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^•iiitelwiiJlS.''' NisrUnited States Department of Commerce

Technology Administration

National Institute of Standards and Technology

NIST Technical Note 1358

Microcalorimeter for 7 mm

Coaxial Transmission Line

Fred R. Clague

—QC-

Microcalorimeter for 7 mm Coaxial

Transmission Line

Fred R. Clague

Electromagnetic Fields Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, Colorado 80303-3328

August 1993

<>^'.s^ °^ c.

*^>1TES O^

U.S. DEPARTMENT OF COMMERCE, Ronald H. Brown, Secretary

TECHNOLOGY ADMINISTRATION, Mary L. Good, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arati Prabhakar, Director
National Institute of Standards and Technology Technical Note

Natl. Inst. Stand. Technol., Tech. Note 1358, 52 pages (August 1993)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1993

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
CONTENTS

1. INTRODUCTION 1

1.1 NIST Microwave Power Standards 1

1.2 Bolometer Mount-Microcalorimeter Operation 2

2. DESIGN CONSIDERATIONS 5

2.Thermopile Assembly 6

2.2 Other Design Features 6

3. FABRICATION AND ASSEMBLY 7

3.1 Mechanical Design and Drawing Reference 7

3.2 Assembly Details 10

3.2.1 Microcalorimeter Base 10

3.2.2 Thermopile 11

3.2.3 Thermopile and Bias Leads 12

4. ACKNOWLEDGEMENTS 13

5. REFERENCES 13

APPENDIX A. Parts List Al

APPENDIX B. Parts Drawings Bl

APPENDIX C. New Thermopile Assembly CI

111
TRADE NAMES AND SI UNITS DISCLAIMER

Certain commercial components used in the microcalorimeter are identified in this document in

order to adequately specify the design. Such use and identification does not imply

recommendation or endorsement by NIST, nor does it imply that the identified items are

necessarily the best available for the purpose.

The International System of Units (SI) is used except in the dimensioned part drawings found in

appendix B. The shop which made these parts was not equipped to use metric dimensions,

Financial resources are not currently available to convert the drawings to SI units.

IV
MICROCALORIMETER FOR 7 mm COAXIAL TRANSMISSION LINE

Fred R. Clague

National Institute of Standards and Technology

Boulder, Colorado 80303

Design, evaluation, and construction details are given for the coaxial

microcalorimeter used by NIST as part of the microwave power standard in 7 mm

coaxial transmission line. Two versions are described: one with a Type N connector

and one with an APC-7 connector. The operating frequency range is 0.01 to 18 GHz

with either connector. The microcalorimeter is used to measure the effective

efficiency of a reference standard, which is then used to calibrate other microwave

power sensors. These reference standards are thermistor mounts designed by NIST

to be compatible with the microcalorimeter. Detailed microcalorimeter drawings and

assembly instructions are included.

Key words: coaxial microwave power standard; microcalorimeter; microwave;

microwave microcalorimeter; microwave power measurement; microwave power

standard.

1. INTRODUCTION

1.1 NIST Microwave Power Standards

The microwave power standards in use at the National Institute of Standards and Technology (NIST)

consist of microcalorimeters and associated reference standards [1-4]. Each power standard is made

up of both a microcalorimeter and a reference standard. The reference standards are substitution

type bolometric power detectors' which have been designed by NIST to give optimum performance

with the microcalorimeters. Figure 1.1 shows the coaxial microcalorimeter and the Type N

thermistor mount used as the reference standard [5]. References [1-5] include detailed descriptions

of typical microcalorimeter and bolometer mount operation. However, for convenience a brief

summary follows.

'These detectors are generally called bolometer mounts or simply, mounts. In this document, the terms

'reference standard," "bolometer mount," and "mount" are used interchangeably.

1
1.2 Bolometer Mount-Microcalorimeter Operation

The bolometric power detector uses a heat sensitive

resistor (bolometer) which terminates the

transmission line and absorbs the microwave

energy. Two types of bolometers are used: a platinum wire with a positive temperature

coefficient called a barretter, and a thermistor bead

with a negative temperature coefficient. The

detectors are biased by an external source of dc

current (power meter) to an operating resistance

that produces a match with the characteristic

impedance of the transmission line. Coaxial

mounts typically use two bolometer elements which

are connected in series for the dc bias, but are in

parallel for the rf. Thus, to match the 50 fl

characteristic impedance of a coaxial transmission

line, the pair is maintained at a series resistance of

200 0. When microwave energy is applied to the

mount, the dc bias supplied by the power meter is automatically reduced to maintain a constant

operating resistance [6]. If all the microwave

energy incident on the mount were absorbed by the

bolometer elements and if the elements were heated

identically by equal amounts of dc and rf power, then the microwave power would be equal to the

amount by which the dc power is reduced. This is called a substitution type power meter, because

the rf power replaces a portion of the dc bias power. The substituted dc power (also called the

bolometric power) is calculated using the equation

Figure 1.1 Coaxial microcalorimeter and

coaxial reference standard.

P^ = vl - vi (1.1)

where V, is the power meter output voltage (the dc voltage across the bolometer elements) with no

rf, Vj is the power meter output voltage with rf, and Rq is the dc operating resistance of the

bolometer pair (200 Q for a coaxial mount).
The microwave energy incident on a mount is not all absorbed by the bolometer elements. The

dielectric and conductor losses in the input connector, the input transmission line, and the bolometer

mounting structure result in a measurement error characterized by a correction factor called the

mount efficiency. This correction is always less than 1. In addition, the bolometer elements are not

heated identically by equal amounts of rf and dc power. This is known as the rf-dc substitution

error. The combination of these two effects, which is measured by the microcalorimeter, is a correction factor defined as the effective efficiency 17^. The rf power absorbed by the mount is

calculated by dividing the substituted dc power by the effective efficiency. The mount effective

efficiency is independent of mismatch corrections, which are treated separately at the time of

calibration transfer to an unknown mount.

The bolometer elements used in the reference standards are thermistors. Thermistors are rugged and

resist burnout in the event of an rf overload. They are available commercially as a conveniently

usable subassembly. Disadvantages to using thermistors include a continuous drift in the bias current even in a constant temperature environment. Also, thermistors are not usable in an alternative

efficiency measurement technique known as the impedance method [7].

The microcalorimeter essentially measures the temperature rise of the bolometer mount connected

to it. In the coaxial microcalorimeter, the temperature increase of the mount is measured with a thermopile. The measurement procedure determines the following at each frequency of interest: the

power meter and thermopile output voltages (Vj and €{) with only dc applied to the mount, and then

again (Vj and ^2) with both rf and dc applied. The effective efficiency 77^ is calculated at each

frequency using the equation

'I* = «

^^1

2 (1.2)

The term g is a frequency dependent correction factor for the microcalorimeter-bolometer mount

combination. It is also called the calorimetric equivalence correction. The uncertainty in the

effective efficiency measurement is determined primarily by the uncertainty in g. The determination

of ^ is a major effort that is described in reference [8].
A typical reference standard calibration is done at approximately 125 frequencies from

50 MHz to 18 GHz. Even with the

automated system described in reference

[8], the measurement takes about 40 h.

Figure 1.2 shows a typical thermopile

output at a few frequencies. The value of

rjg for one of the reference standards,

measured at 125 frequencies, is shown in

figure 1.3. The expanded uncertainty in the

rjg measurement as a function of frequency

is shown in figure 1.4. The uncertainty

components have been combined using the

RSS method (square root of the sum of the

squares.)

>

D

a.D

O

109

Figure 1.2

60 120 180 240

ELAPSED TIME, min

Thermopile output versus time for seven frequencies (in GHz).

too 0.5 I 1 1 1 I I I I l| 1 1 1 I I I M| 1 1 1 I I I M| I

t- 04 -

^ 3 Iz

< 0.2 -

0.1 -

0.01

' I 1 1 li i__i I I I II

FFteOUENCY. GHz

01 1 LOG FREQUENCY. GHz

10 30

Figure 1.3 Effective efficiency of Type

N mount measured at 125

frequencies.

Figure 1.4 Expanded uncertainty for the

Type N coaxial microcalorimeter when measuring

the effective efficiency of a NIST Type N mount.
2. DESIGN CONSIDERATIONS

As noted in the introduction, the

primary function of the

microcalorimeter is to measure the

effect of all microwave energy

dissipated in the reference standard

bolometer mount. This is accomplished by using a thermopile to measure the temperature rise of the

attached bolometer mount with respect

to a thermal reference ring under two

conditions. The first condition is with

dc only dissipated in the bolometer

mount and the second condition is with

both dc and rf dissipated in the mount.

Because the temperature changes are very small (on the order of 0.05 °C),

the microcalorimeter is also designed

to be immersed in a stable temperature

controlled water bath [9, 10] during the

measurement to minimize the effect of

external temperature changes. The

water bath is controlled to about ±20

;i°C at near room temperature.

Figure 2.1 is a partial cross sectional

view of the entire calorimeter with the cover raised. When in use the unit is suspended in the temperature controlled

water bath by the hanger extending

from the top of the cover. The actual

water level when the unit is in the bath

is indicated.

TYPE N CONNECTOR

-DC BIAS LEAD

o

Figure 2.1. View of the entire microcalorimeter

assembly.
2.1 Thermopile Assembly

This part contains a thermal isolation section as well as the thermopile itself. The thermal isolation

section, which is between the baseplate (in close contact with the water bath) and the bolometer

mount, allows the mount temperature to rise with respect to the baseplate. The isolation section is

a short length of coaxial transmission line made with a thin-wall copper outer conductor and a hollow

thin-walled gold-plated stainless steel inner conductor.

The thermopile is a ring made of 66 equally spaced radial turns of Constantan wire; the lower half

of the ring is copper plated giving 66 copper/Constantan junctions around both inner and outer

circumferences. The ring of inner thermocouple junctions is in thermal contact with (but electrically

insulated from) the 7 mm coaxial outer conductor just below the coaxial connector. The circle of

outer thermocouple junctions is in contact with a thermal reference ring which ^proximates the

thermal characteristics of the dummy reference standard used in the earlier twin-joule

microcalorimeter design as described in references [1] and [2].

The thermocouples in the thermopile are connected in series, so the thermopile output can be

increased by increasing the number of junctions. The number of thermocouples in the original

thermopile was limited by the input range, 100 /xV, of the potentiometer used to measure the

thermopile output. Since the microcalorimeter described here uses the original thermopile assembly,

it has fewer thermocouple junctions than it would otherwise. A thermopile with many more

junctions and made of finer wire would be a better match to the 2 mV range of the modem electronic nanovoltmeter now used.

2.2 Other Design Features

The rf input leads, the mount dc bias leads, and thermopile output leads are brought in through the

bottom of the microcalorimeter. This provides a more convenient arrangement for removing the top cover and also keeps the leads in the water bath for a greater distance to provide better thermal

tempering.

The entire assembly, including the cover, is gold plated for corrosion protection. The gold-plated

interior of the cover is polished to provide a high infrared reflectance.
3. FABRICATION AND ASSEMBLY

3.1 Mechanical Design and Drawing Reference

Figure 3.1 shows a cross section of a portion of the Type N microcalorimeter with the major

assemblies identified. Figure 3.2 is an exploded cross-sectional view of the same portion of the

microcalorimeter with an identifier for each part as it is listed in the tables found with figure 3.3.

Figure 3.3 is a view of the complete microcalorimeter with all the parts (except the dc bias and

thermopile leads) identified. The parts that are obtained commercially are listed by letter in the

upper table and with additional parts and detail in table A.l, ^pendix A. Those that must be

fabricated are listed by item number in the lower table and with additional parts and detail in table

B.l, ^pendix B. The table cross indexes the item with a part drawing which is identified by name

and sheet number. All the dimensioned part drawings are found in appendix B in sheet number

order. When viewed in the normal orientation, the sheet number is in the lower right comer.

TYPE N CONNECTOR

THERMAL REFERENCE RING

THERMAL ISOLATION SECTION h'.f^^^^

BASE PLATE

BRACKET

THERMOP I LE

ASSEMBLY

SEMBLY

CABLE CONDUIT

ASSEMBLY

Figure 3.1. Cross section of the Type N microcalorimeter.
Figure 3.2. Exploded cross section of the base and thermopile with part

identification.

8
1 TM DESCRIPTION

A O-ring. Pork«r JJ-OIO (2 r»q'd) B 0-rin9. Parker (12-111 C O-rln,. Park.r #2-346 D In.ulolor. r( conn. dor. H/P #5040-0306 E Mlnlotur. lock*!. AUP |50462-6 (2 r.q'd)' F O-ring. Parkor #2-015 G 4-40 X 1/4 hoiogon lockat tcrti (6 r.q'd)" H 6-32 X 1/2 hxogon lockat acra» (2 rtq'd)"

J 0-80 X 1/2 flol counloriunk tcrow (3 r«q'd)" K 4—40 X 1/2 hexogon iock«t mcrtm (8 roq'd)'* L 8-32 X 1/2 knurl«d knob, broat (3 r.qd) M 8-32 X 3/4 SHSS. (4 r.q'd)" N Connoctor, (yp* N, Miphanol #APC-N 131-10021 P 0.250, I4ml-rlgld coaxlol cobl« Q Pr«s9ur« 8«nsi1iv« poly«Bt4r top*

* R Conatanton sira, #30 AWC

' NOT SHOWN IN FIGURE " STAINLESS STEEL

ITM DESCRIPTION DWG NAME

SHT

NO.

1 RF input 1 ina bp4.tub2 1 2 Uppar & lowar lina ipocar bp4.br2 2

3 Lag (4 roq'd) bp4.br2 2 4 Lowar c 1 orrp ond brockat bp4.br 1 3 5 Cob la condui t bp4.tub1 4

6 Top hongar saction Monger 5

7 Cantar hongar aaction hanger 5 8 Lowar hangar aactlon hanger 5 9 Wotartlghl covor ecover 6 10 Tharnnol rafaranca ring cc1p.tr 7

1 1 Typa N plug cclp.p 8 12 Tharmopi l« form cc Ip.u 9 13 Thormol rafaranca ring baaa CClp.V 10 14 Center conductor ccio.o 1 1 15 Boaep lote bp4.ba 12

16 Tharmop i 1* boaa cclp.r 14 17 Tharmol ring standoff cclp.r 14 18 Uodified 4-40 X 1/2 icre» cclp.r 14 19 Thermal isolation section cclp.q 15 20 Input fastener ring bp4.e 16 21 Input foatener nut bp4.a 17 22 Input troneition bp4.c 18 23 RF input fastener bp4.b 19

Alignment tool' ol 1 tool 20

' NOT SHOWN IN FIGURE

Figure 3.3. Drawing and part reference for the Type N microcalorimeter.

9
3.2 Assembly Details

The following section gives some suggestions

regarding the assembly of the Type N coaxial

microcalorimeter. The assembly of an APC-7

version would be essentially the same process.

Most of the mechanical assembly can be carried

out in a logical manner based on a careful study

of the drawings.

3.2.1 Microcalorimeter Base

The microcalorimeter base consists of the

baseplate (item 15), the legs, and the rf input

and dc lead subassembly (items 1, 2, 4, and 5).

After assembling the individual lines, the

subassembly is put together according to the

layout and dimensions shown in figure 3.4.

Item 4, the lower clamp and bracket, is attached

to the baseplate and then the subassembly can be fastened to the baseplate. Finally, the legs

are attached to the baseplate.

63. 5 rmn (2.5 in)

152.4 rrm (6.0 in)

LINE SPACERS

38. 1 HTn

(1 .5 in)

25.4 mn

(1 .0 in)

Figure 3.4. Rf input and dc lead subassembly.

10
3.2.2 Thermopile

The thermopile subassembly consists of items

10 through 14, and 16 through 19. For a

cross- sectional view of those parts as

assembled, refer back to figure 3.1. The

threaded Type N connector part (item 11) and

the base (item 16), are soldered to the outer

conductor/thermal isolation section (item 19).

The base for the thermal reference ring (item

13) is attached to the base by the standoffs

(item 17).

Refer to figure 3.5. The thermopile form is wound with evenly spaced turns of Constantan

wire (item R, table A.l). The ends of the

wire are secured by looping the wire through

the two holes in the form. The wires on the

top half of the form are masked to prevent

plating, and the exposed wire is copper plated

for 30 min at a current of 100 mA. After

removing the masking material, jacks (item E)

are placed in the holes and soldered to the

thermopile wire. The thermopile is pressed

into place with an insulating film (item Q) on

the inner and outer diameters.

THERMOPILE FORM WRAPPED

WITH CONSTANTAN WIRE

biiiiiii 1 1 1 I I I I I I I 1 1 1 iiiiiid SIDE VIEW WITH TOP HALF MASKED

READY FOR COPPER PLATING

TYPE N CONNECTOR

INSULAT ING

F I LM ( ITEM Q)

JACKS

( ITEM E)

THERMOPILE

REFERENCE RING

COMPLETED THERMOPILE INSTALLED IN THE THERMOPILE SUBASSEMBLY

Figure 3.5. Thermopile top and side views.

The exact length of the stainless center conductor tube (part of item 14) is determined in the final

stages of assembly. The tube is trimmed to give the final connector dimension shown in figure 3.6.

Also shown in the figure is the nylon alignment tool (item 24, table B. 1) that is used to make certain

11
the thermopile assembly is properly centered

with the microcalorimeter baseplate before

tightening the mounting screws (item 18),

3.2.3 Thermopile and Bias Leads

The thermopile output cable is made with

shielded two-conductor cable (item S) with

two small pins (item T, table A.l) that

connect to the thermopile jacks. Thread this

cable through the conduit (item 5), and then

attach the custom made low thermal

connector (for a Keithley model 181 nanovoltmeter). The special low thermal

connector is identified as item 25 in table

B.l and shown in figure B21. The outer

shell parts of the connector are listed in table

A. 1 as items U and V.

The dc bias lead is made of a 1 m length of

the shielded four-conductor cable listed as

item W in table A.l, The connector inside

the microcalorimeter that attaches to the

reference standard is item X. For the wiring

pin-out see figure 3.7, The cable is threaded

through the conduit before the custom Type

rV power meter connector is put on the

cable. This power meter connector is

identified as item 26 in table B,l and shown

in figures B22 through B24.

ALIGNMENT TOOL

5.258 !°:°°°rmi

(0.207 !g:gg°in

Figure 3,6, Connector dimension and alignment

tool detail.

4-PIN MOUNT BIAS CONNECTOR

TOP VIEW

(As seen from the cable side of the connector)

PIN 1 PIN 2 PIN 3 PIN 4WHITE. LO-SIDE CURRENT

GREEN, HI -SIDE CURRENT

RED, HI-SIDE POTENTIAL

BLACK, LO-SIDE POTENTIAL

KEYWAY LOCATION

Figure 3,7, Pin-out for mount connector.

12
4. ACKNOWLEDGEMENTS

The first version of the coaxial microcalorimeter was designed and constructed by Morris E. Harvey.

The microcalorimeter described in this document is based on that original design with some

modifications. It uses the thermopile assembly from the original microcalorimeter. I also

acknowledge the many helpful discussions and suggestions of Neil T. Larsen. Much of the drafting

for the parts and other figures was ably provided by Paul G. Voris. The work was supported in part

by the NIST calibration surcharge development fund.

5. REFERENCES

[1] MacPherson, A.C.; Kerns, D.M. A microwave microcalorimeter. Rev. Sci. Instrum. 26(1):

27-33; 1955 January.

[2] Engen, G.F. A refined x-band microwave microcalorimeter. J. Res. Nat. Bur. Stand. (U.S.).

63C(1): 77-82; 1959 July-September.

[3] Harvey, M.E. WR 15 microwave calorimeter and bolometer unit. Nat. Bur. Stand. (U.S.)

Tech. Note 618; 1972 May. 33 p.

[4] Weidman, M.P.; Hudson, P. A. WR 10 millimeter wave microcalorimeter. Nat. Bur. Stand.

(U.S.) Tech. Note 1044; 1981 June. 11 p.

[5] Clague, F.R.; Voris, P.G. Coaxial reference standard for microwave f)Ower. Natl. Inst. Stand.

Technol. Tech. Note 1357; 1993 April. 46 p.

[6] Larsen, N.T. A new self-balancing dc-substitution rf power meter. IEEE Trans. Instrum.

Meas. IM-25: 343-347; 1976 December.

[7] Engen, G.F. A bolometer mount efficiency measurement technique. J. Res. Nat. Bur. Stand.

(U.S.). 65C(2): 113-124; 1961 April-June.

[8] Clague, F.R. A calibration service for coaxial reference standards for microwave power. To

be published as a Natl. Inst. Stand. Technol. Tech. Note.

[9] Larsen, N.T. 50 microdegree temperature controller. Rev. Sci. Instrum. 39(1): 1-12; 1968

January.

[10] Harvey, M.E. Precision temperature-controlled water bath. Rev. Sci. Instrum. 39(1): 13-18;

1968 January.

13

APPENDIX A. Parts List

The use of the particular components identified in the tables below does not imply recommendation

or endorsement by NIST nor does it imply that the identified items are necessarily the best available

for the purpose. Table A. 1 identifies the parts and a manufacturer.

Table A. 1 . Commercial parts list

Part IDNo.

needed

Part No., Description, Manufacturer

A 1 No. 2-010, 0-ring

Parker Seal Group, 0-Ring Division, Lexington, KY 40512 (606) 269-

2351

B 1 Parker No. 2-111, 0-ring

C 1 Parker No. 2-346, 70D Nitrile 0-ring

D 1 HP No. 5040-0306, insulator, rf conneaor

Hewlett-Packard Co. - contact local sales office

E 2 No. 50462-6, miniature socket i AMP Special Industries, Valley Forge, PA 19482 (215) 647-1000

F 1 Parker No. 2-015, O-ring

G 6 4-40 X 1/4 hexagon socket screw, stainless steel H 2 6-32 X 1/2 hexagon socket screw, stainless steel

J 3 0-80 X 1/2 flat counter sunk screw, stainless steel 1 K 8 4-40 X 1/2 hexagon socket screw, stainless steel L 3 8-32 X 1/2 knurled knob, brass

M 4 8-32 X 3/4 SHSS, stainless steel |

N 1 No. APC-N 131-10021, Type N connector

Amphenol Corp., RF/Microwave Operations, Danbury CT 06810

(203) 743-9272

Al
Table A. 1 . Commercial parts list

P 1 No. BA 5250, 0.250 semi-rigid solid dielectric coax

Precision Tube Co., North Wales, PA 19454 (215) 699-5801

Q

- Pressure sensitive polyester tape

R - #30 Constantan wire

S 1 Type 8640, #28 bare solid copper two-conductor shielded cable, 1 m long

Belden Wire Co.

T 2 Type K24A/M gold-plated connector pin, 0.99 mm (0.039 in) diameter

Vector Electronic Co.

U 1 No. MS-3106A-16-11P, connector shell Amphenol Corp.

V 1 No. 97-3057-1008, cable clamp

Amphenol Corp.

W 1 No. NMVF4/30-404655, #30 AWG four-conductor shielded cable, 1 m

long

Cooner Wire Co.

X 1 DP-4S-1 straight plug, four-conductor miniature connector

Microtech, Inc. Boothwyn, PA 19061 (215) 459-3566

A2
APPENDIX B. Parts Drawings

Table B. 1 cross references the fabricated parts to their drawing.

Table B.l. Fabricated parts list

Item No. Description No.

needed

Figure

number

1 Coaxial rf input line (RG-401AJ) configuration 1 Bl

2 Input lines spacer 2 B2

3 Microcalorimeter legs 4 B2

4 Lower bracket and clamp 1 B3

5 Cable conduit for dc input 1 B4

6 Top hanger section for cover 1 B5

7 Center hanger section for cover 1 B5

8 Lower hanger section for cover 1 B5

9 Watertight cover 1 B6

10 Thermal reference ring 1 B7

11 Type N connector and thermopile contact ring 1 B8

12 Thermopile form 1 B9

13 Base for thermal reference ring 1 BIO

14 Center conductor assembly 1 Bll

15 Microcalorimeter baseplate 1 B12, B13

16 Thermopile baseplate 1 B14

17 Thermal ring standoff 3 B14

18 Modified 4-40 X 1/2 screws 8 B14

19 Type N connector and thermal isolation section 1 B15

20 Fastener ring for input lines 2 B16

21 Fastener nut for input lines 2 B17

22 Center conductor transition 1 B18

23 Fastener for coaxial rf input 1 B19

24 Alignment tool - B20

25 Low-thermal-EMF connector 1 B21

26 NIST Type IV power meter connector 1 B22-B24

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B25

APPENDIX C. New Thermopile Assembly

The body of this report documents the design

and construction of the original NIST coaxial

microcalorimeters. The Type N version of

the microcalorimeter has been modified with

a new thermopile assembly. The thermopile

output voltage is now over an order of

magnitude higher than the original design.

This increased sensitivity allows coaxial

reference standards to be measured at less microwave power than the previous 10 mW

minimum. Measurement of the effective

efficiency of a CN transfer standard at 1 mW is now possible, producing a low power

calibration with a slightly increased

uncertainty. Also, a measurement at 1 mW

and again at 10 mW provides a way to determine the transfer standard nonlinearity

which is caused by the dual-element

substitution error [CI]. Additionally, with

the higher output, thermal emfs are of less concern.

Figure CI shows the Type N

microcalorimeter with the new thermopile

assembly. The thermopile consists of four

commercially obtained modular heat pumps

electrically connected in series and mounted

radially between the outer conductor and the

thermo-pile reference ring. The copper

braid provides the thermal path from the

small heat pump to the ring. Other than

replacing the original thermopile with the

new, the mechanical structure of the

microcalorimeter is unchanged. Thus the

previously determined calorimetric

equivalence correction factor also remains

valid.

Figure C2 is the equivalent of the earlier

figure 1.2 and shows the thermopile output

for five frequencies. This output voltage is almost 15 times greater than with the

original thermopile.

Figure CI. Coaxial microcalorimeter

1610 -

1600 -

1590 -

1580

ELAPSED TIME, mln

Figure C2. Thermopile output versus time for five

frequencies (in GHz).

CI
An example of the effective efficiency

measurement as a function of power is shown in figure C3. Four repeated

measurements and their average are plotted.

The observed change from 1 mW to 10 mW

is about 0. 1 percent.

Figure C4 is similar to figure 3.5 and is a

top and side view of the modified thermopile

assembly. Figure C5, the equivalent of a

portion of figure 3.2. shows the original plus

the new parts which are referenced and

described in table C.l. This table, which

adds to the list found in table A. 1 , appendix

A, identifies the parts and a manufacturer.

0992

O

z

01

Q.

u.&

UJ>

I-o

UJ

LL

U.UJ

0.991 -

0.990

I 1 1 1 1 1 1 I I

RUN »1. STEP

O RUN #2. STEP - V RUN #3. STEP

• RUN #4. STEP - —8^

-1- ^

^^. ...;.. ^""^^^"^T"^

:

:

:

; ^

i i i i i i i i i

POWER. mW

8 9 10 11

Figure C3. Effective efficiency of the CN mount

at 1 GHz as measured at three power

levels.

TYPE N CONNECTOR

THERMOP I LE REFERENCE RING

COPPER BRA

HEAT PUMP

Figure C4. Top and cross-sectional

views of the modified

thermopile assembly. Figure C5. Exploded cross section of the

modified thermopile assembly.

C2
Tabled. Commercial parts list

Part No.

needed

Part No, Description, Manufacturer |

Y 4 FC 0.45-8-05L thennoelectric (Peltier) heat pump |

Melcor, Trenton NJ 08648 (609)393-4178 1 Z 4 No 15-5 "Chem-Wik", 0.150" X 1" piece of untinned copper braid

Chemtronics Inc., Hauppauge, NY 11787

AA ~ Solder, BiSn (58% Bismuth, 42% Tin) 138 C eutectic solder

Indium Tin Corp. of America, Utica NY 13503 (315) 797-1630

BB ~ EG-8050-HC, thermally conductive epoxy (Ag-fiUed)

A. I. Technology Inc., Princeton, NJ 08543 (609)882-2332 |

The following describes the assembly and installation of the new thermopile. It is intended as a rough guide only and should be studied carefully before starting the process. Items in parentheses

are the component and part identifications as shown in figure C5 and table C.l. In making the

thermopile change, it is assumed that the original thermopile as described in section 3.2.2 has been

removed or was never installed. It is also assumed that the thermopile reference ring base (13) and

the reference ring (10) are not installed. If the base is already in place, the heat pumps can be put

in without removing it; it is just more difficult.

1. If using a copper braid made for solder wicking, the flux must be first be completely

removed using a suitable solvent.

2. Cut the braid to the lengths needed and form to the approximate final shape.

WARNING: The

semiconductor materials used in

heat pump construction are

structurally very weak. Avoid

placing any torsional or shearing

stresses on the heat pumps.

Touching the crystals with a tool

may fracture them.

C3
3. Remove all leads, if any, from the heat pumps. Replace with ff26 stranded hookup wire

with thin insulation, for flexibility. Use only one lead per pump for three of the pumps, as the pumps will eventually be connected in series. Use solder and flux available from the heat

pump manufacturer. The heat pump is assembled with a bismuth-tin eutectic solder melting

at about 136°C, available from the heat pump manufacturer. An active zinc chloride-based

flux such as Alpha Metals flux 2(X)L may be used.

4. Saturate a small area of one end of each piece of copper braid with thermally-conductive

epoxy and attach to the smaller plate on each heat pump. Pay attention to orientation, so that

proper polarity is maintained. The braid must point "up" and the leads "down" on each

pump. Small spring clips may be used to clamp the braid to the heat pump, taking great care not to overstress the part. Use only enough force to hold close contact between the braid and

the heat pump.

5. Cure the epoxy using a thermostatically-controlled hot plate or enclosure. The cure temperature must remain below 100°C to avoid melting any of the solders.

6. Complete the lead attachment by connecting all pumps in series, such that the thermoelectric

outputs all add.

7. Bond the larger plate on each of the heat pumps to the lower flange of item (11) with

thermally-conductive epoxy. A rubber band may be used to hold them during the cure cycle,

or, they may be bonded one at a time with spring clips.

8. Install the thermal reference ring (13).

9. Clamp the four copper braids between the base (13) and the reference ring (10) and trim off

any excess braid.

Reference

[CI] Engen, G.F. A dc-rf substitution error in dual-element bolometer mounts. DBEE Trans.

Instrum. Meas. IM-13: 58-64; 1964 June-September.

*U.S. GOVERNMENT PRINTING OFFICE: 1993-774-025/89043

C4


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