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REFERENCI

NBS TYPE ERF POWER METER

OPERATION AND MAINTENANCE

Neil T. Larsen

Electromagnetics Division

fistitute for Basic Standards

ational Bureau of Standards

Boulder, Colorado 80302

October 1977

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

Sidney Harman, Under Secretary

IP Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

NBS TYPE IV POWER METER

OPERATION AND MAINTENANCE

CONTENTS

Page

LIST OF FIGURES AND TABLES iv

ABSTRACT v

SECTION I. GENERAL INFORMATION 1

1.0 DESCRIPTION 1

SECTION II. OPERATING INFORMATION ~ 1

2.0 INTRODUCTION 1

2.1 PRELIMINARY OPERATING INSTRUCTIONS 1

2.2 BASIC POWER MEASUREMENT 1

2.3 PRECISION POWER MEASUREMENT 4

SECTION III. MAINTENANCE AND CALIBRATION 9

3.1 INTRODUCTION 9

3.2 TEST EQUIPMENT 9

3.3 INITIAL SETUP 9

3.4 POWER SUPPLY PERFORMANCE (FIG. 3-1) 10

3.5 AMPLIFIER OFFSET ADJUSTMENT (FIG. 3-3) 10

3.6 CURRENT INDICATOR (FIG. 3-3) 11

3.7 CALIBRATION OF INTERNAL RESISTORS (FIG. 3-3 & 3-4) 11

3.8 GAIN AND NOISE TESTS 12

3.9 TEST OF INTERFACE LOGIC (FIG. 3-2) 13

3.10 NONSTANDARD COMPONENTS 14

APPENDIX 23

m

iii

LIST OF FIGURES AND TABLES

Page

Figure 2.1. Control identification 2

Figure 2.2. Basic measurement system 3

Figure 2-3. Precision measurement system using reference voltage generator (RVG) 5

Figure 2-4. Typical voltage measurement sequence providing first-order

compensation for temperature drifts 7

Figure 3-la. Power supply drawing, serial number 122 and below. 15

Figure 3-lb. Power supply drawing, serial numbers 123 and above 16

Figure 3-2. Interface and mount operating resistance logic 17

Figure 3-3. Analog cicuitry for the self-balancing current loop 18

Figure 3-4. Simplified diagram of analog circuits during calibration procedure 19

Figure A-1. Parts placement diagram, large board — 20

TABLE A-1. MANUFACTURER'S CODES 21

TABLE A-2. PARTS LIST 24

TABLE A-3. CONNECTOR PIN ASSIGNMENTS 27

TABLE 2-1. VOLTMETER-CAUSED ERRORS • 8

iv

NBS TYPE IV POWER METER

OPERATION AND MAINTENANCE

Nell T. Larsen

Operating, maintenance, and calibration procedures are provided for the

NBS Type IV Power Meter. This is a recently developed instrument for

rf power measurement by means of dc substitution in a bolometer (thermistor

type only). Complete parts lists, schematic diagrams, and parts placement

drawings are provided. Mechanical drawings are not included, but are available from: National Technical Information Service, U. S. Department of Commerce, 425 Thirteenth Street, N. W. , Washington, D. C,

20004.

Key Words: Bolometer; dc substitution; microwave; power meter.

IMPORTANT NOTICE

The specific components selected for the prototype instruments were chosen

on the basis of suitability, availability, and cost. They do not necessarily

represent the only possible choices, or even the best choices. The National

Bureau of Standards states only that they were used in the instruments

described here. Substitution of nominally equivalent components meeting the same specifications should normally cause no difficulty; however, NBS has

not tested all such possible choices.

NOTE: References to serial numbers apply only to units built by NBS.

V

SECTION I GENERAL INFORMATION

1.0 DESCRIPTION

The NBS Type IV RF Power Meter is designed to measure high-frequency or microwave power in

conjunction with an external thermistor mount. The mount should be designed to operate at a

dc resistance of either 100 or 200 ohms. A complete power measurement system will also include an accurate dc voltmeter and some means of offsetting the voltmeter so that small voltage changes may be measured more accurately. This offsetting voltage source, or reference

voltage generator (RVG), is particularly important when measuring power levels below a few

milliwatts.

The Type IV, unlike its predecessors, is not a self -balancing Wheatstone bridge. Instead, it

passes a current through the bolometer mount while sensing the voltage across it in such a way that the resistance of the bolometer is defined at the dc terminals of the bolometer,

even at the ends of long leads. This eliminates the lead errors associated with a bridge. In addition, because of this new configuration, it is now possible to change the mount operating

resistance with solid state switches. Therefore, the Type IV is well suited tg applications

requiring high accuracy and where control by a computer is used.

SECTION II OPERATING INFORMATION

2 . 0 INTRODUCTION

The NBS Type IV Power Meter is a standards laboratory quality instrument used to measure

accurately high-frequency or microwave power in terms of substituted dc power. The instrument is not direct reading; the power must be calculated, generally using data obtained from

an external precision voltmeter. The Type IV is intended to be used with non-temperaturecompensated mounts. Two units can be used for compensated mounts (one for measurement and one for compensation). In most automated applications, the speed of measurement is

such that compensation is not required. The frequency and power range is, of course, dictated by the thermistor mount used. The Type IV can supply up to 30 mA of bias current to

the bolometer. This corresponds to 90 raW in 100 ohms, or 180 mW in 200 ohms.

2.1 PRELIMINARY OPERATING INSTRUCTIONS

See Figure 2.1 for the location and function of the front and rear panel controls. Before

connecting the Type IV to the ac power line, check the line voltage selector for the proper

setting and the fuse for the proper value. Remove the line cord, slide the window to the

left and eject the fuse. The line voltage is selected by proper orientation of the small

printed circuit card, which may be pulled after removing the fuse. Line voltages of nominally 100, 120, 220, and 240 may be selected. (Note: In units with serial numbers 122 and

below, and using power transformer part number 13575, the lOO-volt option is not available).

Line voltage tolerance is plus or minus 10%. For 100-120 volt operation, a 1/8 A fast-acting

fuse, 6.35 mm by 31.75 mm (0.25 by 1.25 inches) is used. For 220-240 volt operation, a 1/16

A fuse isquired.

2.2 BASIC POWER MEASUREMENT

1. With the instrument turned off, attach the thermistor mount to either the front

panel binding posts or the rear connector. (See Appendix I for connector pin

designations). If it is desired to use a 4-terminal connection to the mount to eliminate lead resistance errors, open the two links. The current-carrying con- ductors are connected to the red (positive) and black (negative) BOLOM binding

posts. The sense leads are connected to the corresponding blue SENSE binding

posts. Leads should be shielded to prevent noise pickup. Connect the shield

to the black binding post and leave the other end open. See Figure 2.2.

Attach the positive and negative input leads of the external voltmeter to the red

and black binding posts marked DVM. Note that it is necessary that the voltmeter
I
{J RF POWER METER ^^1

ERROR

NBS TYPE BE

DVM

100 REM 200

SENSE BOLOM

LINE

'<§)

—-{D

Figure 2-1, Control identification

le Bolometer current indicator » 0-30 mA.

2o ERROR indicator. Lights red on any fault condition preventing loop balanc

3. Mount operating resistance indicators (green).

4. Mount operating resistance switch i 100 ohms, REMOTE, 200 ohms.

5 . Line power switch (push-push)

6. DVM terminals, for external voltmeter.

7. Positive bolometer voltage sense lead.

8. Positive bolometer current lead.

9. Negative bolometer current lead.

10, Negative bolometer voltage sense lead.

Front panel links connecting respective sense and current leads are not shown.

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be isolated from ground, and note also that while the source resistance presented

to the voltmeter by the Type IV is very low, any current drawn by the voltmeter

flows through the sense lead (if used) connected to the low side of the thermistor mount. This current produces a voltage drop in the lead which causes an error. Since most digital voltmeters have very high input impedances, this is not

a problem; however, it should be kept in mind that the two black terminals on the

front of the instrument are not common to each other, but are connected through

either the link or the sense and current leads of the cable to the mount.

3. Set the slide switch SlOl to the 100 to 200 ohm position, as appropriate for the

mount. On the rear, set the toggle switch S202 to the FLOAT position.

4. Turn the line power on by pressing the LINE switch S201. The correct green lamp

should light, indicating the mount operating resistance. The panel meter should

indicate the thermistor current. The external voltmeter should show a reading

equal to the product of the thermistor current and the mount operating resistance.

The ERROR lamp should be dark. If incorrect, open, or shorted connections have

been made, or if the mount is open (i.e., burned out), the ERROR indicator will

light. It will also light under conditions of rf power overload, or any other con- dition which results in the balancing servo of the instrument being unable to achieve a balance between the mount resistance and the internal resistance

standard.

5. To measure rf power, record the voltmeter reading, apply the rf power, and read

again. If temperature compensation is desired, a third reading may be taken with

the power off again, and the first and third readings averaged. This assumes,

for example, a linear drift in voltage with time, and an equal length of time

between measurements. Then, the power, P, is given by

P = (vj - V2)/Rq , (1)

where V^^ is the output voltage in the absence of rf power (or the averaged readings

mentioned above), is the voltage in the presence of rf power, and R^ is the

mount operating resistance. Thus, the so-called "bolometric power" is simply

the change in the dc power in the mount. The appropriate calibration factor for

the mount must be applied to this number to obtain the true rf power.

2.3 PRECISION POWER MEASUREMENT

It is apparent from equation (1) above, that as the rf power becomes small, approaches V^. This has serious consequences for measurement accuracy. The magnification of the voltmeter

uncertainties when it is used to measure two voltages which differ only slightly can be seen from Table 2-1, below. For this computation, a typical 200-ohm mount with a nominal bias

current of 12,75 mA was assumed. The voltmeter specifications included: ranges of 0.1, 1,

and 10 volts, with uncertainties of 0.01% of reading on all ranges, plus 0.005% of full scale on the 0.1 V range, and 0.002% of full scale on the 1 and 10 V ranges. At 1 mW, for example,

the error due to the voltmeter is only 0.04% when an RVG is used; however, it is 2.3%

without one.

When connected as shown in Figure 2-3, the power equation is modified to

P = (2Vj^ - V2)V3/Rq , (2)

where = V w V2 • The differencing between and is now done electrically, rather than

in the computation, so that the voltmeter is operating at a much larger fraction of fullscale. This minimizes the uncertainty contribution of the percent-of -fullscale specification. In effect, the RVG is used to store V^ . The procedure is similar to that in Section 2.2.

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1. Attach the bolometer mount as described in Section 2.2. Connect the negative side

of a stable voltage source (RVG) to the black DVM terminal, as shown in Figure

2-3. Arrange switching to allow the voltmeter to be connected either across the

DVM terminals, or between the positive side of the RVG and the red DVM terminal.

2. Adjust the RVG so that its output is approximately equal to the output of the Type

IV when no rf power is present. The object is to make any residual offset small

compared to the lowest range on the voltmeter to be used, e.g., 10 mV on a 100 mV

range. The residual offset will not cause an error, being removed in the calculation

of the power from the voltmeter readings. Although it is not necessary to know

exactly what the RVG output is, it is very important that the output remain stable

during the time required for a measurement. Drifts should be held to less than

10 ppm.

3. To measure rf power with the highest accuracy and minimum sensitivity to temperature

effects, the following steps are performed, usually under the control of a calculator

or small computer. (Figure 2-4 summarizes these steps.)

3a. Read and record the Type IV output with no rf power (V^

3b. Read and record the voltage difference between the Type IV output and the

output of the RVG, still with no rf power (Vp

3c. Turn on the rf power. Again read the voltage difference between the Type IV output and the output of the RVG (V^)

3d. With the rf power still off, repeat step 3b to obtain V^.

3e, Turn off the rf power and repeat step 3a to get V^.

4. Compute the boloraetric power using equation (2), as follows:

4a. For V^, use the average value of V^ and V". This provides first-order

compensation for temperature-induced drifts.

4b. For V^, use V^ - (V^ + V|j)/2. The term (V' + V")/2 Is the average value

of the difference voltage with no rf and also gJves first-order temperature

compensation. Subtracting this average from V^ gives the true change in the

output of the power meter, compensated for linear temperature changes and

corrected for any initial offset due to inexact setting of the RVG. Eq, (2) now becomes

P = [(V + V") - + (V^ + v;^)/2] • [V^ - (V^ + v;|)/2]/R^ . (3)

This sequence may be modified in the software to accomodate specific situations.

For example, if the bolometer mount is in a closely controlled environment,

compensation may be unnecessary, and the measurement process can be speeded

by eliminating unnecessary voltage readings. If the RVG is a very accurate

digital-to-analog convertor (DAC) , then it may be possible to rely in its

calibration accuracy. In general, however, the voltmeter will be considerably more accurate than the DAC, and it is better to actually measure V^ and V^

than to trust the DAC. Another possible compromise is to take several

readings with rf power on for every reading without it, especially, if the

temperature is drifiting only slowly.

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o

o

>

RVG

Figure 2-4. Typical voltage measurement sequence providing first-order

compensation for temperature drifts. Symbols shown in the

figure as the same as used in equation (3) . In automatic

systems, software techniques can often substitute for hardware, for temperature compensation, for example. Taking

these five voltage readings with a data logger or minicomputer greatly reduces the need for precise temperature con- trol of the bolometer mounts, while not seriously decreasing the speed.

TABLE 2-1. VOLTMETER-CAUSED ERRORS

P,inW ERR)R,% VI RANGE

0.100 0.1552 10.0000

d.2m 0.0914 10.0000

0.300 0.0702 10.0000

0.400 0.0595 10.0000

0.500 0.0532 10.0000

Id, em 0.0489 10.0000

0.700 0.0459 10.0000

0.800 0.0436 10.0000

0.900 0.0419 10.0000

i.000 0.0405 10.0000

V2 RANGE

10.0000

10.0000

10.0000

10.0000

10.0000

10.0000

10.0000

10.0000

10.0000

10.0000

V3 RANGE

0. 1000

0.1000

0.1000

0.1000

0. 1000

0.1000

0.1000

0.1000

0.1000

0.1000

Table 2-U Errors due to voltmeter when RVG is used, 0.1-1 mW.

P,mW ERRDR,% VI RANGE V2 RANGE

0.100 23.1772 10.0000 10.0000

0.200 11.5746 10.0000 10.0000

'0.300 7,7071 10.0000 10.0000

0.400 5.7734 10.0000 10,0000

0.500 4.6131 10.0000 10,0000

0.600 3.8396 10.0000 10.0000

0.700 3.2871 10,0000 10.0000

0.800 2.8727 10.0000 10.0000

0. 900 Ivi . 0000 10.0000

1.000 2.2926 10.0000 10.0000

Table 2-lb. Errors due to voltmeter wherf no RVG is used, 0.1

P,mW ERROR,

VI RANGE V2 RANGE

1.000 0.0405 10.0000 10.0000

2.000 0.0341 10.0000 10.0000

3,000 0.0442 10.0000 10.0000

4.000 0.0400 10,0000 10.0000

5.000 0.0376 10.0000 10.0000

6.000 0.0359 10.0000 10.0000

7.000 0.0348 10.0000 10,0000

8.000 0.0339 10.0000 10.0000

9.00U 0.0333 10,0000 10,0000

10.000 0,0328 10.0000 10.0000

Table 2-lc. Errors due to voltmeter when RVG is used, 1-10 mW

P,mW ERROR,

VI RANGE V2 RANGE

1.000 2,2926 10.0000 10.0000

2.000 1.1323 10.0000 10.0000

3.000 0.7455 10,0000 10.0000

4.000 0.5520 10,0000 10.0000

5.000 0.4359 10.0000 10.0000

6.000 0.3585 10.0000 10.0000

7.000 0.3032 10.0000 10.0000

8.000 0.2617 10.0000 10.0000

9.000 0.2294 10.0000 10.0000

10.00kJ 0.2035 10.0000 10.0000

V3 RANGE

0.1000

0.1000

1,0000

1.0000

1,0000

1.0000

1.0000

1,0000

1.0000

1,0000

Table 2-ld. I'rrors due to voltmeter when no RVG is used, 1-10 mW.

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SECTION III MAINTENANCE AND CALIBRATION

3.1 INTRODUCTION

This section provides calibration and performance testing information. The adjustment

procedure below assures that the unit will meet specifications. Failure of a unit is

indicated by inadequate response to one or more of the adjustments, and further troubleshooting is required. This section also includes information on replacement of special

components. All details refer to NBS-built units.

l-Zhen it becomes necessary to remove a component from the two-sided printed circuit board, do

so with great care. A solder removing tool must be used to remove solder from around the

component leads. This will minimize the danger of lifting the circuit pattern from the

board, a condition not easily repaired. Generally, for two-lead components, it is safer to remove the defective part by cutting the leads as close as possible to the body of the part.

The replacement is then soldered to the leads of the old component.

3.2 TEST EQUIPMENT

The following test equipment is required:

DC null detector (microvoltmeter

Range: ± 10 microvolts fullscale to ± 30 V fullscale

Input impedance: 100 kohms or greater, resistive

Accuracy: ± 2% of fullscale

General purpose oscilloscope

DC milliammeter

Range: 30 raA Accuracy: ± 2%

Standard resistor, 4-terminal, 100 ohms ± 0.01% or better. The accuracy of the

power meter depends directly upon the accuracy of this resistor.

Thermistor mounts for 100 ohm and 200 ohm operation.

Pulse generator

Required output is 1 ms pulse, negative-going from + 5 V to ground.

Output impedance: 100 ohms or less

Adjustable resistance decade or 10-turn potentiometer, capable of adjustment over range of 0 to 100 ohms with 0.1% resolution. Accuracy is immaterial.

Diode, silicon, 1N4153 or equivalent.

AC voltmeters for line-voltage tests, 105-130 V.

10. Adjustable autotransf ormer for line voltage test, 117; 105-130 V.

11. Power supply, + 5V, 25 mA minimum.

3.3 INITIAL SETUP

1. With the instrument turned off, remove the left side by removing 2 screws each from

the front and rear panels.

2. Connect the links, if necessary, and connect the 1N4153 diode between the BOLOM

terminals, with the anode to the red terminal. Set the lOO-REM-200 switch to 100.

3. Connect the instrument to the autotransf ormer , and set it for 105 volts.

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4. Apply power, allow at least five minutes for warmup stabilization.

5. Set the GND-FLOAT switch on the rear panel to FLOAT. It should remain in that

position throughout all adjustments. NOTE: The red ERROR lamp on the front panel

should remain dark throughout the entire adjustment and calibration procedure, ex- cept for momentary indications when changing the setting of the 100-REM-20G switch.

It is impossible to perform the adjustments if the lamp is lit, and the cause must

be found and corrected.

6. Check, the jumpers for proper placement: brown to Ell, red to E2, green to E15,

blue to E6, and gray to E8. The ribbon cable connector should be seated securely

in JlOl.

3.4 POWER SUPPLY PERFORMANCE (FIG. 3-1)

1. There are two dual isolated supplies, each providing plus and minus 15 V. They are designated supply "A" and supply "B". The common of the A supply is connected to the black BOLOM terminal. Measure the A+ output at E7 (gray jumper) while changing

the line voltage from 105 to 130 volts. It should remain in the range 14.4 to 15.6 V.

2. Repeat the measurement for the A- supply; this is accessible at the case of ARIOI.

The voltage should remain in the range - 14.4 to - 15.6 V.

3. Repeat the measurement for the B+ supply. The B supply common is connected to E5

The B+ is accessible at the collector tab of power transistor Q115. The range of

voltage is 14.4 to 15.6 volts.

4. Repeat the measurement for the B- supply, which is accessible at the case of AR102.

The voltage should remain in the range - 14.4 to - 15.6 V.

Any out-of -tolerance indications from the above tests indicate failure of one or more of the

voltage regulators, filter capacitors, rectifiers, or transformer. Also, check for incorrect

insertion of the line voltage selector card.

3.5 AMPLIFIER OFFSET ADJUSTMENT (FIG. 3-3)

The two operational amplifiers, ARlOl and AR102, are provided with null balancing potentiometers. In making these adjustments, it is important that the amplifiers be in thermal

equilibrium; heat from fingers, etc., must be allowed to dissipate before attempting a balance. In addition, the null detector used should be very well isolated from ground,

preferably battery operated. The leads used with it should not exhibit large thermal emfs

when handled. Proper operation of the instrument depends upon the cancellation of the

emfs arising from dissimilar metals in the circuit, such as solder, copper, socket contact

alloys, integrated circuit lead alloys, and so forth. This cancellation occurs only if there are no large temperature gradients across the circuit board; therefore, it is advisable to protect the unit from drafts and strong light sources with a loose cover after attaching the test leads. When a stable indication is obtained on the null detector, the adjustments may

be made.

1. Connect the null detector between Ell and E2. It is essential that the measurement

not be made between Ell and El, because El carries current, and an error will be

introduced. Adjust R128 for a null. A stable indication of less than ± 10 microvolts should be obtained.

2. Repeat for AR102 by connecting the null detector between E6 and E15 and adjusting

R140.

Failure to obtain a proper null may be due to defective ARlOl or AR102, or to any condition

which prevents the servo from balancing, as indicated by the ERROR lamp.

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3.6 CURRENT INDICATOR (FIG. 3-3)

The front panel milliammeter serves only to indicate the approximate bolometer current; it is not intended to be used for measurement. It is adjusted as follows:

1. Turn off the power.

2. Remove one end of the diode across the BOLOM terminals and insert in series both

the 0 to 100 ohm adjustable resistor and the dc milliammeter against which the adjustment is to be made. Set the lOO-REM-200 switch to 100. Turn the power on.

3. If the ERROR lamp is lit, reduce the value of the series resistor. It should be

possible to obtain an indication of 30 mA on the external meter by adjustment of

the resistor. Increasing the resistance increases the current, because in order to maintain the series combination of diode, resistor, and external meter constant at 100 ohms, the servo must decrease the apparent resistance of the diode by increasing the current through it. Use of the diode eliminates the danger of damage to a valuable thermistor mount during this procedure.

4. After obtaining a current of 30 mA on the external meter, adjust R143 to bring the

internal meter to full scale indication. Turn off power.

3.7 CALIBRATION OF INTERNAL RESISTORS (FIG. 3-3 & 3-4)

The absolute accuracy of the instrument depends upon the accuracy of the resistance against

which the thermistor is compared by the servo. This resistance is composed of two adjustable

resistances, connected in such a way that separate potential and current leads are available

J5g^ both ends and at the junction of the two resistances. Thig makes it possible to define

|wBch of them (very accurately) , without concern for the resistance of the circuit board pat-

"Tiern. The adjustment procedure is based upon the comparison technique shown in the Appendix,

Figure 2. Section 3.5 must be completed before the following steps are undertaken:

1. Turn the power off. Remove ARIOI. Transfer the gray jumper from E8 to E9. Set the lOO-REM-200 switch to 100.

2. Open the links. Attach the 100 ohm standard resistor to the front panel terminals. The current terminals must be connected to the BOLOM binding posts of the

instrument, and the potential terminals must be connected to the respective blue

SENSE binding posts.

3. Turn the power on and allow at least 5 minutes for stabilization.

4. Attach the null detector between Ell and E2.

At this point, in accordance with the discussion in Appendix I, current is flowing through

Q109 into E3, the junction of the two resistors. This current is now fixed, limited by R144A

to about 16 mA. The null detector is reading what would be the input signal to ARIOI. If the internal resistor comprised of R133-R136 is not identical to the external standard, the

difference in the voltage drops across the two will appear on the null detector. The sensitivity of the measurement is about 1.6 parts per million per microvolt. Since no current

flows through R129-R132, the detector is connected to E2 for convenience.

5. Adjust R135 for a null indication on the null detector. A null of less than 10

microvolts should be obtained.

6. Turn the power off. Transfer the blue jumper from E6 to E4. Set the lOO-REM-200

switch to 200. Turn the power on.

i?^^rent is now flowing through all of R129-R136, but AR102 is now sensing the potential at instead of E6. Therefore, R133-R136 are now outside of the loop, as far as the calibration is concerned.

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7. Adjust R131 for a null of less than 10 microvolts.

8. Turn the power off. Replace ARIOI. Return blue jumper to E6, Return gray jumper

to E8. Remove external standard.

Inability to make the above adjustments indicates failure of AR102, misadjustment of amplifier offset, failure of Q109 or Q112, or out-of-tolerance resistors in the network R129-R136.

The adjustment range of each half of the resistor network comprising R129-R136 is ± 0.05%

around the nominal 100 ohm value. The nominal tolerance on the principal resistors, R130

and R134, is ± 0.01%, so that it is unlikely that they will ever drift so far that adjustment is impossible. Worst-case accumulation of limit tolerance resistors will still allow

proper adjustment. The precision of the adjustment, limited by the setability of the

trimmers, is about h ppm. Because of the 4-terminal connection to the external standard,

lead resistances of as much as 1 ohm produce no detectable error.

If trouble is suspected, certain checks may isolate the cause. When the instrument is operating with a thermistor or diode, measure the voltage between E2 and E4 in the 100 ohm con- dition with a null detector. It should be essentially zero, less than 10 microvolts. A higher value indicates failure of Q112 to turn off. A similar test can be applied to Q109,

by interchange. These transistors are MOS field effect devices with no gate protection

diodes, so they may be easily damaged by static discharge. Do not allow them to contact

polyethylene or polystyrene sheet or foam. Transistors Q114 and Q113 connect the red DVM

terminal to the appropriate point, depending upon the setting of the lOO-REM-200 switch. If either has excessive leakage in the off state, errors will result.

3.8 GAIN AND NOISE TESTS

The error analysis of the servo is based on a model which assumes that the amplifier

differential input voltages never exceed a certain value under any condition of operation,

including temperature effects and finite amplifier gain. The resulting equation is given in

Appendix I equation (1) . In order to ensure that the instrument is actually performing to specifications, it is necessary to measure the amplifier gains.

1. Set up the instrument as described in Section 2.2, preferably with a 100 ohm thermistor mount.

2. Connect the null detector between Ell and E2. If offset adjustment. Section 3.5

has been carried out properly, this reading should be less than 10 microvolts.

Record this reading.

3. Record the reading of the external voltmeter.

4. Apply rf power to the point where the external voltmeter reading has been reduced

to about 0.2 volts. Record the readings of both the null detector and the voltmeter.

5. Connect the null detector between E6 and E15. Repeat steps 1-4 above.

The gain of each amplifier is found by dividing the change in the external voltmeter reading

by the change in the null detector reading when rf power is applied. The amplifiers used in

the Type IV have a minimum gain specification of 120,000. The value will typically be larger

than this. An amplifier showing a gain of less than this should be replaced and Section 3.5

should be repeated for that amplifier.

At low power levels, noise ultimately limits the usefulness of the instrument. The servo noise may be observed by connecting an oscilloscope to the DVM terminals with a diode across

the BOLOM terminals, anode to the red terminal. The trace should show no "flicker" or "popcorn" noise—only a steady display of about 1 mV peak-to-peak amplitude. There should be no components present which are synchronous with the line frequency. A true-rms wideband voltmeter connected across the DVM terminals should indicate not more than 100 microvolts in a band from 10 Hz to 10 MHz.

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If noise measurements are made with a thermistor, the noise due to a defective component is

likely to be obscured by the excess noise generated by the thermistor bead itself. On the

other hand, the diode is not a good dynamic simulation of the thermistor, because it has a virtually zero response time. Therefore, it is also informative to examine the noise with a typical mount. When this is done, line-synchronous noise will usually be seen. This is due

to magnetic field leakage from the power transformer coupling with the printed circuit board

pattern. Because the bandwidth of the servo is now limited by the thermistor, it cannot maintain balance for these disturbances (mostly 120 Hz and higher line harmonics). However, measurement accuracy will not be affected with most voltmeters because of the very high linefrequency rejection inherent in their design. Typical amplitudes are on the order of 3 mV

peak-to-peak. The noise will now also show flicker characteristics. There is considerable

variation in excess noise from one thermistor to the next.

3.9 TEST OF INTERFACE LOGIC (FIG. 3-2)

The Type IV power Meter is provided with circuitry to allow change of mount operating resistance by an external controller, and to give indication of certain kinds of malfunction to the

controller. The interface is simple; most of the timing and "handshaking" circuitry must be

provided by the system controller. Since it must be possible to FLOAT the unit, optoisolators

are used for all digital signals in and out.

1, Turn the power off. It is necessary to provide 1 kohm pullup resistors for PlOl,

pins 7, 9, and 10, in order to give an observable signal. Connect three resistors

between these pins and PlOl-2. Connect the + 5 V supply to PlOl-2 also. Connect

the negative side of the supply to PlOl-5.

2. Connect a diode to the BOLOM terminals, anode to red terminal. Turn the power on. Set 100-REM-200 to REM.

Apply a negative pulse to PlOl-1, from 4- 5 V to 0 V. The 200-ohm indicator

should light and stay lit after the pulse is removed.

4. Apply the negative pulse to PlOl-3. The 100 indicator should light and stay lit

after the pulse is removed.

If the thermistor mount is used in place of a diode, the ERROR lamp will flash momentarily

during the transition from the 100 ohm state to the 200 ohm state, or vice versa. This is

due to the time lag occasioned by the thermal capacity of the thermistor bead. When going

from 200 ohms to 100 ohms, the lag is short because of the ability of the instrument to supply additional current rapidly; the temperature rise is also rapid. On the other hand,

when cooling from 100 ohms to 200 ohms, the cooling rate is dependent solely on the mount

characteristics and is determined mostly by the conduction of heat from the bead along its

leads. Therefore, the ERROR light will stay on for a large part of a second. This is normal.

Monitor PlOl-9 with the oscilloscope while applying pulses to first PlOl-1 and

then PlOl-2. A negative-going + 5 V to 0 V pulse should appear at PlOl-9 following

every positive-going transition of the test pulse. The presence of this pulse con- firms that the instrument has received the test pulse.

Apply the test pulse to PlOl-1 and PlOl-2 simultaneously. No response pulse should

be seen at PlOl-9. This ensures that ambiguous commands do not produce an acknowledging pulse at PlOl-9.

Monitor PlOl-10. When the test pulse is applied to PlOl-1, no pulse should appear

at PlOl-lO. When the test pulse is applied to PlOl-2, a negative-going + 5 V to

0 V pulse should appear at PlOl-10. This pulse provides information on the state

to which the instrument has been set: it appears only when set to 100 ohms. If present, it is coincident with the pulse appearing at PlOl-9.

Monitor PlOl-7. This pin goes low whenever an out-of-balance condition occurs in

the servo. This should occur after every transition from 100 to 200 ohms, or vice versa, but at no other time. This pulse may be difficult to observe unless a thermistor mount is used instead of a diode. Short the diode or thermistor mount. PlOl-7 should go low. This tells the system that data taken while PlOl-7 is low

is invalid, and that a fault condition exists, such as open or shorted bolometer

leads, or rf overload.

13
c
3.10 NONSTANDARD COMPONENTS

Use of unusual or nonstandard components has been kept to a minimum; however, certain components must meet critical specifications. These are:

1. R130 and R134, 100 ohms ± 0.01%, wirewound, specified at 23°C and negligible power.

Drift: ± 50 ppm/yr

Temperature coefficient: ± 5 ppm/ C Temperature rise (power coefficient): less than 25 C rise per watt.

2. T201, power transformer. The primary is split for 117/234 volt operation. One

primary is tapped for 100 volt operation; this partial winding must be able to carry the entire burden with a 100 volt supply line. Secondaries are split and

shielded, 36 volts center tapped, 90 mA per winding to fullwave rectifier. The

critical specification is for interwinding capacitance. Three-terminal capacitance

between primary and either secondary, or between secondaries must be less than

0.01 picofarad with all shields grounded.

3. AT101-AT105 optoisolators . While these are not critical in the same sense as the two components mentioned above, it is desirable that the current transfer ratio be

adequate to allow direct interf acting with TTL logic, while maintaining a collector

swing meeting the usual 0.8 V low and 2.4 V high logic levels. Common Darlington

optoisolators may not meet the 0.8 V specification.

The specific components selected for the prototype instruments were chosen on the basis of

suitability, availability, and cost and do not represent the only possible choices. The

National Bureau of Standards does not represent that they are the best possible for the application, only that they were used in the instruments described here. Substitution of nominally

equivalent components meeting the same specifications should normally cause no difficulty;

however, the National Bureau of Standards has not tested all such possible choices. In case

of question, please consult with the designer.

14

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(
TABLE A-1

MANUFACTURER'S CODES

3M

3M Company, Electronic Products Division

3M Center

St. Paul, Minnesota 55101

AB

Allen-Bradley Company

1201 S. Second Street

Milwaukee, Wisconsin 53204

ALCO

Alcoswitch Division of Alco Electronic Products, Inc.

P. 0. Box 1348

Lawrence, Massachusetts 01842

AMPH

Bunker Rairo Sales Division

2875 South 25th Street

Broadview, Illinois 60153

API LFE Corporation, Process Control Division

1601 Trapelo Rjad

Waltham, Massachusetts 02154

AZTE

Aztec Electronics, Incorporated

1240 Blue Gum Street

Anaheim, California 92806

BELD

Belden Corporation, Electronic Division

P. 0. Box 1331

Richmond, Indiana 47374

BOUR

Bourns, Incorporated, Trimpot Division

1200 Columbia Avenue

Riverside, California 92507

CA

Circuit Assembly Corporation

3169 Red Hill Avenue

Costa Mesa, California 92626

CAMB

Cambridge Thermionic Corporation

445 Concord Avenue

Cambridge, Massachusetts 02138

CORC

Cor com. Incorporated

2635 North Kildare Avenue

Chicago, Illinois 60639

CORN

Corning Glass Works, Electronic Products Division

Houghton Park, Bldg. A-2

Corning, New York 14830

21
I
TABLE A-1

MANUFACTURER'S CCDES, CONTINUED

DALE

Dale Electronics, Incorporated

Box 609

Columbus, Nebraska 68601

DEST

Destek Industries

P. 0. Box 24163

Los Angeles, California 90024

ELCO

EDCO

Maryland Road & Ccsnputer Avenue

Willow Grove, Pennsylvania 19090

ERIE

Erie Technological Products

644 West 12th Street

Erie, Pennsylvania 16512

ESI

Electro Scientific Industries

13900 N, W. Science Park Drive

Portland, Oregon 97229

GE

General Electric Co., Semiconductor Products Department

Electronics Park, Box 1122

Syracuse, New York 13201

GENE

General Instrument Corp., Semiconductor Products Division

600 West John Street

Hicksville, New York 11802

HHSM

Herman H. Smith, Incorporated

812 Snediker Avenue

Brooklyn, New York 11207

HP

Hewlett-Packard, Optoelectronic Division

640 Page Mill Road

Palo Alto, California 94304

JBT

J-B-T Instruments, Incorporated

Box 1904

New Haven, Connecticut 06509

KEME

Union Carbide Corporation, Components Department

Box 5928

Greenville, South Carolina 29606

KODA

Eastman Kodak Company

343 State Street

Rochestor, New York 14650

22
(
TABLE A-1

MANUFACTURER'S CODES, CONTINUED

LITR

Litronix, Incorporated

19000 Homestead Road

Cupertino, California 95014

MOTO

Motorola Semiconductor Products

2002 West 10th Place

Tempe, Arizona 85281

PMI Precision Monolithics, Incorporated

1500 Space Drive

Santa Clara, California 95050

RCA

ICA Solid State Division

Route 202

Sotierville, New Jersey 08876

SCHA

lEE/Schadow, Incorporated

8081 Wallace Road

Eden Prairie, Minnesota 55343

SIQi

Signetics Corporation

811 East Arques Avenue

Sunnyvale, California 94086

SPRA

Sprague Electric Company

481 Marshall Street

North Adams, Massachusetts 01247

SUPR

Superior Electric Company

1000 Middle Street

Briston, Connecticut 06010

TOER

Thermalloy, Incorporated

2021 West Valley View

Dallas, Texas 75234

ULTR

Ultronix, Incorporated

P. 0. Box 1090

Grand Junction, Colorado 81501

USEC

USECO Division, Litton Industries

13536 Saticoy Street

Van Nuys, California 91409

23
I , /
TABLE A-2

CATEGORY NO. 1

PARTS LIST

-RESISTORS1 R101 3 CARBON, 0.25w, 5% 1000 OHMS AB FSN 5905-681-6462

2 R102 11 CARBCW, 0,25W, 5% 33 KOHMS AB FSN 5905-577-9602

3 R103 SAME AS R102

4 R104 6 CARBON, 0.25W, 5% 750 OHMS AB FSN 5905-807-4954

5 R105 SAME AS R104

6 R106 SAME AS R104

78 R107

R108

SAME AS R102

SAME AS R102

9 R109 SAME AS R102

10 R110 2 CARBC»^, 0.25W, 5% 15 KOHMS AB FSN 5905-681--8818

11 Rill SAME AS R102

12 R112 SAME AS R102

13 R113 SAME AS R102

14 R114 SAME AS R102

15 R115 3 CARBON, 0,25W, 5% 100 KOHMS AB FSN 5905-686--3129

16 R116 SAME AS R104

17 R118 SAME AS R102

18 R119 2 CARBON, 0.25W, 5% 330 KOHMS AB FSN 5905-686--3131

19 R119A SAME AS R115

20 R120 SAME AS R119

21 R121 SAME AS R104

22 R122 SAME AS R125

23 R123 1 CARBON, 0,25W, 5% 16 OlMS AB FSN 5905-835--1630

24 R124 SAME AS R104

25 R125 SAME AS R110

26 R126 SAME AS R102

27 R127 2 CARBON, 0.25W, 5% 1 Mom AB FSN 5905-681--8817

28 R128 2 TRI^WER, METAL Filial 20 KOHM BOUR 3009P-1-203

29 R129 2 METAL FILM, 0. 125W, 1% 9530 OHMS CORN

30 R130 2 WIREWOUND, 0,01% 100 ams ESI RF6R

31 R131 2 TRIMMER, METAL FILM 1 KOHM BOUR 3009P-1-102

32 R132 2 WIREWOUND, 1%, 0ol5W 1 cm DALE RB55CE1R000F

33 R133 SAME AS R129

34 R134 SAME AS R130

35 R135 SAME AS R131

36 R136 SAME AS R132

37 R137 SAME AS R101

38 R138 1 CARBON, 0.25W, 5% 22 KOHMS AB FSN 5905-687--0002

39 R139 SAME AS R101

40 R140 SAME AS R128

41 R141 1 CARBOI, 0,25W, 5% 10 OHMS AB FSN 5905-817--7971

42 R142 1 CARBCW, 0.25W, 5% 220 OHMS AB FSN 5905-683--2240

43 R143 1 TRIMMER, METAL FILM 100 OHMS BCXJR 3329H-100

44 R144 SAME AS R127

45 R144A 1 CARBON, 0.5W, 5% 820 OHMS AB

CATEGORY NO.

1 C101 ; 2 C102

3 C103 ] 4 C201

5 C202

6 C203

7 C204

2 DISC CERAMIC, IKV

SAME AS C101

1 METALLIZED FILM

4 ALUMINUM, 35V

SAME AS C201

SAME AS C201

SAME AS C201

—CAPACITORS100V

1000 PF ERIE 801-000-X5FO-102K

0.047 MF

220 MF

SPRA

SPRA

225P47391WD3

503D227G035DK

24
r
•

TABLE A-2

PARTS LIST, CONTINUED

8 C205 4 SOLID TANTALIM,

9 C2m SAME AS C205

10 C207 SAME AS C205

11 C208 SAME AS C205

12 C209 4 SOLID TANTALUM,

13 C210 SAME AS C209

14 C211 SAME AS C209

15 C212 SAME AS C209

35V

25V

1 MF KEME T330A105K035AS

4,7 MF KEME T330A475K025AS

CATEGORY NO. -TRANSISTORS1 Q101 9 SILICON NPN

2 Q102 SAME AS Q101

3 Q103 SAME AS Q101

4 Q104 SAME AS Q101

5 Q105 SAME AS Q101

6 Q106 SAME AS Q101

7 Q107 SAME AS Q101

8 Q108 SAME AS Q101

9 Q109 4 MOSFET, N-CHANNEL

10 Q110 2 SILICOJ Nm DARLINCnON

11 QUI SAME AS Q101

12 Q112 SAME AS Q109

13 Q112A 1 SILICCW PNP

14 Q113 SAME AS Q109

15 Q114 SAME AS Q109

16 Q115 SAME AS Q110

2N3904 MOrO FSN 5961-892-8706

SD210

MPS-U45

2N3906

SIGN

MOTO

MOro FSN 5961-931-0372

CATEGORY NO. 4 DIODES1 CR101 1 ZENER, 4.7V, 10%, 0.5W UJ5230A MOTO

2 CR102 7 SILICm SIGNAL 1N4153 GE

3 CR103 SAME AS CR102

4 CR104 SAME AS CR102

5 CR105 SAME AS CR102

6 CR106 1 ZENER, 24V, 10%, 0.5W 1N5252A MOTO

7 CR107 SAME AS CR102

8 CR108 SAME AS CR102

9 CR109 SAME AS CR102

10 CR201 2 RECTIFIER, FVJ BRIDGE W02M GENE

11 CR202 SAME AS CR201

12 DS101 2 LED, GREEN HP

13 OS 10 2 SAME AS OS 101

14 OS 10 3 1 LED, RED HP

5082-4955

5082-4655

CATEGORY NO. -INTEGRATED CIRCUITS1 AR101 2 OPERATIONAL AMPLIFIER OP-05CJ PMI

2 AR102 SAME AS AR101

3 AT101 5 OPTO-ISOLATOR IL-5 LITR

4 AT102 SAME AS AT101

5 AT10 3 SAME AS AT101

6 AT104 SAME AS AT101

7 AT105 SAME AS AT101

8 Ul 1 CMOS, QUAD 2-INPUT NAND CD4011AE RCA

25
I
TABLE A-2

PARTS LIST, COOTINUED

9 U2 1 CMOS, QUAD EX-OR CD4030AE RCA

10 VR201 2 VOLTAGE REGULATOR, 15V MC7815C MOTO

11 VR202 2 VOLTAGE REGULATOR, -15V MC7915C MOTO

12 VR203 SAME AS VR201

13 VH204 SAME AS VR202

CATEGORY NO. 6- —CCWNECTORS1 2 8-PIN PTFE SOCKET AMPH 631 26011 082

2 5 6-PIN DIP SOCKET CAMB 703-3771-01-03-16

3 2 14-PIN DIP TEFMINATION 3M 3406-1000

4 4 4-PIN PTFE SOCKET AMPH 631 26011 042

5 J101 3 14-PIN DIP SOCKET CA CA-14LS1-10SD

6 NONE 2 MATE TO P101, P102 3M 3473-3000

7 NONE SAME AS J101

8 P101 2 10 PIN MALE SCOTCEIFLEX 3M 3491-1002

9 P102 SAME AS P101

CATEGORY NO. -TERMINALS1 El-15

2 El-15

3 E201

4 E202

10 PC VERTICAL HE^dAPHRO

5 CRIMP HERMAPHIO

2 BINDING POST, BLACK

2 BINDING POST, BLUE

5 E203 SAME AS E202

6 E204 2 BINDING POST, RED

7 E205 SAME AS E204

8 E206 SAME AS E201

9 E16-21 6 TERMINAL, FORK, STAKED

ELCO 62-5201-02-13-00-011

ELCO 00-60-8017-0313

SUPR DF21BC

SUPR DF21BLC

SUPR DF21RC

USEC 200 IBl

CATEGORY NO. 8 SWITCHES1 S101 1 SP3T SLIDE ALCO MSS-1300

2 S201 1 2PDT PUSH-PUSH , SCHA ZFFA200BLK/GR2UNEI5

3 S202 1 SPOT TOGGLE JBT JMT 123

CATEGORY NO. 9 METERS1 Ml 1 0-1 MA, 32 cm COIL API 1015-0003-0000

2 1 METER SCALE MATERIAL KODA CAT. NO. 157 8285

CATEGORY NO. 10 HARDWARE —

1 6 SPACER, HEX BRASS 6-32X0.25 HHSM 2320 (GOLD FLASH ADDED)

2 1 CHASSIS AND PANELS DEST K-33-9 (NO OPTIONS)

3 6 MACHINE SCREWS 6-32X3/16 NPBH BRASS

4 6 WASHERS #6 91ALL BRASS (GOLD FLASH ADDED)

5 2 SWING LUGS, GOLDPLATED ESI 3247

6 1 STOP, 1X4.5X0.062 IN 6061-T6 ALUMINUM, SET W/EPOXY

26

TABLE A-2

PARTS UST, CaJTINUED

CATEGORIES 11 AND 12 ARE EMPTY.

CATEGORY NO. 13- -MISCELLANBOOS1 1 LINE CORD BEI£) 17250

2 WIRE, PRIMARY rcfWER 2-#24 mC BELD 8461

3 A201 1 LINE CCMJECTOR/SELECTOR CORC 6J4

4 T201 1 TRANSFOreiER, POWER 117/36/36 AZTE 13575 (SER

5 T201 1 TRANSFORMER, POWER 117/36/36 AZTE 13886 (SER

6 4 MOUNTING PAD, C201-C204 THER 7717-204

122

123

BELOW)

ABOVE)

TABLE A-3

CONNECTOR PIN ASSIQJMENTS

^01:

Wl Pull low to set mount operating resistance to 200 ohms.

2 External +5 volt power for opto isolators. Drain is less than 5 itiA. 3 Pull low to set mount operating resistance to 100 ohms,

4 Not used.

5 External common.

6 -DVMs Bolometer negative sense lead, black front panel DVM post E206 (also E202)

7 Goes low on any error condition; data is invalid when pin 7 is low.

8 +DVM: front panel DVM post E206.

9 Goes low to acknowledge completion of setting mount operating resistance (approximately 1 millisecond pulse, +5 volts to 0 and back to +5 volts.

10 Cneck signal to acknowledge that specified mount operating resistance was set.

If mount was set to 100 ohms, a pulse identical to and coincident with the pulse on pin 9 appears. If the mount was set to 200 ohms, no pulse appears.

P102:

1 BolcHTieter negative sense lead, in parallel with front panel blue post E202.

2 Not used.

3 BolcHneter negative current lead, in parallel with front panel black post E201.

4 Same as pin 1, P102.

5 Bolometer positive current lead, in parallel with front panel red post E204.

6 Not used.

7 Bolometer positive sense lead, in parallel with front panel blue post E203.

8 Not used.

9 Not used.

13 Not used.

27
€(
IEEE I KANSACTIONS ON INS I RUMEN I A TION AND MEASUHKMEN I , VOL IM 25, NO 4, DECEMHEK 1976 343 A New Self-Balancing DC-Substitution RF Power Meter

NEIL T. LARSEN, member, ieee

Abstract—Problems intrinsic in self-balancing Wheatstone

bridges have led to the development of a new dc substitution mi- crowave power meter. The new instrument allows four-terminal measurement of bolometer resistance and affords improved accuracy and lower noise at a lower cost than earlier instruments. Measurement errors due to imperfect behavior of the servo system

are typically less than 0.01 percent.

Introduction

FOR MANY YEARS the highest accuracy in microwave power measurements in the range from 0.1 to 10 mW has been achieved with bolometers, using dc

substitution techniques. The first such precision instru- ment was a self-balancing Wheatstone bridge described

by Engen in 1957 [1]. In 1970, we described an improved

instrument which also was a self-balancing Wheatstone

bridge but included a power-leveling capability and a

differential voltmeter [2]. We now describe a new dc substitution power meter which provides certain advantages

over the earlier instruments in lower cost, greatly simplified circuitry, improved signal-to-noise ratio in the output

signal, and the reduction or elimination of certain errors.

Background

Fig. 1 shows the form of the original self-balancing

Wheatstone bridge, with a bolometer in one arm. The

operational amplifier senses the error voltage across the

horizontal diagonal of the bridge and drives the top of the

bridge to heat the bolometer to a point which closely ap- proximates bridge balance. The voltage across the vertical diagonal is measured before and after the RF power is applied, and the unknown RF power is related to the change in the computed dc power in the bolometer through

the constants of the bolometer mount.

This straightforward approach is subject to certain problems, however. First, as RF power is dissipated in the bolometer, the common-mode voltage at the amplifier

input changes; therefore, an amplifier having a high com- mon-mode rejection ratio (CMRR) is required for accurate measurement. Second, the bolometer is a two-terminal

device which typically must be located at some distance

from the other three arms of the self-balancing bridge,

since in most systems, the terminal surface of the RF

portion of the system dictates the physical location of the bolometer mount. It is not common practice, nor is it usually feasible, to separate the resistances associated with

Manuscript received June 30, 1976. The author is with the Electromagnetics Division, National Bureau

(if Standards, Boulder, CO 8();W)2.

+

Fig. 1. .Self-balancing Wheatstone Ijridge.

the leads to the bolometer from the bolometer itself. This

causes a first-order error in the measurement of the substituted power unless the actual lead resistances are measured separately, and corresponding corrections cal- culated for each different set of leads. The third problem is more subtle. Consider equivalent

noise-voltage generators in series with each of the input

leads of the balancing amplifier. If there is no feedback

from the output of the amplifier to the inverting input,

then the noise will be amplified by the full open-loop gain

of the amplifier and will appear at the top of the bridge

the point from which the output signal is taken. Since it

is the purpose of the servo system to maintain the bridge

in balance as exactly as possible, the result is a closed-loop

gain for noise that is higher than the closed-loop gain for the dc signal (at those frequencies where the attenuation

of the feedback path is high and the amplifier has gain).

This conflict is an intrinsic problem in all systems which

use a self-balancing dc bridge.

The Basic Circuh

Active measuring circuits of various kinds incorporating

operational amplifiers and isolated power supplies are not new. In 1954 Hoge [3] proposed a manually balanced cir- cuit for the intercomparison of two four-terminal resistors. Although his system was manually operated, his concept

bears some interesting similarities to the circuit described

below. Others [4], [5] have reported on resistance ratio measurement techniques involving operational amplifiers.

However, the prior work has dealt exclusively with linear systems involving linear resistances. In Fig. 2, an operational amplifier and associated power supply are connected

with two four-terminal resistors and an external voltage source in such a way that a current /, flows through R\.

Under the assumption of an ideal amplifier, the current

will flow as shown, and because by hypothesis no currents flow in the amplifier input leads, exactly the same current

flows out of the power supply common terminal ba.'k

28
(
IKKK. TRANSACTIONS ON INSTRUMKNTATION AND MEASUKKMKNT. 1>KCEMBKH 1976

Fig. 2. A method for intercomparing two four-terminal resistors.

R2 -— I

Fin- A self-adjusting current loop.

(^jj^nigh /?o to the adjustable source. The amplifier mainfains the current at such a level as to keep a zero potential

(inference between its input leads at all times. A voltmeter

connected between the other two potential terminals of R i and R > will display a reading which is proportional to the

difference in resistance between R] and /?2- Now consider replacing R2 by a negative temperature

coefficient bolometer (a thermistor). If the source is varied, the current / will vary. This will cause the bolometer re- sistance to change and will produce a varying indication on the voltmeter. This immediately suggests closing a second feedback loop in such a way that the source voltage

is controlled by the voltmeter reading to produce a volt- meter reading of zero. This concept is shown in Fig. 3, where the voltmeter and source have been replaced by a

second operational amplifier and its power supplies. If a

current / flows through R\,hy the previous argument, the

current flowing through R > is identically the same. The

amplifiers maintain a zero potential difference across their respective inputs by controlling these identical currents. Thus, since R \ and R2 have identical voltage drops across them for identical currents, R^ must be equal to R\. This

circuit therefore forces the bolometer resistance into

equality with R\, and it does so on a four-terminal basis. This allows the elimination of the lead-error problem in- trinsic to a Wheatstone bridge. The circuit has two stable

states, since the current can, in principle, flow in either «ion. However, once the current flow has been eslied, the self-balancing servo is quite stable. It is a

1 matter to provide for a predetermined direction of How in an actual instrument.

Fig. 4. Error sources in the loop. Current /) is the desired holometer

current.

The circuit also has interesting symmetry properties. For example, in one configuration the amplifier com- mon-mode voltages are identically zero if the lead resis- tances are zero. This would allow the use of chopper-stabilized three-terminal amplifiers instead of differential amplifiers. Finally, the system works equally well with

positive temperature coefficient bolometers (barretters)

by interchanging the bolometer and Ri. In all, a total of

eight configurations result from various combinations of amplifier configurations and bolometers.

Error Analysis

The error sources in the system due to the limitations

of the operational amplifiers and the reference resistor Rj

are shown in Fig. 4. The assumptions are that the amplifier

input bias currents are small but finite, that leakage currents flowing into or out of each operational amplifier and

its power supplies are negligibly small, and that the amplifier input offset voltages are small but finite. The amplifier gain is assumed to be large but finite. Equation (1) summarizes the results of the analysis.

P =

{2yj - V3)V3

Ro

1 - X

£1

(

E'l I . . Ro

- v.o)

2V,

^ Roil, - IrJ 2V, - V:, 2

'^(1+ —^—

)

-VA yViiVi-V:,)/

Rl

)

^V,ai -I- S'l/j, -I- (V, - U;,)<v:,

(V, - V;,)S:,/i:A (1)

where

V| - output voltage with no RF power

V-i = output voltage with RF power

7 = bolometer coefficient 12/W

29
I
I.ARSEN: DC SUBSTrrUTION HF METER .'145

0 5 10 15 20 25 50 55 RF Power. mW

Fig. .5. Voltmeter errors with and without offsetting reference voltage

generator. Total servo errors with bias currents of 6 nA and amplifier

offsets of 10 mV.

Si,S2,S'i = range (full scale) of the external voltmeter

when measuring Vi, V2, and V3, respectively

«i.a2,«3 = fraction of reading error in the external voltmeter when measuring Vi, V2, and V3, respectively

0iy02<^3 = fraction of full scale error in the externa, voltmeter when measuring Vi, V2, and V3, respectively.

The remaining terms in the equation are shown in Fig. 4. Error voltages £1 and E2 are due to the combined effect of ampUfier offset and finite gain. The loop current is Ii, and 13 through le represent the amplifier input bias currents. The nominal resistance of the reference resistor is

/?(), and it may be in error by the fractional amount X. The

bolometer is represented by the element T, which has an ohms-per-watt coefficient 7. The first error term is due to the reference resistor and

is not dependent on the RF power level. The second term

is due to the finite amplifier gains and offsets and is a

function of the RF power level. The third term is due to the

amplifier input bias currents. Note that two of the four bias currents, 14 and /e, do not appear in the equation because

they do not flow through either of the resistors in the

configuration which was shown in Fig. 4. The servo errors

in (1) are plotted in Fig. 5 for a typical coaxial dual-element

thermistor mount with an ohms-per-watt coefficient of 13 000 and a bias current of 12.8 mA. The normal maximum RF power level for this type of mount is limited to 10 mW because of the dual-element substitution error; however, the curves have been continued to current cutoff

to show the behavior of the errors. Only the power-dependent errors are shown here. The errors due to the ex- ternal voltmeter are also shown in the upper two curves.

If a stable variable voltage source (i.e., a "reference generator") is used to offset the voltmeter so that it is always

used at as great a fraction of full-scale indication as possible, the middle curve results. If the reference generator

is not used, then the voltmeter errors are magnified by

using it to measure two voltages (Vi and V2) which are nearly the same, and then taking the difference of the

squares. We have described the errors due to the limitations of operational amplifiers and resistors. Unfortunately, the

principal source of error in an actual powjr measurement

is the limited accuracy of the external voltage measuring

equipment. The errors due to the voltmeter alone are

typically 20 times those due to the servo, with voltmeter

specifications typical of high quality digital instru- ments.

Practical Realization

The unique features of the self-balancing circuitry de- scribed above make this instrument especially attractive

in applications where automatic control is desired. It is often necessary to assemble several power meters in a given

system, one for each of several output ports. The use of four-terminal connections to both the reference resistor and the bolometer make it possible to bring long leads from

CR201 ASSY W02M

PRI : 120/240 V, 50-60 Hi EACH SECONDARY : 35 VCT, SOoiA isolation: less than 0.01 pF, PRI -SEC OR SEC-SEC GREATER THAN lO'O n DC LEAKAGE

RESISTANCE.

F. 35V ALUMINUM

35V SOLID TANTALUM

F. 25V SOLID TANTALUM

Fig. 6. Dual isolated power supplies.

30

346 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT. DECEMBER 1976

RI02 EXT»5V(A>-,

LO-SET /S\ TO zoon

LO-SET /p\ TO lOon

SET DONE

100 a SET

200flfl SET _XJ'^^J_

LO- ANALOG^

DATA

INVALID

EXT COM @-

RMS

100 kfl A

15V

C0403OAE

. inno I"'*

*;'05 «,06

0108 NPN's 2N3904

PNP'5 2N3906

Resistors '^W. 5% Cofbon

Fig. 7. Interface logic.

the bolometer units to a central instrumentation console without introducing errors. In addition, it is now possible

to control the operating resistance of the bolometer unit

by changing the reference resistor in response to an electrical control signal. This is much more difficult to do in a Wheatstone bridge because the contact resistance of the

switches cannot be entirely removed from the bridge circuit. Now, however, it is possible to use field-effect transistor switches with no degradation in the measurement

accuracy. A complete schematic diagram is shown in Figs. 6, 7, and

8. The bulk of the circuitry is involved with the interface between the power meter and an external controller. The

self-balancing portion is shown in Fig. 8. The reference

resistor is comprised of R129 through Rue. It is arranged

in such a way that by switching the current terminals, the

effective resistance can be changed from 100 to 200 fi. This

switching is accomplished by the field-effect transistors.

In addition, by removing ARlOl from its socket, these resistors can be calibrated against an external four-terminal

standard attached to the bolometer terminals using es- sentially the technique shown in Fig. 2. The remote control inputs are applied at the left side of

Fig. 7 through optical isolators. They allow an external

controller to set the mount operating resistance to either 0 or 200 U. The mount operating resistance is deter- ined by SlOl: 100 Q, REMOTE, and 200 fi. When in the REMOTE position, a negative-true pulse applied to terminals B or C will set the operating resistance accordingly.

An elementary "hand-shake" return signal is provided at Pin D and a logic level confirming that the correct resis- tance was set appears at Pin E.

If the balancing servo goes out of control due to RF

overload, broken or shorted lead wires, switching transients

in changeover from one operating resistance to another,

or a burned-out bolometer, then a front panel warning light appears, and a control signal appears between Pins F and

G of the interface. This signal may be used to protect bo- lometer mounts against RF overload or to prevent the data

acquisition portion of the system from taking invalid

data. The instrument as shown is designed to accept only

negative temperature coefficient bolometers. The addition

of a barretter capability requires only the incorporation

of a 4PDT switch to interchange the amplifier input leads

for example.

The signal output to the external voltmeter may be

taken either from a terminal pair which is switched automatically as the mount operating resistance is switched,

or from terminals connected directly to the reference re- sistor if the on-resistance of the switches cannot be toler- ated. This resistance is less than 50 ft. The Thevenin

equivalent source resistance measured, for example, be- tween terminals 2 and E— in Fig. 8, is very small (microohms) because of the large amount of feedback in the

loop.

31
rIi
LAKSKN: DC SlIBSfllllllON RK MKTKK 347

0112

SD2I0

CR107 t i

RI29

9 53kn

^NA^ W^^^ 3g

R131 RI30

i 1 Mfi

HI--

C103 R138 0047^F 22kn

Fiji. 8. Self-balanring current loop. The bolometer is connected to the terminals idenlitied hy /— , E+. and / + .

Perfokmance

Performance of the instrument was measured in two

ways. First, the magnitudes of the offset voltages and bias currents were measured under conditions of changing RF

power to verify that the values used for these terms in the

error analysis were realistic. Second, a system was assembled that allowed a comparison of the new instrument with an NB3 Type II power meter. Mounts for the two power

meters were attached to two arms of a "magic tee" or other power divider. The Type II was used to control and measure the power level from a source, while the Type IV measured the power on the other arm. The two power

levels were nominally equal. The entire measurement

system was under the control of a programmable calculator. Power levels were selected at random by the calculator, typically over the range of 0.1 to 15 mW, and the ratio of the bolometric powers at the two ports computed. The

independence of this ratio was a measure of the precision of the two power meters and of their relative linearity. The

standard deviation of a series of 1024 measurements of randomly selected levels was about 0.008 percent. The

series was repeated several times for different combinations of single and dual-element mounts, with similar re- sults. Since the two power meters were quite different in- ternally, this is also indicative of the ahsolule linearity of each.

Summary

We have described a new dc substitution self-balancing

KF power meter which is not a form of Wheatstone bridge.

Its advantages include reduced cost, lower noise, improved

ease of adjustment, and adaptability to automatic control. A disadvantage is the requirement for a highly isolated power supply. The technique may also be useful with

hot-wire anemometers. A patent is pending.

ACK NOWLEIXJMENT

The author wishes to acknowledge the contributions of G. Reeve in early discussions, and the construction of the prototype instrument by W. Boone. This work was supported in part by the U.S. Army, U.S. Navy, U.S. Air Force, and the Energy Resources Development Administration. References

(I I (i. F. I'Migen, "A seK halancinjj direct-current bridge for accurate bolometric power measurement," NHS J Res., vol. h9, no. 2, pp.

101 105, Aug. 1 IT)?.

|2) N. T'. l,arsen and F. R. Clague, "The NB.S type II power measurement

system," Ado. Inslmm, vol. 2,'i, Pt. ;i, paper no. 712-70, in Prnc. 25th Annu. ISA Cimf (Philadelphia, PA), Oct. 2f. 29, 1970.

|.!| H. .1. Hoge, "A two enil method I'or the comparison of resistances." Hfr. Si i Inslriiiii.. vol. 2.''), no. 9, pp. 902 907, Sept. 19M.

|4| I,. Crovini and ('. 1 1. M Kirby, "Resistance comparisons at nanovolt

levels using an isolal ing current ratio generator," Rev. Sci. Instrum.,

vol. 41, no. -I, pp. 49.! 497. Apr. 1970.

|.5| M I,. Roberts, 'l'olenliom( lri<- bridge for resistance thermometry." ISA 7'nni.v.. vol. K. no. 2, pp l,S7 1()2, I9ti9.

32