FEB1

PB 161624

^o. 123

FUNCTIONAL AND DESIGN PROBLEMS

OF THE NBS RF VOLTAGE BRIDGE

BY

L. F. BEHRENT

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
\^3,Q0^

THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

technical ^^ote

123

January 1961

FUNCTIONAL AND DESIGN PROBLEMS OF

THE NBS RF VOLTAGE BRIDGE

by

L. F. Behrent

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

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Price $ 1 . 00

TABLE OF CONTENTS

Page

ABSTRACT

1,00 Introduction 1 2,00 General Description 1

3. 00 Design Requirements 3

3, 10 Bridge Resistors, Rrp and R, 3

3,20 Shorting Relay and Associated Resistances 4

3. 30 Measurement of a 6

3.40 Bridge Sensitivity 6

3,50 Measurement of V , ^r2 and Voi 7

4,00 Operating Essentials 7

4, 10 Measurement Procedure - High Voltage

Levels (0. 1 to 1. 9 v) 8

4,20 Low RF Voltage Level Procedure (20 to 100 mv) 9

4,30 Calculations 11

4. 40 Precautions 12

5.00 Symptoms of Bridge Malfunctioning 14

Erratic or Continuous Rapid Drift of the Bridge 14

Drift in Vq 15

Unbalance Microammeters 15

6, 00 Construction Details 16

Thermistor Mount Cabinet (Figure 3) 16

Bridge Cabinet 17

Potentiometer Working Voltage Supply 17

a Monitoring Vacuum- Tube Voltmeters 18

DC Bridge Construction 18

REFERENCES 20

TABLE I 21

TABLE II 22

LIST OF COMPONENTS 23

CAPTIONS FOR ILLUSTRATIONS 24

5. 10

5. 20

5. 30

Oons

6. 10

6. 20

6. 30

6. 40

6. 50

11

FUNCTIONAL AND DESIGN PROBLEMS OF THE NBS RF VOLTAGE

BRIDGE

ABSTRACT

A detailed presentation is given of the practical solutions to the

design and operating problems encountered in constructing a Thermistor Bridge similar to that used by the NBS for RF Voltage

Standardization, Measurement and operating techniques, critical

structural features, as well as the proper use of available components are discussed.

FUNCTIONAL AND DESIGN PROBLEMS OF THE NBS RF VOLTAGE

BRIDGE

by

L, F. Behrent

1. 00 - Introduction

A little over ten years ago, a thernnistor bridge was developed

by the NBS for rf voltage standardization. (Selby & Behrent, 1950)

Recently this bridge was modified and reconstructed to make its operation

simpler and more trouble-free, and to innprove its physical appearance.

Basically, however, its design and operation remain the same. The

numerous requests for detailed information about its design, and the

design and functional problems associated with the construction and use

of such a bridge motivated the preparation of this paper as a supplement

to the earlier one. Therefore, a familiarity with the previous paper is

assumed.

Z. 00 - General Description

The rf voltage bridge is a bolometer bridge in which two thermistors are used as the bolometer elements. The thermistors used

have an average diameter of 0. 015 inch and 1. mil platinum leads.

These thermistor beads are mounted symmetrically in 0. 030 inch gaps

in a specially-designed thermistor mount. The bolometer bridge is

not operated as an equal-arm Wheatst^e Bridge. Two arms of the

bridge, the thermistor arm and an adjacent arm, R , are 200 ohms

each, while the other two bridge arms, R , are equal and have
- 2 -

resistances around 300 ohms. See Figure 2. These latter bridge

elements are adjustable to permit balancing the bridge when no rf is

applied.

The principle of power substitution is used in making measurements with the bridge. After the initial dc balance, rf power is applied

to the thermistors and balance is restored by removing an essentiallyequal amount of dc power by inserting resistance between the dc biasing

source and the bridge. Since the rf resistances of the thermistors are

essentially equal to their dc values, the rf voltage across the thermistors can be calculated using (1) for rf voltages of 0. 1 volt and above,

and (2) for voltages below 0. 1 volt.

^RF = (R, . RJ (1 f a) ("%2 ^'^o " ^RZ^)* ^^

^T \i

^RF = (S-r^;)(rT7-)(-t%2-%i][^^-%2-^RiJ/ (2)

The bridge is housed in two cabinets except for the biasing

batteries which are kept in covered carts beneath the table on which

the bridge is placed. The larger cabinet contains the dc bridge components, the dc measuring potentiometer, standard cell, and associated

ac, dc, and rf controls necessary to operate the bridge. The smaller

cabinet houses the thermistor mount and the microammeter s which

continuously monitor the condition of the thermistors.
3 -

Figure 1 presents a front view of the larger cabinet. The

functions of the front panel connectors and controls are outlined in

Table I.

3.00 - Design Requirements

The earlier paper contains a detailed analysis of the allowable

errors for all component parts and all measured dc quantities. For

convenience these results are summarized in Table II so that a comparison may be made between design requirements and the components

selected for use in the bridgee

3.10 - Bridge Resistors, R_ and R,

° T b

The actual, measured values of all resistors used in the bridge

arms, both fixed and adjustable, are within ± 0. 1% of their nominal

values. Experimental data compiled by the National Bureau of Standards have shown that the combined effects of both aging and ambient

temperature variations for resistors of the type used here are not

likely to exceed 100 parts per million per year. Therefore, the

nominal values for R and R can safely be used in ( 1) or

(2) for calculating the magnitude of applied rf voltage.

Each R arm consists of a fixed resistor with adjustable

resistance decades connected in parallel with it to provide vernier adjustment. See Figure 2. Good quality decades are used having

multi-leaf contacts and wiping action at all contact surfaces to minimize

contact resistance. Inasmuch as both R arms must be adjusted

b

simultaneously by equal amounts in maintaining bridge balance, like

decades are mechanically coupled and driven from a single knob control

on the bridge panel. The fixed resistors are bolted to terminal blocks
4 -

on the rear of the R, decade assembly, and short, heavy copper leads

b

connect them to the bridge circuit so that lead resistance is negligible

compared with the bridge-arm resistances.

3.Z0 - Shorting Relay and Associated Resistances

To maintain bridge balance when rf voltage is applied to the

thermistors, an equivalent amount of dc power is withdrawn from the

bridge by inserting resistance in series with it. With the withdrawal

of rf, full dc bias is applied to the bridge by shorting out the series

resistance with a relay connected across it. Unless the voltage drop,

V , across these relay contacts, when closed, is no more than a few

R

1

microvolts, at rf levels between ZO and 100 mv, a correction must be

applied to the calculations of rf voltage. In the original version of the

bridge the relay contacts used had optically -flat silver-plated contact

surfaces. During the years that it was in use it had an essentially

constant voltage drop of Z5 |j.v across closed contacts. Amalgamation

of the contacts with mercury was tried. It did reduce the drop considerably but, because of the maintenance required with such surfaces,

amalgamation was not resorted to.

In the reconstructed bridge, a mercury relay replaces the

earlier model, primarily because a relay mounted in a cabinet is rather

difficult to service, and the mercury relay, with its totally enclosed

contacts, is more trouble-free. A consistently reproducible 70 |jlv

drop exists across all pairs of paralleled, closed contacts of this

mercury relay. For rf levels of 100 mv and less, then, another term,

V , must be included in this equation for calculating rf voltage

applied. To reduce V to this low a level required connecting in Rl
parallel all three pairs of contacts of the mercury relay, a raodel

which was origirxally manufactured for use in controlling three-phase

power circuits.

The resistive network across which the mercury relay contacts

are connected consists of four ten-turn potentiometers connected in a

series -parallel arrangement. A, B, C, and D, Figure 2. Resistor A

and B are for coarse adjustment, while C and D provide the vernier

adjustment. Wide copper strap is used to interconnect these components. With the resistors shown, the dc bias can be adjusted to

balance the bridge adequately for rf voltage input levels between 0.2

and 1. volts. The overall network resistance needed within this rf

voltage range is from 6 to 85 ohms. To measure higher rf voltages,

greater resistance must be connected in series with the bridge to obtain

an rf balance. For exanaple, with 1.5 volts rms applied to the thermistors approximately 1100 ohms are needed to produce a balance.

For the range, then, between 1.0 and 1.5 volts, an external 1000-ohm,

ten-turn potentiometer is connected in series with A, B, C, and D

externally. This resistor has not been permanently connected to the

system because its minimum resistance is too high and unstable to

obtain a bridge balance at low rf levels.

Below 0.2 volt rf, an external slide-wire is connected across

the shorting relay contacts in parallel with the whole network of A, B,

C, and D so that a very low resistance can be obtained, adjustable in

increments of 1 milliohm or less, and which remains acceptably stable.

Rough adjustment is made using the slide -wire, and the potentiometers

provide vernier control.
- 6 -

3.30 - Measurement of a

The microammeter s used in the earlier bridge to indicate the

thermistor unbalance, a, have been removed. Very-high impedance

dc vacuum-tube voltmeters connected across the decoupling capacitors

in the thermistor mount are now used to indicate the unbalance, by

monitoring the voltage drops across the thermistors to warn the operator of malfunction in the thermistor mount.

The magnitude of the unbalance is now measured directly using

the same dc measuring potentiometer provided for determining V > V > and V with an accuracy of ± 0. 0Z%. To measure a, an adR2 Rl

justable resistive divider comprised of resistors R through R , R ^, R ^» and switch SW , is connected across either thermistor and

a2 a3 4

the dividing ratio is adjusted until the dc measuring potentiometer indicates 1.000 volts. Without altering the divider ratio, the divider

and potentiometer are transferred to the other thermistor by SW , and

the voltage drop is measured by the precision dc potentiometer. This

new voltage is the value of a. The only requireraents placed on the

divider are that its input impedance be at least one megohm to eliminate

error due to shunting of the thermistor by the dividing network, and

that the short-time stability shall be of the order of ± 0. 01%. Using

this approach to measure a nnakes the error in a negligible, and

eliminates one step previously necessary in calculating VRF

3. 40 - Bridge Sensitivity

The original bridge, with an R^ = 200 ohms and R, of approx- T b

imately 300 ohms, using a moving coil or D'Arsonval galvanometer could

detect a change in R of . 01 ohnns in 200 ohms. Considerable difficulty has been experienced in using this type of galvanometer because
- 7

of vibration, even when specially mounted. Therefore, in the present

bridge, electronic galvanometers with 1000 ohms input impedance and

-9 a current sensitivity of 10 amperes per mm are used for both the

bridge and the dc measuring potentiometers. Not only has this removed

the vibration problem but it has also improved the bridge sensitivity.

A change in R of 0. 0005 ohms in ZOO ohms can now be detected.

3.50 - Measurement of V , V ^j and V , o RZ Rl

At low rf voltage levels, the difference betw^een the bias battery

voltage, V , and the voltage applied to the bridge itself becomes quite

small. Therefore the measurement points at which these voltages are

determined must be carefully chosen. In determing V , (see Figure

2) one connection to the dc measuring potentiometer should preferably

be located at (1) and the other at (3). In this way the total voltage

drop occurring between the bridge and terminals (1) and (3) is

measured. Then the bias voltage applied to the bridge itself can be

accurately found. To determine V and V , the measuring

potentiometer should be connected between (2) and (l).

4.00 - Operating Essentials

The operational procedure for making rf voltage measurements

was briefly outlined in the previous paper. Many small details which

would be obvious to the experienced operator were omitted. Since an

objective of this paper is to provide information for those who are

interested in using such a bridge but may lack experience with it, a recommended procedure will be given here in detail. Some steps are

included to guide the operator in completing the measurement more

quickly. Inadequate attention to each step could prolong the
- 8

measurement interval, and possibly result in a deterioration of measurement accuracy because of the additional delay. To illustrate, in

step 4 of the following procedure, when operating the bridge around

1 volt and higher, if the operator applies rf voltage to the bridge in

large increments, he may find upon rebalancing the bridge that the

change in loading on the source has raised the rf input level too high

for the test voltmeter indication he desires. Then he must reduce the

rf level and rebalance, repeating these steps until the bridge balances

at the desired rf voltage level. In addition to the time spent in repeated

adjustments, it is conceivable, that with some rf voltmeters, damage

to the voltmeter could occur since the operator would have no warning

of the extent of the over voltage.

4.10 - Measurement Procedure - High-Voltage Levels (0.1 to 1.9v)

The procedure to follow in using the bridge is outlined in the

following steps.

1. Connect rf voltmeter to the thermistor mount and energize the

voltmeter.

2. Energize bridge by closing switch SW . 5

3. With SW open, balance the bridge with dc by adjusting resistK.-b ors R, ,• Continue periodic adjustment until bridge drift has

essentially ceased, a process which may require an hour or more.

4. With A, B, C, and D at maximum resistance - highest dial readings

close SW^^ and apply about one-third the desired rf voltage to the RF ®

bridge. With A and B, roughly balance the bridge. Then raise

the rf level toward the desired rf voltmeter indication. Readjust

A and B to restore balance. When the desired rf level is reached,
- 9

-

open SW and recheck dc bridge balance. Where V„^ is ^ RF RF

expected to exceed 1. volt, add a 1 K ohm, ten-turn potentiometer

in series with the network A, B, C, and D.

5. Correct dc balance by adjusting R .

6. Check potentiometer standard cell balance. Adjust according

to instructions furnished with the potentiometer.

?• Check rf voltmeter zero - make necessary adjustments,

8. Close SW and obtain final rf balance using C and D (SW, in

RF * ^ 1 V position, SW in V position). R^ ^

9. Measure V and record - if outside range of potentiometer, R2

switch SW, to V ^ X 10 and ineasure.

1 R2

10. Change SW to V position and measure and record voltage.

11. Turn SW to "a" position.

12. By means of controls a, a. , and a , adjust the voltage across the

potentiometer to read exactly 1 volt. SW corresponds to "a".

13. Change SW to its other position and measure the dc voltage.

Record this value as a.

14. Record R, . bl

15* Open SW and recheck bridge balance and rf voltmeter zero, RF

If either or both are off, repeat the measurement,

4,20 - Low RF Voltage Level Procedure (20 to 100 mv)

16, With the bridge energized, adjust for a dc balance as in steps

2 and 3,
10 -

17. Check rf voltmeter zero. Make necessary adjustments.

18. With A, B, C. and D at maximum resistance, connect a good

quality low resistance slide -wire (1 to 2 ohms) across the terminal blocks at the top center of the bridge panel.

19. Apply sufficient rf voltage to the bridge to give the desired rf

voltmeter indication. With the external slide -wire, roughly

balance the bridge. Examination of the rf voltmeter indication

may show a change in the level; readjust the rf and again rebalance the bridge. Repeat as often as necessary until the rf

and dc balances of the bridge coincide at the proper rf level.

20. Remove the rf by opening SW and recheck voltmeter zero RF

and bridge balance. Make the necessary adjustments to both.

21. Close SW„„ and if the rf level indication on the rf voltineter RF

is correct, balance the bridge until the rf and dc balances

coincide as exactly as possible by means of A, B, C, and D.

(SW in V position, SW in V position)

22. Check dc potentiometer standard cell balance. Adjust according to manufacturer's instructions.

23. Measure and record V ^.

R2

24. Change SW to V position; measure and record V . i o o

25. Turn SW to a position and SW to either position.

26. With controls a, a , and a adjust the voltage at the potentiometer

to read exactly 1. 0000 volt.

27. Turn SW to its other position and measure the dc voltage.
11

This is equal to a. For some pairs of thermistors, it may be

possible to adjust to 1. 000 volt only when switched to the thermistor of lower resistance,

28. Record R,,.

bl

29. Open SW and recheck bridge balance and rf voltmeter zero.

RF

If either or both have changed, repeat the measurenaent.

30. When measuring rf voltages between 20 and 50 mv, maximum

galvanometer sensitivity is required. The bridge should be kept

in balance within ± 3 large-scale divisions of the galvanometer if

measurement accuracy is to be maintained.

4.30 - Calculations

The equation used in calculating the value of the applied rf voltage

for levels above 0. 1 volt is presented in M).

where

R = 200 ohms

T

R, = the combined resistance of R, ^ in shunt with the b b2

adjustable decade resistors R, , ;

R, ^ will be either 300, 340 or 400 ohms depending upon the

battery condition. R, , is read directly from the dials

bl

a is determined from steps 11, 12 and 13;
12

V is determined in step 9;

R2

V see step 10. o

At levels below 0. 1 volt the voltage drop, V , across the

Rl

mercury relay MS must be included in the calculation.

^RF = (R . Rjil . a)

^ [ ^R2 ' Vl^^ ^^o " ^R2 " ^RlO^ ^'^

V can be measured with no rf applied and SW, in the V ^ position. Rl 1 R2

4.40 - Precautions (in anticipation of some of the operating difficulties

which may arise, a few precautions are included at

this point. )

1. The rf voltage applied to the bridge when calibrating rf voltmeters must be filtered sufficiently to reduce the harmonics

50 to 60 db below the fundamental.

2. In measuring to an accuracy of ± 1%, the amplitude stability of

the rf source must be ± 0. 1% or better.

3. The batteries biasing the bridge must be serviced and recharged

once each year. When the bridge is not in use, a load equal to

the bridge resistance must be connected across these batteries

to prevent a false charge build-up.

4. Periodically the voltage drop across the mercury relay in its

closed condition must be measured. If this drop varies by more

than 5-10 microvolts with repeated closing of the relay, the relay

should be replaced.
- 13 -

5. R should normally have a resistance value around 5000 to 6000

bl

ohms for proper resolution in balancing the bridge. In adjusting

for a balance, this resistance should never be reduced below

2500 ohms or damage to the thermistors is possible. When no

balance can be obtained with R, = 5000 to 6000 ohms, the fixed

b

resistances R should be replaced with higher or lower ohmic

b2

values depending on the bias battery voltage --a larger R for

b2

V > 8 volts and a smaller R, _ for V < 8 volts.

o bZ o

6, While the microammeter s in the thermistor mount cabinet cannot

be used to measure a, they serve to indicate the general condition of the thermistors. If one meter reading goes to zero or both meters read less than 20 jxa, de -energize the bridge at once

or damage to the thermistors may result. When one or both

meters read close to zero, there may be excessive rf applied to

the thermistors, driving their resistances down to a dangerously

low value, leading to burn-out of one or both. When one meter

goes to zero and the other to full scale, a thermistor has opened

or one of the decoupling condensers in the mount has become

shorted,

7, In connecting the external slide-wire resistance to the bridge,

use heavy low-resistance leads or it may not be possible to

balance the bridge for rf voltage levels around 2 mv.

8, When calibrating an rf voltmeter, the meter should be connected

to the mount with the same connector or connectors which will

be used when measurements are made with the voltmeter. In

any case, clip-lead connections are to be avoided at radio

frequencies. Remember that the rf voltage is accurately known
- 14 -

to ± 1% at the surface of the core in which the thermistors are

mounted. The use of connectors will make the voltage at the

face of the probe different.

9. Check for rf leakage when the rf voltmeter has been connected

to the bridge and rf applied. Some rf voltmeter probes are not

adequately shielded. An external shield is oftentimes necessaryover the probe and cable to the voltmeter. Make certain that

the rf source and cables connecting the source to the bridge are

properly shielded also. In some instances the source may also

require shielding by placing it within a double -screened cabinet.

10. There must always be a dc return path for the thermistors

through the rf source if a is to be measured.

5. 00 - Symptoms of Bridge Malfunctioning

5.10- Erratic or Continuous Rapid Drift of the Bridge

1. RF and dc instability : If the bridge drifts on both rf and dc

balance in an erratic or excessively rapid manner, the instability

can be due to the following causes:

a. Poor control of the ambient temperature of the room in

which the equipment is located.

b. Batteries need recharging or replacement.

c. Excessive heat is being transferred from the rf probe to

the thermistor mount.

d. A faulty resistor in the bridge circuit or dirty contacts in

the decade resistance units R

bl
- 15 -

e. A faulty connection in the dc bridge and/or leads from the

biasing source to the bridge.

2. RF instability only: When the erratic behavior or drift is observed only on rf, the causes may be the following:

a. RF leakage currents,

b. Instability of the rf source.

c. A faulty rf cable.

d. SW may require repair, RF

5.20 - Drift in V

When the battery voltage applied to the bridge fails to remain

4

constant to 1 part in 10 per measurement interval, the batteries may

need recharging or replacement.

5.30 - Unbalance Microammeter

s

In order to measure the degree of unbalance, a, between the

thermistors, there must be a dc return path through the rf source. The

two microammeter s should indicate the same current within ± 20%.

Departures greater than this should be investigated. Following are the

more commonly occurring conditions and their causes:

1. One meter indication decreases to zero, or near zero. If this

condition prevails only when rf is applied, excessive rf is being

applied to the thermistors. When the condition prevails on dc,

one of the decoupling capacitors in the mount has shorted. If at

the same time the other meter goes to full scale, one thermistor

has opened.
16

2. Both meters read approximately the same but there is no decrease

in their reading with rf applied. When this happens, even though

there is other evidence of rf at the thermistors, look for lack of

a dc return path through the rf source.

3, On dc balance the meters read widely different. If potentiometric

measurement of a indicates that both the thermistors are well

matched, one of the vacuum tube voltmeter circuits in which the

microammeters are connected is not functioning or requires

readjustment.

6. 00 - Construction Details (Some construction details are given to

assist anyone interested in duplicating

the thermistor bridge for his own

laboratory.

)

6. 10 - Thermistor Mount Cabinet (Figure 3)

This cabinet houses the thermistor mount and the microammeters

which continuously indicate the condition of the thermistors in the mount.

The external appearance of the front, top and rear panels is shown in

the views of Figure 3. The square opening in the center of the top

panel provides access to the surface of the thermistor mount core,

where the rf voltage applied to the bridge is standardized. Here a wide

variety of adapter plates can be fastened to the mount to permit connecting rf devices to be calibrated with any type of rf connection desired.

Figure 4 shows the circuitry in the smaller cabinet. Two shielded

six-conductor cables (see Figure 3 also) are used to connect the components to the remainder of the system. Fanning strips FS and FS

which terminate the cables are connected to terminal strip TS on the

1 rear of the main cabinet.
17

A doable -shielded, 50 -ohm coaxial cable connects the Type N

connector on the under side of the thermistor mount (Figure 5) to the

rf connector, also Type N, on the rear panel. Another rf cable between

the latter and a similar Type N connector on the rear of the main cabinet

transmits rf voltage from the output of the special coaxial switch in the

main cabinet to the thermistor mount.

Since the 0. 01 \ii (C in Fig. 4) mica blocking capacitors built into

the thermistor mount introduce series impedance between the test rf

voltmeter and the thermistors at low frequencies, enough additional

capacitance, C , . is connected in parallel externally to make the

ext

mount usable down to 30 kc. In Figure 6 a typical method of mounting

and connecting external capacitors is shown.

6.20 - Bridge Cabinet

The general arrangement of components within the larger bridge

cabinet is shown in Figure 7. The cell supplying dc current for the

precision dc measuring potentiometer consists of Ni-Cd cells on continuous trickle charge to stabilize their output. Details for this circuit are given in Figure 8, and the text of Section 6. 30. The schematic

for the vacuum-tube voltmeters is shown in Figure 9 and the descriptive

text is presented in Section 6.40. Details regarding the construction

of the Wheatstone Bridge are given in Figures 11 and 12 and Section

6.50.

6.30 - Potentiometer DC Current Supply

When the Ni-Cd cells need replacement, the new cells should be

brought to full charge according to the manufacturer's instructions.

Then the cells are connected to the trickle charger with a milliameter
- 18 -

in series with the cells temporarily. It would also be well to connect

a milliameter in series with the dc measuring potentiometer to which

the dc current supply is connected. Usually it will be necessary only

to vary R-,o (figure 8) to adjust the charging current to 40 ma and

28

R to set the current at the measuring potentiometer to 1 ma.

When the output from the supply does not remain steady, the

charging current and the load current should be checked. Readjustment of either or both currents might be required. Ordinarily, once

the dc current supply has been placed in service, R-,-, does not require

readjustment unless there has been a large change in the 110 volt

60-cycle ac line input.

6.40 - a Monitoring Vacuum-Tube Voltmeters

Figure 9 presents the complete schematic for the simple vacuum

tube voltmeters used to continuously monitor the condition of the thernnistors, while Figure 10 represents that part of the voltmeter circuits

in the main cabinet. Connections to the indicating microammeter s and

the measuring points on the thermistor mount in the other cabinet are

made through terminals h n of ternninal strip TS and the corresponding termiinals of fanning strip FS (see Figure 4).

6.50 - DC Bridge Construction

The schematic for the portion of the dc bridge in the main cabinet

is included in Figure 11, and the physical arrangement of the components

is shown in Figure 12. The resistors comprising the bridge arms are

mounted on two bakelite panels. The adjustable decade resistance units

which connprise the vernier adjustments for the R arms are nnounted

b

on the lower half of both the front and rear panels. The upper half of
- 19

the front panel is occupied by the variable resistors A, B, C, and D,

while the fixed resistors R and R are attached to the upper half

of the rear panel. Each pair of decades having the same ohmic range

is coupled mechanically together with insulated shaft couplings. In

this way, one knob for each pair adjusts both R arms by the same

amount.

The rf cable from the thermistor mount cabinet connects to

rf connector RF , and the signal generator supplying the rf voltage

1 < connects at RF .

Terminals o, p and q of fanning strip FS connect to the corresponding terminals of terminal strip TS .
- 20

References

Selby, M. C. , and L. F. Behrent, A bolometer bridge for standardizing

radio-frequency voltmeters, J. Research NBS 44 , 15 - 30 (Jan. 1950).
Connectors

and

Controls

- 21 - TABLE I Panel Control Functions (Figure 1)

Purpose

RF Input A Type N connector through which rf power from

an rf source is supplied to the bridge.

SWRF A coaxial switch connected between the rf input

connector and the thermistor mount. With this

switch in the "out" position, no rf is applied to the

bridge and a microswitch actuated by the switch

handle causes mercury relay MS, Figure 2, to

close, applying full dc bias to the bridge. When

the rf switch is closed, rf passes to the mount and

the microswitch causes the mercury relay to open,

placing the combination of resistors A, B, C, and D

in series w^ith the bridge to keep the bridge in balance

PWR Controls 60 cps mains to the auxilliary equipment

associated with the bridge such as the mercury relay,

electronic galvanometers, and the VTVMS. When

in the "off" position, the switch places a dummy load

across the biasing batteries.

A

B

C

D

The coarse and vernier variable resistors which

constitute R in series with the bridge when the

mercury relay MS is open.

R

bl

These four resistor decades are the adjustable

vernier resistors of the R, arms of the bridge.

b On dc, adjusting them balances the bridge.
Table I continued

Connector s and

Controls Purpose

SW.

SWSW.

SW

Voltage divider for use when the do voltages applied

to the dc measuring potentiometer exceed 1.5 voltSo

(For measuring Vf(2 S-"^*^ ^o* ) ^^ position "Vd2"

the potentiometer measures that voltage directly.

In the Vj^2 X 10 position, the potentiometer measures only ]/lO of Vj^2- In the Vq position l/lO

of Vq is measured.

Connects the potentiometer either to SW]^ for

measuring dc bridge voltages Vq and Vd^ or

to SW3 for determining a, the unbalance ratio

between the two thermistors in the mount. At

position "a" the potentiometer measures the voltage

drops across the thernai stor s. In the other position,

V, bridge voltages are measured.

Connects the potentionaeter across each of the thermistors for determining a.

This switch, labeled "a" on the bridge, together

with the controls labeled a2 and a^, provides a convenient means of measuring a. SW^ is adjusted m steps until with the continuous adjustment

of a^ and a the potentiometer reads 1 volt across

either R-j^ or R^p^- Switching to the other thermistor, the potentiometer voltage measurement

equals the value of a directly.
- 2Z - TABLE II

Allowable Errors in Component Parts and Measured Quantities

Component Measured Quantity Allowable Error in %

R ±0.17 For R = 2 00 ohms

T T

Rb ± 0. 17 R = approx.

300 ohms

a ±1.2 for a = 1. 4 ^ ± 0. 6 for a - 2,

V ±0, 17 - 0.20 ^

o V ^ 0, 17 - 0.24 ^

R2

Rl

Q

V , 1.1

* For a between the ideal 1. 00 and 1.4, high accuracy is not

needed.

A For rf voltages between 20 mv and 1.5 volts.

D With the dc measuring potentiometer used 110 \xv can be meas

ured to ± 1 |j.v which represents about 1%.
- 23 - LIST OF COMPONENTS

PART NO, RESISTANCES QUANTITY

Rb2

Rr

Rbi

Rl

Rdi

Rd2

A

B

C

D

Ra2

Ra3

Rvl

Rv2

R.

R

V3

v4

300-ohm working std. type, ± 0, 1% 2

340-ohm working std, type, ± 0. 1% 2

400-ohm working std, type, ± 0.1% 2

200-ohm working std. type, ± 0. 1% 1

9 X 1000-ohm decade resistance

assembly, ± 0. 1% 2

9 X 100-ohm decade resistance

assembly, ± 0. 1% 2

9 X 10-ohm decade resistance

assembly, ± 0. 1% 2

9 X 1-ohm decade resistance

assembly, ± 0, 1% 2

200-ohm w.w. 1/2 watt resistor 1 100 K ohm, 1 watt, precision, ± 1.0%,

calibrated after installation 2

10 K ohm, 1 watt, precision, ± 1.0%,

calibrated after installation 2

1 K ohm, 1 watt, precision, ± 1.0%,

calibrated after installation 2

100-ohm, 1 watt, precision, ± 1.0%,

calibrated after installation 2

(10 K + 1 K + 100 ohm in series constitutes Rfj?)

1.15-ohm, 10-turn potentiometer

100-ohm, 1 0-turn potention^eter

25-ohm, 10-turn precision potentiometer

500-ohm, 10-turn potentiometer

1 K ohm, 10-turn precision potentiometer

250 K ohnn carbon potentiometer

250 K ohm 1/2 watt carbon, ± 5%

100 K ohm 1/2 watt carbon, ± 5%

100 K ohm 1/2 watt carbon, ± 5%

100 K ohm 1/2 watt carbon, ± 5%
List of Components (continued)

Page Two

PART NO. RESISTANCES QUANTITY

^v5

Rv6

Rv7

Rv8

Rv9

RvlO

^vll

^13,^14

Rl5'Rl6

R17, R^s

^19' ^20

^21' ^22

^23' ^24

R25'R26

R27'R28'R29

100 K ohm 1/2 watt carbon, ±5%

100 K ohm 1/2 watt carbon, ±5%

100 K ohm 1/2 watt carbon, ± 5%

100 K ohm 1/2 watt carbon, ± 5%

100 K ohm 1/2 watt carbon, ± 5%

100 K ohm 1/2 watt carbon, ± 5%

100 K ohm 1/2 watt carbon, ± 5%

510 K ohm 1/2 watt carbon, ± 5%

10-meg, carbon, 1/2 watt

1-meg, carbon, l/2 watt

2 00 -ohm, carbon, 1/2 watt

10 K ohm, carbon, 1 watt

3 K, W.W., 2 -watt potentiometer

100-ohm, W.W., 1 -watt potentiometer

PART NO. CAPACITORS QUANTITY

C^, C2

C3, C4

c

,

ext

1000 [j.f, 12 -volt, electrolytic

16 \xf, 200-volt, electrolytic

0.25 [xf-mica, external additional capacitance, 3 placed in parallel with each of

the built-in capacitances of the mount

PART NO. GALVANOMETERS QUANTITY

Bridge &

Potentiometer

Electronic, current sensitivity 1 X 10"

amp/mm, internal resistance approx,,

1000 ohms, both input terminals off

ground
Last of Components (continued)

Page Three

PART NO. BATTERIES QUANTITY

Low discharge, 2 volts per cellj

600 ampere-hour

PART NO, D. C» MEASURING POTENTIOMETER Q UANTITY

Precision, having ± 0.01% accuracy

and voltage ranges of 0-0.01601,

0-0. 1601, 0-1,601 volts

PART NOo MERCURY RELAY QUANTITY

Three circuit, normally open, with

solenoid operated from 110-volt,

60-cycle, a. c. line

PART NO. TRANSFORMERS QUANTITY

Tl'T^ Filament transformers, 110 ac

input, 6. 3 volt, 2 amp output

Filament transformer, 110 ac

input, 6. 3 volt, 1„2 amp output

PART NO. MISCELLANEOUS QUANTITY

D4.D5

L

M, , M2

E

Diodes, type 1 N 1084, M 500

Lamp, 6„ 3 volt

0-100 |j. anrip dc meter

Nickle -cadmium cells, 1«30 volts per

cell fully charged, 4-amp/hour rating

for 5 hourso
- 24 - CAPTIONS FOR ILLUSTRATIONS

Figure 1 Front view of the 1% thermistor bridge control cabinet

showing the location of controls, indicators, and connectors.

Figure 2 Schematic diagram of the 1% bridge.

Figure 3 Three views of the cabinet housing the thermistor mount

showing the location of meters and connectors.

Figure 4 Schematic of the circuitry in the thermistor mount

cabinet.

Figure 5 Cross-sectional view of the thermistor mount.

Figure 6 A typical method of mounting and connecting external

capacitors in parallel with the built-in blocking capacitors

in the thermistor mount.

Figure 7 Interior view of the bridge cabinet from the rear to illustrate the placement of major conaponents of the bridge.

Figure 8 Schematic of the dc current source for the precision dc

measuring potentiometer.

Figure 9 Schematic of the vacuum-tube voltmeters used for monitoring the condition of the thermistors.

Figure 10 Schematic of that portion of the vacuum-tube voltmeter

circuits contained in the main cabinet.

Figure 11 Schematic of the part of the dc bridge located in the main

cabinet.

Figure 12 Physical arrangement of components of the bridge assembly.

A

?(II)§

BRIDGE

GALVANOMETER

A

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B Ba c @ ©D

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c

XIOO

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GALVANOMETER

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000

R| R2 R3/"^ ^s

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Figure

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TYPE N

CONNECTOR

BRIDGE

FANNING

STRIP

FS2

VTVM FANNING STRIP

FS,

Figure 4

in

^-

=3

UPPER GROUND

PLATE

UNGROUNDED PLATE OF

C^ IN MOUNT

THERMISTOR MOUNT

SUPPORT

Figure 6

ELECTRONIC

GALVANOMETER

PRECISION

POTENTIOMETER

SWJ BWJ BWd ^W

I

ELECTRONIC

GALVANOMETER

n

s^^

STANDARD CELL /

a WORKING CELL

MERCURY

RELAY MS

REAR UNITS

OF Rb

RF CABLES

VTVMs'

TS,

Rf^ CONNECTOR

Figure 7

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r. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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