THE MBS WR62 and WR90 REFERENCE

NOISE STANDARDS

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

May 1984

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NBSIR 84-3005

THE MBS WR62 and WR90 REFERENCE

NOISE STANDARDS

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

May 1984

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

C.K.S. Miller

W.C. Daywitt

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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CONTENTS

Page

Abstract '1

1 . THE BASIS FOR A REFERENCE NOISE STANDARD 1

1.1 Requirements of a Reference Standard 4

2. THE WR90 AND WR62 REFERENCE NOISE STANDARDS 4

2.1 Fundamentals 4 2.1.1 Transmission Line Effects 4 2.1.2 Resistor Design 6 2.1.3 Oven Design 6 2.2 Application to the WR90 and WR62 Reference Standards 9 2.2.1 The Waveguide Transmission Line 9 2.2.2 The Microwave Resistor 11 2.2.3 Accuracy of the WR90 Standard 11

3. COMPARISON OF REFERENCE STANDARDS 13

3.1 International Comparison with Sweden 14

3.2 International Comparison with England 15

3.3 International Comparison with Australia and Japan 16

APPENDIX. NOISE TEMPERATURE DERIVATION 20

THE NBS WR62 and WR90 REFERENCE NOISE STANDARDS

C. K. S. Miller and W. C. Daywitt

National Bureau of Standards

Boulder, Colorado 80303

The basis for the National Bureau of Standards (NBS) WR90 and WR62 Waveguide

Reference Noise Standards and the corresponding error analyses are described. The

standards are heated (1270 K) thermal noise generators, and a derivation of their

output noise temperature equations is also presented. Results of comparisons of

the NBS WR90 standard with those of Sweden, England, Australia, and Japan are included. The text is extracted from course notes presented at NBS in 1970, and hence

does not include descriptions of standards constructed at NBS since that time.

Key words: error analysis; international comparisons; microwave; noise standard.

1. THE BASIS FOR A REFERENCE NOISE STANDARD

Temperature as a measure is one of the six internationally agreed upon Basic Standards [1]

defined by concerted agreement of the member nations to the treaty signed and known as the Metric

Convention; the other five are, "the meter" (defined in terms of the wavelength of krypton

light - 86), "the second" (defined in terms of a transition of the cesium atom), "the kilogram"

(defined in terms of a cylindrical mass of platinum-irridium alloy and kept at the International

Bureau of Weights and Measures in Paris), "the ampere" (defined in terms of the magnetic field

induced between two parallel wires separated by one meter), and "the candela" (defined in terms of

the intensity of light radiated from a cavity maintained at the temperature of freezing platinum).

The thermodynamic or kelvin scale of temperature used in the International System of Units (SI),

has its origin or zero point at absolute zero and has a fixed point at the triple point of water

defined as 273.16 kelvins. The Celsius and Fahrenheit scales are derived from the kelvin scale,

and the triple point of water in each of these is respectively 0.01°C and 32.02°F.

According to the international System of Units, the unit of the International Practical

Temperature Scale is the kelvin which by definition is the fraction 1/273.16 of the thermodynamic

temperature of the triple point of water [2]. The kelvin is abbreviated by the symbol K and is

equal to a degree in the Celsius temperature scale. A difference of temperature is expressed in

kelvins; it may also be expressed in degrees Celsius.

The six mentioned quantities are the only Basic Standards defined by international agreement,

and all other standards are derived from these six. These derived standards are referred to either as Derived Standards or as Reference Standards , and should be preceded by an identification

as to whom they belong; hence NBS Reference Standards.

By 1875 the United States was one of 17 countries that signed an international treaty in

France, the Metric Convention (today there are 40 countries who are signees). This treaty set -p defined metric standards of length and mass, and established the International Bureau of Weights

and Measures (BIPM). Also established was the General Conference of Weights and Measures (GCWM),

1
which would meet every six years to consider any needed improvements in the standards, and to serve as the authority governing the International Bureau. An International Committee of Weights

and Measures (ICWM) was also set up to implement the recommendations of the General Conference,

and to direct the activities of the International Bureau: this Committee meets every two years.

Figure 1 gives a pictorial display of the relationship of the National Laboratories to the three

functioning international organizations, differentiating between the diplomatic and technical

levels of activity in the chain.

From figure 1 we can see that the only contact any country has with these international or- ganizations is through their own National Laboratories. This limited contact removes the burden

of negotiating agreements in measurements and standards from privately owned industrial concerns

when dealing in international trade, and avails them of a means of coordinating results through

each of the countries' respective National Laboratories. The burden of coordinating measurements

and standards is a concern of these international organizations.

In 1866 the metric system of measurement was established by law to be an acceptable system of

measurement in the U.S., and in 1902 an effort to make it the only system of units for Federal

usage was defeated in Congress. There are at present only four major countries in the nonmetric

world (the United States, Canada, Australia, and New Zealand), and Australia has recently com- pleted a study which recommended conversion to the metric system. Such a move on Australia's part

would have a great deal of influence on New Zealand. At present a study is in progress to determine what course of action should be taken in the United States.

In the United States the National Bureau of Standards was designated by a law enacted in 1901

as the sole National Laboratory to represent this country to the international organizations, and

to maintain the United States measurement standards. This law when enacted, however, did not

carry with it any authority to enforce the use of or reference to these standards. It was left up to individuals and organizations to decide whether they would cooperate with NBS or not.

In October 1968, the International Committee of Weights and Measures approved a new International Practical Temperature Scale. The Committee had been authorized to take this action by the

13th General Conference of Weights and Measures which had met in October 1967. The new International Practical Temperature Scale of 1968 (abbreviated EIPT-68 in French, and IPTS-68 in English) is defined in terms of fixed points and interpolation formulas for specified instruments

used to realize the scale. There were two main reasons for revising the International Practical

Temperature Scale. First, the national laboratories had established national scales for the temperature range 10 K to 90 K, and these efforts needed to be unified. Second, the Scale had not

been revised over the temperature range -183°C to 1063°C since its inception in 1927, and improved

gas thermometer measurements showed serious differences between it and the thermodynamic temperatures [2]. The new Scale differs from the previous scale, the International Practical Temperature

Scale of 1948 (IPTS-48), in the following ways:

(1) The new scale extends down to 13.8 K; IPTS-48 terminated at 90.18 K.

(2) New values have been assigned to most of the defining fixed points and to the second

radiation constant, C2, in order to more closely approximate the thermodynamic temperature scale [3].

2
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1.1

Requirements of a Reference Standard

If a Reference Standard is to have the quality of being reproducible, then it must be based

on established physical laws and accepted Basic Standards. To satisfy this requirement the measure of thermal noise has been tied into the accepted International Practical Temperature Scale.

The second requirement of a Reference Standard is that its physical realization be thoroughly

evaluated, both theoretically and experimentally.

2. THE WR90 AND WR62 REFERENCE NOISE STANDARDS

Basically the NBS Reference Standards are built about a single concept, but before describing

the design a short discussion will establish the fundamentals upon which the designs are based.

2.1 Fundamentals

If a resistor that terminates a transmission line is heated to a prescribed temperature, then

under ideal conditions the resistor will generate a noise power that will be propagated down

the transmission line. In this matched and lossless case P„ = kTB, where k is Boltzmann's con- stant, T is the temperature of the resistor on the kelvin scale, and B is the limiting bandwidth

of the system used to compare the standard to a device under test. Since the bandwidth is con- stant, the noise power is usually referred to by the temperature of the resistor. Difficulties

are quickly encountered however, when a practical model is visualized. The work at NBS to over- come these practical problems is primarily based upon the work done by Wells and his associates

[4,5].

2.1.1 Transmission Line Effects

Wells sought to 1) calculate the noise output of the standard by taking into account both the

attenuation and generation effects of the transmission line, and to 2) check the calculated output

by building two models in which the parameters were varied in a known way to see if the measurement results obtained were consistent with the calculations. The resulting two transmission lines

and resistors were made from different materials, the resistors being formed into different shapes

(fig. 2). Results showed that both models gave essentially the same results. The attenuations of

the heated waveguide were calculated from dc resistivity values versus temperature. To check the

calculations a gold waveguide was constructed and truncated at the heated end, and its reflection

coefficient measured with a tuned reflectometer. The measured and calculated values showed re- markable agreement, being well within the estimates of error of measurement and calculation.

This approach calculates both the attenuation effect and the additional noise generated by

the transmission line losses. It requires that the resistivity of the material be known as a function of temperature, limiting the number of available materials from which the transmission

line can be made. The equation used to calculate the temperature correction to the measured tem- perature of the resistor for the transmission line effects and its contribution to the error are included in Appendix A.

4
5

Figure
2. 1. 2 Resistor Design

Two different shapes (fig. 2) for the design of the microwave resistor were used, both of

which had low reflection coefficients. They were made from lossy materials especially chosen 'to be compatible with the material of the transmission lines. Since noise radiated from a surface is

a volume effect, the spear-pointed shaped resistor had the advantage that three thermocouples

could be located in the material along the entire length of the noise generating volume.

2. 1. 3 Oven Design

The requirements of each reference standard necessitates that: 1) a uniform heat input be

supplied about the microwave resistor to maintain the gradient of the resistor to less than one- half kelvin; 2) a designated temperature setting for the resistor be maintainable to less than 0.1

kelvin during 24 hours, and to less than 0.2 kelvin for a week; 3) the gradient established for

the designated temperature in the transmission line must be held constant -- this requirement is

fairly easily accomplished if heat input is uniform and fairly constant and the hot and cold ends

of the transmission line are maintained at constant temperatures; and 4) the heating and cooling

rates for the oven must be low enough to avoid cracking the heat distributor during heating and

cooling the oven. The oven design (fig. 3) is a simple muffle type furnace which has three nichrome heating

coils wrapped about a dense, non-porous, high-purity alumina tube that extends through the length

of the oven. The larger central heating coil is servo-controlled, and the other two smaller coils

are controlled manually. The central heating coil is controlled by a proportional network that

selects a portion of each half cycle of the full wave rectified line voltage as the heating duty

cycle (phase controlling a silicon-controlled rectifier). A emf from a thermocouple located on the inside surface of the alumina tube serves as the sensing element. The central heating coil is

further controlled by a fail-safe device to protect from over-heating in the event of a malfunctioning thermocouple, and by a programmable device to raise and lower the temperature at a set

rate. Inside the alumina tube and centrally located is a dense high-purity beryllia cylinder which

serves as a heat distributor. The beryllia cylinder has a rectangular hole into which is placed

the microwave resistive generator mounted in one end of the transmission line. Alumina insulators

are located in the tube to support the transmission line and thermocouple leads. Figure 4 shows

the thermocouple leads extending out of one end of the oven, and the water-cooled flanged end of

the transmission line extending out at the front end. The whole control system is located in the

base.

An apparent disadvantage to using beryllia as a heat-distributor is its toxicity. The hardfired beryllia heat distributor has been evaluated for toxic dust that could be airborne by vaporization at the surface and, although air samples did indicate the presence of such dust in the

vicinity of the operating oven, the traces observed were generally about 1/20 the safe limits (at

worst 1/10 the safe limits, and that only for the ovens operated close to the gold point

1064. 43°C). The proven safe limit by the AEC is designated to be 2 pg per cubic meter of air for

persons exposed to beryllia dust for an average 8-hour working day. At this time beryllia heatdistributors are used in this connection at NBS, and have been regularly employed in their role

with reference standards for over five years without incident. The advantage to using beryllia is

6
1 ALUMINA TUBE(NON-POnOUS)

2 REFRACTORY MATERIAL

3 INSULATORS OF HIGH PURITY

A meat distributor

5. RESISTIVE MATERIAL WEDGE(NOISE GENERATOR)

6 WAVEGUIDE

7. WATER JACKET ATTACHED TO FLANGE

8 MANUALLY CONTROLLED HEATING COILS

9 SERVO-CONTROLLED HEATING COIL

Figure 3.

7
Figure 4.

8
its high thermal conductivity, thereby maintaining a uniform temperature in the proximity of the

microwave resistive generator -- in addition variations in the heat input from the heating coils

are damped out.

2.2 Application to the WR90 and WR62 Reference Standards

These two reference standards are quite similar (the major difference is in the size of waveguide employed), so they will be considered together. The description is based on the WR90 model,

remarks being injected where the WR62 model is sufficiently different to warrant comment.

2.2.1 The Waveguide Transmission Line

Both pure gold and an 87 percent platinum -- 13 percent rhodium alloy for the waveguides were

used; the resistivity of both materials has been determined as a function of temperature [4]. For

a permanent reference standard it was found that gold was unsatisfactory because: 1) it was too

soft and easily distorted, especially at elevated temperatures; 2) when operated within 60 kelvins

of its melting point for an extended period of time it is prone to recrystallization and therefore

a change in its resistivity; and 3) for the WR90 waveguide size gold was not available in an ex- truded waveguide form -- an extruded waveguide is commercially available in the WR62 waveguide

size. Due to the dissatisfaction with gold both the WR90 and WR62 reference standards use only

the platinum alloy waveguide at this time.

The platinum alloy waveguide is approximately 51 cm long, and has a water-jacket constructed

onto its flange to maintain the flange at approximately room temperature (297 K); this is the output flange of the reference standard. The microwave resistor is inserted into the other end of

the waveguide and is operated at about 1273 K. The controlled and manually operated heating coils

are set for a minimal gradient in the microwave resistor (the gradient of the resistor must be

less than 0.5 K). With the heat input and the heat output maintained at a constant rate a steadystate condition is established, and a set gradient exists between the hot and cold ends of the

waveguide that can be maintained for long periods of time.

When the gradient is set, it is measured by inserting a calibrated platinum versus platinum

10 percent rhodium thermocouple (the gradient thermocouple) through the output port. Figure 5 shows a typical gradient for the platinum alloy waveguide. Using the tip of the resistor as a zero point, the gradient thermocouple is withdrawn by 2.5 cm increments; each increment including

initial and ending points in the waveguide are measured to ±0.16 cm. Each location that a temperature measurement is made is a difference measurement relative to its preceding measurement, and

these difference measurements can be made to ±0.1 K. Since the gradient of the waveguide is started at the tip of the resistive generator, the temperature measurement at this location is an absolute measurement and this measurement is made to ±0.3 K, the accuracy to which the thermocouple is calibrated. The gradient curve is dependent upon the temperature measurements, and if

these measurements are not carefully made, this gradient can be upset, causing an error in the

output calculation.

For the calculation of the attenuation and noise generation effects of the waveguide and the errors associated with this calculation, the resistivity values are assumed to be in error by less

than ±20 percent. ,

9
MEASURED

TEMPERATURE

OF

WAVEGUIDE

IN

KELVINS

51 cm. 76 cm,

.—

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DISTANCE ALONG

WAVEGUIDE IN cm.

Figure 5.

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10
2.2.2 The Microwave Resistor

The resistive generator is fabricated from a ceramic formed by imbedding silicon-carbide in

beryllium-oxide. The material has the good thermal properties of beryllia and the electrically

lossy qualities of silicon-carbide.

The shape is experimentally determined by a cut-and-try process with a cheap substitute that

at room temperature has comparable characteristics to the Carberlox at elevated temperatures. The

shape is intended to give a low reflection coefficient magnitude across the band. Placing the

thermocouples initially was done more as a matter of convenience to the designed shape, but their

placement has proved very satisfactory.

Using the gradient thermocouple a check is made of the thermal gradient on the resistor's

exposed surface (or face) to correlate the temperatures recorded by the three thermocouples imbedded in the resistor with the measured surface temperatures. Two restrictions imposed on these

gradient measurements are: 1) the temperature gradient of the exposed surface must be less than

0.5 K; and 2) the greatest discrepancy between the three imbedded thermocouples cannot exceed

0.5 K. Both of these restrictions must be met before the reference standard is usable. Using a pyrometer to determine the gradient on the face is not satisfactory since the sensitivity of pyrometers is not adequate.

Figure 6 gives an idea of the temperature conditions found in a typical set of WR90 resistor

measurements (the numerical values are very similar to the WR62 resistor). The largest gradient

is usually between the tip temperature Tpj and the imbedded tip thermocouple temperature Tj. The

exposed surface temperatures are generally about 0.4 K cooler than a plane parallel to the face

that includes the imbedded thermocouples. The error in assuming the whole resistor to be at the

temperature T„ is AT„ and must be determined.

The noise contribution from the region about Tg is negligible in determining either T^^^ or

AT|^; this implies that temperatures measured at Tj^ are necessary only to insure that the gradients

are minimized. The contribution to the error from temperatures T^ and Tj is half the difference

between them; since ITq-Tj| < 0.5 K, we can assume this portion of the error will always be less

than +0.25 K. The amount of noise generated at the tip of the resistor will be small since the mass is small, so we assume an additional error of 1/2 |Tj-Tpyl = +0.4 K. The sum of these two

effects (+0.25 K) + (+0.4 K) = +0.65 K encompasses most of the thermal gradient in the resistor

(the shaded portion in fig. 6). All thermocouples are calibrated by the Heat Division, NBS,

Gaithersburg, Maryland, to a reported accuracy of +0.3 K. Adding the thermocouple accuracy to these effects gives AT <1 K. ^ m

2.2.3 Accuracy of the WR90 Standard

The Appendix gives a convenient variation of eq (1) [4] that is programmed for a computer. The

computer program also includes an error analysis of the same equation, so that an indication of

the contributions of the component errors are read out with the output data. The input data for

the program, besides the previously discussed sources of error, are the measured thermal gradient

of the waveguide, the constants related to the frequencies selected for operation of the reference

standard [4], and the measured temperature of the microwave resistive generator T^. For an actual

measured gradient of the waveguide, table 1 shows the comparative results of input and output

data. The values in table 1 are unique to the gradient; since the gradient will change with each

11
I

I

I

Measured Data

|T^ - Tp^l = 0.8k |Tj - T^l < 0.5k

|Tc - Tp^l = 0.4k |T(. - T^l < 0.5k

iTj - Tpjl = 0.4k |Tj -

i ^cl < 0.5k

Accuracy Determination

T

T

- ^

^ ^T

2

’|Tr Ml IM ’p-rl

+ AT 1

AT = ± m

^ + L 2 2 calib J

“ ± [(0.25) + (0.4) + (0.3) < ± 1.0 k

Therefore, T ± AT is shown by shaded area,

m m

Figure 6.

12
Table 1.

FOR A GIVEN WAVEGUIDE GRADIENT AT 9.6 GHz, THE SOURCES OF ERROR OF THE

WR90 NBS REFERENCE STANDARD ARE:

Measured Temperature of Resistor T^ = 1275.6 K

Input Data

Resulting

Error in Standard

(Kelvin)

The Error in Knowing the Absolute

Temperature of the Microwave

Resistive Generator 1.00 K 0.96

Error in Establishing Zero Point

for Length Measurement 0. 16 cm 0.04

Error in Establishing Each Inch

Increment of Length—This is

Cumulative to the Whole Length 0.16 cm 1.63

Error in Establishing the End Point

for the Total Length Measurement 0.16 cm 0.23

Error in Difference Temperature

Measurements for Each Increment

of Length 0.1 K 0.03

Error Due to Assuming Resistivity

Values of Waveguide to be

Inexact 20% 0.02

Total Error for NBS Reference Standard

At 9.6 GHz 2.91

The Correction to T^ Due to the Waveguide Gradient = -13.05 K

operation (extreme gradients are shown by dotted lines in fig. 5) the resulting accuracy will also

change. Generally, for a given fixed gradient the accuracy is poorer at the lower frequencies in

the band. It should be noted that changes in frequency will affect the contributions from each

source error identified in table 1 influenced by the gradient in the waveguide, while the error of

the resistive generator AT^^ does not change.

The correction to the measured temperature of the resistive generator due to the influence of

the waveguide loss is also calculated by the computer, and for 9.6 GHz amounted to a negative

13.05 kelvins as shown in table 1. This in turn implies that of the microwave noise generated by

the resistor only (T|^-13.05) is available at the output flange of the standard.

3. COMPARISON OF REFERENCE STANDARDS

There are several countries that have either developed or purchased their own standards.

Intercomparing these standards assures a degree of measurement agreement between countries.

13
NBS has been a partner to three international comparisons in WR90 waveguide. Unfortunately,

to compare reference standards the outputs of the standards have been compared to an argon gasdischarge noise source having a noise output that is a great deal higher than the reference standards. This implies that the errors due to transferring the noise output of the reference standard

to the argon source are part of the comparison error. A degradation of accuracy occurs in this

transfer, and this in turn implies that an evaluation of the comparison radiometers must be made.

Evaluation of the radiometers differs not only with the mode of operation and design, but also

with the experience of the respective metrologists. Some of the difficulties would be eliminated

if the noise power of an argon noise source were attenuated to the level of the reference standards of concern, as was once suggested [6]. With interlaboratory standards of comparable noise

output to the reference standards being intercompared, errors could be reduced +15 kelvins and the

results would be independent of most of the effects of the comparison radiometers used.

3.1 International Comparison with Sweden

In January 1965 an intercomparison was initiated with the Swedish national laboratory, the

Research Institute of National Defense (FOA) at Stockholm. The FOA WR90 Reference Standard was fabricated for them by a Japanese firm and is designed to operate at 1000 K, and their radiometric

comparator was fabricated by a U.S. firm [7]. The reference standard and the radiometer are each

capable of +0.05 dB accuracy. Therefore, the comparison measurements of argon noise sources with

outputs of about 11,000 K is estimated by FOA to be +0.1 dB. At the time of this comparison all of the sources of error were not known at NBS, but it was

felt that a conservative estimate of the total error was +0.1 dB. FOA sent NBS three waveguide

noise sources (argon) directly from the manufacturer. The three were measured in January 1965.

Two of the three were compared to the NBS WR90 Reference Standard indirectly by way of using the

NBS Working Standards, and one was compared directly. The reference standard used was the gold

waveguide with a silicon-carbide resistive generator. In June 1965 the results of the Swedish

measurements on the same units were received [8]. In May 1966, two of the three waveguide noise sources were returned to be measured a second

time. At this time both were compared directly to the NBS WR90 Reference Noise Standard, this

time using the platinum alloy waveguide and the Carberlox resistive generator. The results showed

a significant difference in noise output at 9.0 GHz. An investigation did not reveal the cause

for the change. It was not until long afterwards that it was discovered that recrystallization of

the gold waveguide used for the comparison in 1965 had been the primary contributor to this discrepancy.

From table 2 (the values are given in Excess Noise Ratio above 290 K with effective noise

temperature for NBS measurements included parenthetically) it can be seen that the agreement

achieved at 9.8 GHz and 11.2 GHz never was very good between NBS and FOA although their results

are quite self-consistent from one comparison to the next. The large difference between the

values measured in 1965 and 1966 by NBS should also be noted. Later FOA learned that a discrepancy was found in the calibration of their temperature measuring thermocouple, and when it was recalibrated their measured values dropped by approximately 0.1 dB, with the final result that now moderately good agreement exists between NBS and FOA. FOA also found some small improvements made

to the radiometer improved things on their end.

14
Table 2.

Comparison With Sweden

Comparison of January-June 1965 [8]

9.0 GHz 9.8 GHz 11.2 Ghz

ENR ENR ENR

NBS Test No. 40020

NBS 15.77 dB

(11,240 K*)

15.73

(11,150

dB

K*)

15.75 dB

(11,190 K*)

FOA 15.80 dB 15.86 dB 15.83 dB

NBS Test No. 40021

NBS 15.77 dB

(11,230 K*)

15.74

(11,160

dB

K*)

15.75 dB

(11,190 K*)

FOA 15.80 dB 15.86 dB 15.83 dB

NBS Test No. 40022

NBS 15.78 dB

(11,260 K*)

15.75

(11,180

dB

K*)

15.76 dB

(11,210 K*)

FOA 15.80 dB 15.88 dB 15.86 dB

Comparison on May- June 1966 [9]

NBS Test No. 40021

NBS 15.70 dB

(11,060 K)

15.73 dB

(11,140 K)

15.74 dB

(11,170 K)

FOA 15.79 dB 15.82 dB 15.83 dB

NBS Test No. 40022

NBS 15.71 dB

(11,090 K)

15.74 dB

(11,170 K)

15.76 dB

(11,200 K)

FOA 15.81 dB 15.84 dB 15.86 dB

*A11 effective noise temperature measurements are related to the IPTS-48

temperature scale.

3.2 International Comparison with England

In May 1966 an intercomparison was initiated with the Services Value Test Laboratory (SVTL)

of England, which serves as the national laboratory in England. A comparison had been initiated

in 1964, but the waveguide noise source became unstable and therefore unsuitable for the purpose.

The SVTL WR90 Reference Noise Standard operates at 673 K and was constructed at SVTL [9]. The

SVTL radiometric comparator is an IF switched radiometer [9]. The waveguide noise sources con- taining argon noise tubes used for the comparison were an SVTL design [9] but made commercially in

15
England. The evaluation SVTL had made of their reference standard and their radiometer resulted

in "an absolute accuracy of +130 K (+0.05 dB)" [9] capability.

In November 1966 NBS received one mount and three argon noise tubes for the comparison. When

each tube was placed in the mount it comprised a one-port waveguide noise source to be measured.

The repeated removal and reinsertion of the tube in the mount proved to have a good repeatable

effective noise temperature. For this reason the results will be referred to by the tube number

although the combination of tube in mount is implied in each case. For the NBS measurements the

platinum alloy waveguide with the Carberlox resistive generator was used since by this time the

gold waveguide had been shelved.

In January 1967 SVTL remeasured the three noise tubes in the same mount and returned them to NBS, who in turn measured them a second time in April 1967. They were returned to SVTL the last

time in April where they were remeasured in May and June 1967.

The connectors used in England are different to the type common to the U.S., and SVTL allowed

for this by providing a mount that had a UG 39/U cover flange for the convenience of NBS. Therefore, it became necessary for SVTL to account for the adapter loss, which they did by measuring

two such adapters back to back, and assuming the loss to be half the loss measured [10].

Table 3 lists the values obtained in the comparison [10], and is recorded in terms of the

effective noise temperature T^^ (kelvins) and the excess noise ratio ENR (dB). During the course

of the comparison SVTL encountered difficulties using a choke flange at 9.8 GHz, finding it to have resonances at this frequency. They corrected data accordingly. SVTL also became aware of a deficiency in their calibration method for their precision attenuator. This was eliminated and

corrections to the data taken were made. In the end SVTL estimated that the improvements to their

measurements system increased the accuracy to +100 K, or +0.04 dB in the ENR. In the interim, no changes or modifications to the NBS equipment or measurement methods were made except that the NBS

WR90 Reference Noise Standard was disassembled and reassembled for the second set of measurements

with a new set of thermocouples for the resistor temperature measurements. The accuracy NBS

claimed was +135 K, or +0.05 dB in the ENR. Table 3 shows the good agreement achieved in the final results, but does not indicate the

original data taken. Several peculiarities are noticeable however. The differences between the

SVTL and the NBS measurements show: 1) at 9.0 and 11.2 GHz the values given by SVTL are higher

than those by NBS; 2) at 9.8 GHz the values are reversed; and 3) the SVTL results indicate an increasing measured value of each waveguide noise source with increasing frequency of operation

while the NBS results indicate the highest values were obtained at the center frequency of measurement. It is worth noting that since SVTL uses a resistive generator at 673 K their values

should be increased by approximately 2 kelvins to comply with the IPTS-68 temperature scale while

NBS values should be increased by approximately 14 kelvins. This improves the agreement.

3.3 International Comparison with Australia and Japan

In 1966 three organizations in Japan had microwave reference standards. They had intercompared these reference standards through the use of a waveguide noise source with an argon tube.

Unfortunately they each measured the same waveguide noise sources at three separate frequencies;

their results showed little agreement. The University of Tokyo (UT) has a cryogenic reference

standard, the Electrotechnical Laboratory (ETL is the national laboratory of Japan for noise) a

16
9.0 GHZ

Table 3. Comparison With England

9.8 GHz 11.2 GHz

T

* ENR ENR ENR

ne

in Kelvin in Decibels

ne

in Kelvin in Decibels

ne

in Kelvin in Decibels

Tube No. 261

SVTL 11178 15.745 11192 15.751 11234 15.763

NBS 11160 15.738 11216 15.761 11196 15.753

SVTL 11178 15.745 11199 15.754 11228 15.765

NBS 11150 15.734 11201 15.755 11191 15.751

SVTL 11183 15.747 11197 15.753 11230 15.766

Tube No. 277

SVTL 11213 15.759 11229 15.766 11271 15.782

NBS 11188 15.750 11245 15.772 11236 15.768

SVTL 11210 15.758 11232 15.767 11266 15.780

NBS 11178 15.745 11240 15.770 11223 15.762

SVTL 11211 15.758 11229 15.766 11264 15.779

Tube No. 302

SVTL 11336 15.808 11349 15.814 11384 15.826

NBS 11315 15.800 11370 15.821 11354 15.815

SVTL 11334 15.808 11352 15.815 11386 15.827

NBS 11303 15.795 11368 15.821 11342 15.811

SVTL 11335 15.808 11351 15.815 11388 15.828

*A11 effective noise temperature measurements are related to the IPTS-48 temperature scale.

heated reference standard that is operated at the freezing point of gold, and the Tokyo Electrical

Engineering College (TEEC) a heated reference standard.

In March 1967 NBS participated in a comparison with the Japanese and the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia. Not much is known of the

Japanese reference standards and measurement system. However, the Australian reference standard

and radiometric comparator is described by Somlo and Hollway [11]. The Australians used a heated

reference standard with the resistive generator operated at 1300 K [12].

Three frequencies were chosen for this comparison, one each to coincide with each of the

frequencies of measurement used by the three organizations in Japan. The results obtained are detailed in table 4 with the respective accuracies indicated with the results. It is worth noting

that agreement with University of Tokyo was very good at one frequency only. Apparently, the

waveguide noise source stayed quite stable as indicated by the measurements at the University of

Tokyo, taken first in September 1966 and again in September 1967. Agreement between NBS and CSIRO

was not as good as was expected, being just within the +90 kelvins accuracy limits. At 11.1 GHz

the value obtained by CSIRO is in disagreement with the other values reported. Unfortunately, the

values obtained at 9.375 and 10.0 GHz are only useful for the comparison between CSIRO and NBS.

17
Table 4.

Comparison With Australia And Japan

Date Organization T

* ne ENR

(in Kelvin) (in Decibels)

For 9.37E) GHz

Sept. 1966 ETL 11,280 ± 440 15.79 + 0.16

March 1967 NBS 11,590 ± 145 15.907 + 0.06

Aug. 1967 CSIRO 11,509 ± 90 15.876 + 0.0035

For 10.0 GHz

Sept. 1966 TEEC 11,700**

March 1967 NBS 11,626 ± 130 15.921 + 0.05

Aug. 1967 CSIRO 11,550 ± 90 15.891 + 0.035

For 11.1 GHz

Sept. 1966 UT 11,670 ± 190 15.94 + 0.07

March 1967 NBS 11,650 ± 135 15.930 + 0.05

Aug. 1967 CSIRO 11,560 ± 90 15.895 + 0.035

Sept. 1967 UT 11,650 ± 220 15.93 + 0.082

*A11 effective noise temperature measurements are related to the IPTS-48

temperature scale.

**This value is unofficial and is only included to give a perspective to the problem in Japan.

REFERENCES

[1] National Bureau of Standards (U.S.), Special Publication 304, 1968.

[2] "The International Practical Temperature Scale of 1968," Int. Jr. of Sci. Metrology, Vol. 5, No. 2, pp. 35-44, April 1969.

[3] National Bureau of Standards (U.S.), Technical News Bulletin, Vol. 53, No. 1, pp. 12-13,

1969.

[4] Wells, J. S. , W. C. Daywitt, and C. K. S. Miller, "Measurement of Effective Temperatures of

Microwave Noise Sources," IRE International Conv. Rec. Pt. 3, pp. 220-238, March 1962; also

in IEEE Trans, on Instrumentation & Measurement, Vol. IM-13, pp. 17-28, March 1964.

[5] Miller, C. K. S. , W. C. Daywitt, and M. G. Arthur, "Noise Standards, Measurements and Re- ceiver Noise Definitions," Proc. IEEE, Vol. 55, No. 6, pp. 865-877, June 1967.

[6] Miller, C. K. S. , W. C. Daywitt, and E. Campbell, "A Waveguide Noise-Tube Mount for use as an Interlaboratory Standard," Acta IMEKO III, pp. 371-381, 1964.

[7] Liedquist, L. , "Absolutkalibrering av bruskallow inom Mikrovagsomradet" ("Absolute Calibration of Noise Sources in the Microwave Field"), Elteknik, Vol. 9, Pt. 3, pp. 43-47, March

1966.

[8] Per-Olaf Lundbom, private communication.

18
[9] Halford, G. J. , "Noise Comparators and Standards for S and X Bands," IEEE Trans, on Instrumentation and Measurement, Vol. IM-15, pp. 310-317, December 1966.

[10] G. J. Halford, private communication.

[11] Somolo, P. I., and D. L. Hollway, "The Australian National Standards Laboratory X-Band Ra- diometer for the Calibration of Noise Sources," IEEE Trans, on Microwave Theory and Techniques, Vol. MTT-16, No. 9, pp. 664-669, September 1968.

[12] F. J. Lehany, private communication.

19
APPENDIX. NOISE TEMPERATURE DERIVATION

The noise temperatures T^ of the WR62 and WR90 thermal standards are calculated from the

equation

Tc = + AT

S fH (A-1)

where T^ is the temperature of the resistive element and AT is the correction added to T to acin m count for losses in the waveguide lead. This correction is

''T = - \ni - «„) + / t; (1 - a^)dx

0

where the quantities appearing here are defined in the following diagram.

The "waveguide lead" starts a little in front of the resistive element at x = 0 and extends to

X = £ at the output waveguide flange. The lead is assumed to be uniform and reflectionless, and

the point x = 0 is chosen far enough away from the resistor to be certain higher order modes are absent in the waveguide, but close enough that the waveguide between x = 0 and the resistor is at

approximately the resistor temperature T^^. is the reflection coefficient of the resistor; T^

is the reflection coefficient looking from point x in the line towards the resistor; and = T^_q

is the reflection coefficient of the standard. T„ is the measured temperature of the line at

point X and T^ is its derivative with respect to x. is the available power ratio of the line

from point x to x = £.

where

_

(1 - a =

"" 1 - ir^|2

£

-Jx^ydy/lO

L^ = 10 XiCy is the attenuation per unit length of the line in dB per inch, and y is a dummy variable,

is found from a first-order perturbation (in skin depth) calculation [4] and is proportional to

20
(-1 ,' vMK'-h.ilf power of the waveguide resistivity, varies with x because the resistivity is a

fiinotion of the transmission line temperature.

An approximate expression from AT is used in actual calculations and is given by

4T ;

(T„ - T„)(l - a„) + / T^a - »^)dx (A-2)

The error incurred from this approximation is negligible, and is given by

(To - T„)0-o - %) * / T>, - o,,)dx

0.5 |r,i 2 (a)(T^ - T„)

where A is the average total attenuation of the line in dB and is the average line temperature.

The parametric error is T^ is found from the equation

6T^ = 6T„ + 6AT

s m (A-3)

+ T 6L + 6 ; T^dl

0 m mo 1 X X

0

where 6T is the uncertainty in T , 6L is the uncertainty in the total line attenuation L , and

the last term in eq (A-3) arises from uncertainties in actual values of T^ and L^. Equations (A-1), (A~2), and (A-3), along with the data on T^ and are fed into a computer

for reduction and calculation. The T^ data amounts to from fifteen to twenty carefully measured

values of the temperature on the inside surface of the waveguide in one- inch increments from the

tip or front face of the termination to the output flange, is then obtained from a leastsquares-fit curve for the one-half power of tabulated resistivity data over the temperature of

interest. The proportionality constant is calculated from the expression given by Wells [4],

21
NBS-114A IREV. 2-RC) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See Instructions)

1. PUBLICATION OR

REPORT NO. NBSIR 84-3005

2. Performing Organ. Report No. 3. PubI ication Date

May 1984

4. TITLE AND SUBTITLE

The NBS WR62 and WR90 Reference Noise Standards

5. AUTHOR(S)

C.K.S. Hiller and W. C. Daywitt

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction s) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered WASHINGTON, D.C. 20234 Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

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

bi bliography or literature survey, mention it here)

The basis for the National Bureau of Standards (NBS) WR90 and WR62

Waveguide Reference Noise Standards and the corresponding error analyses

are described. The standards are heated (1270 K) thermal noise generators,

and a derivation of their output noise temperature equations is also presented. Results of comparisons of the NBS WR90 standard with those of

Sweden, England, Australia, and Japan are included.

The text is extracted from course notes presented at NBS in 1970,

and hence does not include descriptions of standards constructed at NBS

since that time.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) error analysis; international comparisons; microwave; noise standard

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

Unlimited

1 1 For Official Distribution. Do Not Release to NTIS 26 nn Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

15. Price 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $8.50

USCOMM-DC 600-P80

U.S. GOVERNMENT PRINTING OFFICE;1984—779-709 / 9370
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