use before making a measurement. Also, be certain that the ac reference

generator and the dc null detector meet the required specifications described in

section II.

The comparator power supply need only be energized if the 3 V max comparator is to be used.

CAUTION : The diodes used in the low level (3 V max) comparator can be damaged by static

discharge. Since coaxial connecting cables can store energy, one should momentarily short

the inner and outer conductors together before connecting any cables to either the ref

input or the rf inputs of the comparator. In general, avoid any situation that could cause a static discharge through either detecting diode. Additional precautions against

static discharge are given in the discussion covering diode testing and replacement in

section V.

CAUTION : Any severe stress or force that could tend to twist the center conductors in the

rigid 50 ohm line sections could cause mechanical failure of the rf detector assemblies.

Therefore, whenever connecting to these lines always hold the mating connectors securely

while turning only the tightening sleeve of the male connector.

CAUTION : Before turning on the power supply, always be certain that the rf channel of the

3 V max comparator is terminated. Also, when removing a termination or terminating device

from this channel, always turn off the power supply first.

B. Measurement of rf Voltages Between 0.01 and 0.1 Volt

The offset voltage between the reference and rf detectors of the comparator must be

determined before any voltage measurement can be made at the 0.01 volt level. This

is accomplished in the following manner.

1. Set the frequency and output level of the reference generator to minimum.

2. Turn the power supply on and allow sufficient warmup before proceeding.

3. Connect the equipment as shown in figure 6.

Note: Obtaining a null will be easier if the sensitivity of the null detector

is initially decreased; however, the final null detector sensitivity setting

should be on the proper range, as indicated in the subsequent sections. This

procedure should be followed during any null zero process.

4. Set the null detector to zero, on the 100 pV range, by adjusting the bias pots.

5. Set the reference generator to 0.01 volts at 100 kHz, and record the magnitude

and sign of the null detector indication for future use. This indication is

the measure of the offset voltage and must be used when making measurements

below 0.1 volt. This value should not be more than 20 pV.

6
100 KHz REFERENCE

GENERATOR

Type N Male Input

Ref

Null

NULL DETECTOR

(100 /xV Range)

50 Q, Termination

RF

Voltage

Comparator

AC

Input

Figure 6. Offset Voltage Measurement Diagram

C. RF Signal Generator Calibration

1. RF Input voltages between 0.01 and 0.1 volts rms.

a. Determine the offset voltage value as outlined in section IV. B.

b. Set the frequency and output level of the rf and reference generators

to minimum.

c. Connect the equipment as shown in figure 7.

d. After a sufficient warmup period, set the null detector to zero, on

the 100 yV range, by adjusting the bias pots.

e. Select the desired calibration frequency on the rf and reference generators.

7
100 KHz REFERENCE

GENERATOR

NULL DETECTOR

RF SIGNAL GENERATOR

Ref.

Null

BIAS "POT

3V —^20V

BIAS "POT

( 100 /iV Range)

soil Termination

RF

Vol tage

Comparator

AC

Input

Figure 7. Block Diagram for Calibration of

RF Signal Generators (0.01 -0.1V)

CAUTION : If the audio alarm sounds while either the rf generator level or the ac reference

generator level is increased, the respective level has reached an unsafe level for the

detecting diode. Turn down both levels immediately and recheck the equipment and layout.

f. Slowly advance the rf generator level until the desired calibration point

is reached on either a calibration dial or an output meter of the rf generator.

NOTE: Avoid pegging the meter on the null detector. As the rf generator

level is increased, reduce the null detector sensitivity to keep a near-full

scale indication.

g. Slowly advance the ac reference level until the null indication returns to the

offset value noted in step IV. B. 5.

h. The calibrated rf voltage level (V ^) is: V ^ = V ° rf rf ref

i. Turn the rf and ac reference generators to their minimums and check if the

null voltage returns to zero.

j. If the null does not return to zero, record the magnitude of the drift.

k. The percentage drift, D(%), relative to the reference value can be

calculated by:

8
D(%) =

( Magnitude of Drift ^

V reference

NOTE: The drift should be less than 0.5%. If it is greater, repeat steps d through k. Refer to the Troubleshooting and Maintenance Section if this

problem persists.

2. RF input voltages between 0.1 and 3 volts rms.

Perforin the same procedure as outlined for the 0.01 to 0.1 volt range, with the

exception that the null zero need not be rechecked once initially set. The

final null detector sensitivity settings should be set to the 1 mV position from

0.1 V to 1 V levels and to the 10 mV position from 1 V to 3 V levels.

3. RF input voltages between 3 and 20 volts rms.

a. Set the frequency and the output level of the rf and reference generators

to minimum.

b. Connect the equipment as shown in figure 8.

RF

Voltage

Comparator

100 KHz REFERENCE

GENERATOR

50 Q, Termination

AC

Input

Figure 8. Block Diagram for Calibration of

RF Signal Generators (3-20V)

CAUTION : Be certain that the 20 V max comparator section is being used for measurements in

this range. Since the audio alarm is incorporated only in the 3 V max comparator circuitry, use caution when increasing the input levels to avoid exceeding the 20 volt rms limit

.

CAUTION : Be certain that the termination has the proper wattage rating.

9
RF

VOLTMETER

100 KHz REFERENCE

GENERATOR

NULL DETECTOR

Type N Male or Female

Type N Female

Type N Male

Ref.

Null

n

BIAS " POT --

—^20V

BIAS "POT --

100 KH; r-| REFERENCE

J I^GENERATOR J

RF~

Voltage

Comparator

50ii Termination

NULL I

I DETECTOR |

I I

AC

Input

Figure 9. Block Diagram for Calibration of

RF Voltmeters (0.01 -20V)

10
Select the desired calibration frequency on the rf generator.

d. Slowly advance the rf generator level until the desired calibration point

is reached on either a calibration dial or an output meter of the rf

generator

.

e. Slowly advance the ac reference level until a null is obtained on a sufficiently sensitive scale of the dc null meter. The final null detector

sensitivity settings should be set to 3 mV from 3 V to 10 V, and to 100 mV

from 10 V to 20 V.

f. The calibrated rf voltage level V^is: V^=V

rf rf ref

g. Turn the rf and ac reference generators to their minimums

.

D. RF Voltmeter Calibration (Feed Through Tee Types)

1. RF input voltage between 0.01 and 0.1 volts rms.

a. Determine the offset voltage value as outlined in section IV. B.

b. Set the frequency and output level of the rf and reference generators

to minimum.

c. Connect the equipment as shown in figure 9.

After a sufficient warmup period, set the null-detector to zero, by

adjusting the bias pots.

Select the desired calibration frequency on the rf and reference generators.

Select the voltage range to be calibrated on the rf voltmeter.

NOTE: Avoid pegging the meter on the null detector. As the rf generator

level is increased, reduce the null detector sensitivity to keep a nearfull scale indication.

CAUTION: If the audio alarm sounds while either the rf generator level or the ac reference

level is being advanced, the respective level has reached an unsafe level for the detecting

diode. Turn down both levels immediately and recheck equipment and layout.

11
g. Slowly advance the rf generator level until the desired calibration point

is reached on the rf voltmeter.

h. Slowly advance the ac reference level until the null indication returns to the offset value noted in section IV.D.l.a.

i. The calibrated rf voltage level (V ^) is V ^ = V

rf rf ref

j . Turn the rf and ac reference generators to their minimums and check that the

null voltage returns to the zero.

k. If the null does not return to zero, record the magnitude of the drift.

1. The percentage drift, D(%), relative to the reference value can be

calculated by:

D(%) = ( Magnitude of Drift

^ ^ reference

NOTE : The drift should be less than 0.5%. If it is greater, repeat steps

d through 1. Refer to the Troubleshooting and Maintenance Section if this

problem persists.

2. RF input voltages between 0.1 and 3 volts rms.

Perform the same procedure as outlined for the 0.01 to 0.1 volt range, with the

exception that the null zero need not be rechecked once initially set. The

final null detector sensitivity settings should be set to 1 mV from 0.1 V to 1 V, and to 10 mV from 1 V to 3 V.

3. RF input voltages between 3 and 20 volts rms.

a. Set the frequency and the output level of the rf and reference generators

to minimum.

b. Connect the equipment as shown by the dotted lines in figure 9.

CAUTION: Be certain that the 20 V max comparator section is being used for measurements in

this range. Since the audio alarm is incorporated only in the 3 V max comparator circuitry,

use caution when increasing the input levels to avoid exceeding the 20 volt rms limit.

CAUTION: Be certain that the termination has the proper wattage rating.

c. Select the desired calibration frequency on the rf generator.

d. Select the voltage range to be calibrated on the rf voltmeter.

NOTE: Avoid pegging the meter on the null detector. As the rf generator

level is increased, reduce the null detector sensitivity to keep a near

full scale indication.

e. Slowly advance the rf generator level until the desired calibration point

is reached on the rf voltmeter.

f. Slowly advance the ac reference level until a null is obtained on a sufficiently sensitive scale of the dc null meter. The final null

detector sensitivity settings should be set to 3 mV from 3 V to 10 V

and to 10 mV from 10 V to 20 V.

g. The calibrated rf voltage level (V ^) is: V ^ = V ^.

h. Turn the rf and ac reference generators to

12

their minimums.
V. TROUBLESHOOTING AND MAINTENANCE

A. Troubleshooting Chart

SYMPTOM

Unable to initially zero 1. the low voltage comparator 2. as outlined in IV.

3.

POSSIBLE CAUSE

Power supply is OFF. 1 RF channel has no return 2 path for bias current.

Diode CRl and/or diode CR2

not receiving current bias.

4. Bias currents present, but one or both cannot be varied.

5. CRl and/or CR2 failure.

Audio Alarm Sounding Rf channel of 3 volt max

comparator not property

terminated (no dc return

path)

.

Rf input level and/or reference

input level too high

(3 V max comparator only)

.

CRl and/or CR2 failure.

4. FET Op Amp circuitry failure. 4

Low Sensitivity 1. Wrong comparator section being 1 used

.

2. Faulty null indicator. 2

3. Rf and/or reference diode 3 failure

.

Unable to obtain a null 1. with rf and reference

voltages applied. 2.

(a) Low voltage 3. comparator

(b) High voltage 4. comparator

.

Faulty rf or reference 1 generator

.

Faulty null indicator. 2

Same as listed under initial 3 zero problem.

CRll and/or CR12 failure. 4

REMEDY AND CHECKS

Turn on power supply.

Check cable connecting

rf and reference channels.

Check dc resistance of

rf termination.

Check bias currents as outlined in this section.

Check connecting wires

between circuit boards

Bl & B2. Check the + 15 volts at Ql.

Check trimpots R24 & R25. Check Ql for proper

operation.

Check diodes and replace

as needed in accordance

with procedure outlined

in this section.

Turn OFF power supply

and properly terminate

rf channel.

Reduce level or levels

immediately

.

Check diodes and replace

as needed in accordance

with procedure outlined

in this section.

Check input, output and

supply voltages of Al

and A2 . Check Al and

A2 for proper operation.

Check connections and

change if needed.

Check null indicator

sensitivity

.

Check diodes and replace

as needed in accordance

with procedure outlined

in this section.

Check rf and reference

generators

.

Check null indicator.

Same as listed under

initial zero problem.

Check diodes and replace

as needed in accordance

with procedure outlined

in this section.

13
B. Maintenance

1. General

Keep all input and output connectors clean. Clean with a low deposit solvent

and blow out with compressed air.

CAUTION : When cleaning the rf and reference connectors, avoid any contact or disturbance

to the rf detector assembly. Also, do not allow cleaning solvent to penetrate this region

of the connector.

2. Diode bias current supplies

There are two general methods of checking the bias currents; one method can be

done externally while the other method requires opening the comparator housing

to allow internal measurements.

a. External current check

1. With the power supply turned OFF, terminate the rf channel of the low

voltage comparator with a 1 kilohm load.

2. Turn on the power supply.

3. Obtain a dc millivoltmeter with a floating input and an input resistance

>> 1 kilohm.

4. Connect the voltmeter across the 1 kilohm resistor and note the voltage

reading. It should be between .5 and 1.5 millivolts. This value should

be variable via the screwdriver adjustment access hole.

5. Connect the voltmeter to the reference input connector. This voltage

should be approximately the same magnitude as measured in step 4 but

will have the opposite polarity. This value should also be variable

via the screwdriver adjustment access hole.

6. If one or both of these voltages are not present, the internal current

check must be performed. Such a condition could indicate a diode

failure, a connection problem between the current source and the

detecting diodes or a failure of the current source or sources.

b. Internal current check

1. With the power supply turned OFF, terminate the rf channel.

2. Unscrew holding nut on audio alarm and remove top cover of voltage

comparator

.

3. Obtain a dc voltmeter with a floating input having an input resistance

>> 1 megohm.

4. Turn on the power supply.

5. Connect the voltmeter across R22 and note reading. The voltage should

be somewhere between .5 and 2 volts.

6. Vary R24 while observing the voltage across R22. This voltage should

change as R24 is varied. If no change is observed, check both R24

and Ql.

14
7. Measure voltage across R23. It should be close to that across R22, but

of opposite polarity.

8. Use same procedure as specified in step 6 —only using R23 and R25.

9. If one or both of the voltages checked in steps 5 and 7 are zero, check Ql and its associated circuitry. Also check for the presence

of the ± 15 V supply voltages to Ql. Change Ql if indicated.

3. Detector diode replacement

CAUTION : When testing or handling the detector diodes in the low voltage comparator (CRl

and CR2), observe the following precautions to avoid static discharge damage:

The operator, tweezers, or any other pick-up tool must be grounded to the test or inspection station.

All test fixtures should incorporate a short across the terminals. The

short may be removed once the diode is in place. Where shorts are not practical, a series resistor of approximately 10 kfi can be used.

Spurious pulses generated by test equipment must be eliminated.

When a diode is passed from one operator to another, the receiving operator

should grasp the lead held by the passing operator.

e. All soldering apparatus should be transformer isolated from the power line

and should be free of leakage.

CAUTION: The rf and reference detector diodes in each of the comparator sections are matched pairs. CRl and CR2 are the matched pair for the low voltage comparator and CRll

and CR12 are the matched pair for the high voltage comparator. When a diode failure is

discovered, it will be necessary to replace both diodes of the appropriate comparator

with a new matched pair. Also, it is important to use the right type for replacement

diodes since the detector diodes used in the low voltage comparator are a different type

than the pair used in the high voltage comparator.

The rf and reference detector diodes can be replaced using the following

procedure

:

f. Unscrew lock nut holding the audio alarm Tl and remove the top and

bottom covers of the comparator housing.

g. Unsolder the two wires connecting the circuit boards together at their

connecting points on R20 and R21.

h. Unsolder ± 15 V and ground connection from modular power supply.

i. Remove the holding screws on circuit board B2 and carefully remove the

board from the comparator housing.

j . Unsolder the f ormex lead which attaches the rf detector (in the precision

type "N" male connector) to the comparator circuit board Bl. Unsolder

this wire at either R3 or R13, depending on the diode pair that is being

replaced.

k. Unscrew holding screws and remove the appropriate small aluminum shielding

can.

1. Unsolder miniature coax cable coming from the reference input connector.

Unsolder inner and outer connectors of this cable at the circuit board end.

15
m. Unsolder the fine wire from the reference detector assembly where it

attaches to the appropriate 1 kfi resistor (RO or RIO)

.

n. Remove holding nut for reference detector assembly from the bottom side

of Bl and remove detector assembly.

CAUTION: Before replacing a pair of diodes, make certain that they are the proper type

for the comparator section being repaired, and make certain that the two diodes constitute

a matched pair.

CAUTION: Always observe the static discharge precautions, previously prescribed, when

handling or checking the diodes used in the low voltage comparator section (CRl & CR2)

.

o. Place a small piece of conductive foam over components of the detector

assembly

.

p. Replace reference detector assembly and reassemble this section in reverse order to steps 5 thru 8.

q. Remove body of precision type "N" female connector from the appropriate

50 ohm line section.

r. Loosen and remove holding collar on type "N" male end of line.

s. Remove line from voltage comparator housing.

t. Obtain a termination, attenuator, etc., that has a precision type "N" female connector on it. Screw this female connector into the lines

type "N" male connector—holding both connectors rigid and turning only

the locking sleeve. The female end of the 50 ohm line section of the

voltage comparator could be used if necessary. This will serve as a "supporting connector."

u. Using a pin vise to grasp the female center pin of the type "N" female

connector, gently pull until the internal center conductor joint disengages.

v. Hold male type "N" connector rigidly while unscrewing outer conductor of

50 ohm line section.

w. Remove rf detector assembly from connector.

X. Unscrew type "N" female connector ("supporting connector") from the

precision type "N" male connector.

y. Place a small piece of conductive foam over components of the detector

assembly

.

z . Feed f ormex wire of new rf detector assembly into rear of type "N" male

connector; loop wire back on itself enough to enable feeding it through

the small slanted hole inside the connector. Gently feed assembly into

connector while pulling formax wire through hole. Make certain connector

bead properly seats into the undercut region inside the connector body.

aa. Hold type "N" male connector and detector assembly firmly while screwing

outer conductor of 50 ohm line into the connector. Tighten firmly.

bb. Connect "supporting connector" to type "N" male connector.

cc. Check female center pin—bead—center conductor assembly for tightness and

feed it into open end of 50 ohm line section.

dd . When properly aligned, carefully push on this assembly axially until it

seats into the mating center conductor section.

16
ee. Feed the 50 ohm line section back into the voltage comparator housing. Be certain to feed the formex wire into the comparator housing properly while

performing this step.

ff. Replace and tighten holding collar after properly positioning line and

formex wire.

gg. Replace and tighten type "N" female connector. Do not allow the line

section to rotate while performing this step.

hh. Cut formex wire to the proper length, strip end and solder to appropriate

resistor (R3 or R13) on comparator board (Bl) .

ii. Tighten holding screws to Bl, if they had to be loosened.

j

j . Reposition circuit board B2 in housing while carefully feeding the two connecting wires from Bl through the proper holes in B2 . Resolder these

wires. Tighten the holding screws that secure B2

.

kk. Remove pieces of conductive foam used on replacement detector assemblies.

11. Replace top and bottom covers to voltage comparator housing. Replace and

tighten lock nut for alarm Tl.

mm. Test the general operation of the voltage comparator section just repaired.

If it was the low voltage comparator, perform the null zeroing procedure

described in section IV. If the high voltage diodes were replaced, feed

the 100 kHz reference signal into both channels (rf and reference) and

check the diode tracking (as indicated by the null output reading) at several levels between 3 and 20 volts rms.

17
VI. PARTS LIST AND MANUFACTURER LIST

Parts List

Reference

Sjnnbol

Total

Quantity Description

Manufacturer Manufacturer

type and/or Code No. Part Number

Al, A2

Bl

B2

C1-C4,

C11-C14

C20,C21

C22-C25

C26,C27

CRl, CR2

CRll, CR12

CR21, CR22

Fl

H1,H2

J1,J2

J3-J6

LI

FET Operational Amplifier

Comparator circuit board

Current source & alarm

circuit board

Monolythic chip capacitors,

0.1 yf, required dimensions

L = 0.175 ± 0.015",

W = 0.125 ± 0.005", and

T = 0.045 ± 0.005"

Polycarbonate capacitors,

0.1 yfd, 250 VDC

Disk capacitors, 0.01 \if

Tantalum capacitors,

4.7 yf, polarized 35 V

Matched Schottky barrier

diode pair, matched Vp

within ± 1 mV at 10, 100 & 1000 ya If, Vr > = 12 V at Ir = 10 ya, mounted in "P" package outline

Matched Schottky barrier

diode pair, matched Vp

within ± 5 mV at 10, 100 & 1000 ya Ip, Vr > = 70 V at

LH0042CH

NBS

NBS

11C1812X7R104M

50A

(with specified

dimensions)

B32-541

5GA

150D

A2X1126-1

A2X1127-5

lr 10 ya, mounted in "P' outline

Silicon diode, axial lead

115 VAC receptacle/line

filter module, 250 V at

3 a. 50-60 Hz

Comparator and power supply

housings, metal enclosure

Precision type "N" connector, 7 mm jack

1N4153

3EF1

3301

131-10004

Panel receptacle, BNC female UG-290 A/U

Neon lamp with cap, flush CM22-1000-00A1A

mount

9

10

18
Parts List

Reference Total Manufacturer Manufacturer

Symbol Quantity Description type and/or Code No. Part Number

Ml

Nl

N2

P1,P2

Ql

RO

R1,R2

R3,R4

R5,R6

RIO

R11,R12

R13,R14

R15,R16

R20,R21

1 Modular ± 15 V dc power 279-015-03

supply, ± 25 ma

1 115 VAC power cord assembly 17250

1 ± 15 V hookup cable between 8451

comparator & power supply,

8 foot length

2 - Precision type "N" 131-10003

connector, 7 mm plug

1 Matched dual N-channel 2N5045

silicon junction field

effect transistors, T071 CAN

2 Metal film resistor, 1 YSi, lOOlF

1%, 1/8 W

2 Cermet pellet resistor, 50A5T

50 5%, 1/10 W

4 Carbon resistor, 200 J2, RCR05

5%, 1/8 W

4 Metal film resistors, 1 Mfi, 1004F

1%, 1/8 W, (hand picked to be within 0.05% of each

other)

Metal film resistor, 1 YSl, lOOlF

1%, 1/8 W. Same as RO.

2 Cermet pellet resistor, 200A5T

200 5%, 1/10 W

Carbon resistor, 200 n, RCR05

5%, 1/8 W. Same as R3,R4.

Metal film resistors, 1 Mfi, 1004F

1%, 1/8 W, (hand picked to be within 0.05% of each

other). Same as R5,R6.

2 Carbon resistor, 4.3 Wl, RCR05

5%, 1/8 W

11

12

12

13

14

15

16

14

14

15

16

14

16

19
Parts List

Reference Total Manufacturer Manufacturer

Symbol Quantity Description type and/or Code No. Part Number

R22,R23

R24,R25

R26,R27

R28,R29

R30,R35

R36

SI

Tl

Metal film resistor, 1 KQ,

1%, 1/8 W

Variable resistor, Cermet

trimpot, 500 KQ, 20 turns,

3/4 W, screwdriver adjust

Carbon resistor, 10 KU,

5%, 1/8 W

Carbon resistor, 4.3 KQ,

5%, 1/8 W

Carbon resistor, 100 KU,

5%, 1/8 W

Carbon resistor, 39 KQ,

5%, 1/4 W

Miniature SPOT switch,

125 VAC, 5a

Subminiature audible

signalling device, 1 kHz,

5 to 25 V dc

1004F 14

E-Z TRIM 17 3009P-1-504

RCR05 16

RCR05 16

RCR05 16

RCR07 16

JMT 123 18

Mini-bleeptone 19

Model 525

20
Manufacturer List *

Manufacturer

Code No. Manufacturer Manufacturer's Address

1 National Semiconductor Corporation Santa Clara, Calif.

2 Sprague Electric Company North Adams, Mass.

3 Siemens Corporation Iselin, N.J.

4 Aertech Industries Sunnyvale, Calif.

5 General Electric Company Syracuse, N.Y.

6 Corcom Chicago, 111.

7 Pomona Electronics Co., Inc. Pomona, Calif.

8 Bunker-Ramo Corporation

Amphenol RF Division Danbury, Conn.

9 Bunker-Ramo Corporation

Amphenol Connector Division Broadview, 111.

10 Chicago Minlight Works Chicago, 111.

11 Bunker-Ramo Corporation

Amphenol Cadre Division Endicott, N.Y. 12 Belden Corporation Chicago, 111.

13 Siliconix Incorporated Santa Clara, Calif.

14 Corning Glass Works Corning, N.Y.

15 CTS Microelectronics, Inc. Lafayette, Ind

.

16 Allen Bradley Company Milwaukee, Wis.

17 Bourne, Inc., Trimpot Prod. Div. Riverside, Calif.

18 JBT Instruments Incorporated New Haven, Conn.

19 C.A. Briggs Company, Cybersonic Div. ' Glenside, Pa.

* Certain commercial equipment, instruments, or materials are identified in

this paper in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement by

the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.

21
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIQviKAPnH, DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 78-871

2. Gov't Accession

INo. 3. Recipient's Accession No.

4. TITLE AND SUBTITLE

iNlBS RF VOLTAGE COMPARATOR

5. Publication Date December 19 78

6. Performing Organization Code

724

7. AUTHOR(S)

L. D. Driver. F. X. Ries and G. Rebuldela

8. Performing Organ. Report No.

9, PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 7241171

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Supported In part by

Army Metrology and Calibration Center

Redstone Arsenal, Huntsville, Alabama

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15, SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary ol most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This manual describes a wideband rf voltage comparator which covers

the frequency range of 100 kHz to 1 GHz from 10 mV to 20 volts. This

device uses a pair of matched Schottky-barrier diodes in each independent

channel of a 2 dual-channel configuration. The coaxial line sections are impedance compensated to assure a VSWR less than 1.03 up to 1 GHz.

Applications covered are calibration of signal generators and rf voltmeters.

A troubleshooting and maintenance section is also included as well as illustrations and circuit diagrams to facilitate repair.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Comparator; rf voltage; rf voltmeters.

18. AVAILABILITY [^Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

1 1 For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED

28

1 1 Order From Sup. of Doc, U.S. Government Printing

Washington, D.C. 20402. SD Cat. No. C13

Office 20. SECURITY CLASS

(THIS PAGE)

22. Price

[XE^ Order From National Technical Information Service

Springfield, Virginia 22151

(NTIS)

UNCLASSIFIED

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