MIL-STD-202H.pdf - 第269页

MI L - S TD - 202 - 308 M E TH O D 308 CURRE N T - N O I SE T ES T F O R F I X ED R ESI ST O R S 1. SCOPE 1. 1 P urpose . T his resi s t or no i s e t e s t m et h od i s p er f or m ed f o r t he p ur po s e o f e s t a…

MIL-STD-202-308

CONTENTS

PARAGRAPH PAGE

FOREWORD…………………………………………………………. ii

1. SCOPE 1

1.1 Purpose………………………………………….……..…………. 1

1.2 Precautions……………………………….…….……..…………. 1

1.3 Checks for conflicts………………..……………………………. 1

2. APPLICABLE DOCUMENTS 1

3. DEFINTIONS 2

4. GENERAL REQUIREMENTS 2

4.1 Apparatus……………….…….……..…………….….........……. 2

4.1.1 Test system............................................................................ 2

4.1.1.1 DC measurement considerations…….………………..………. 2

4.1.1.2 AC measurement considerations….…………………..………. 2

4.1.1.3 Calibration technique…………………..…………………….…. 2

4.1.2 Synopsis…….….…….………….………….………...…………. 2

4.2. Procedure…………….………………………..….….………….. 3

4.2.1 Operating conditions………………………..……..……………. 3

4.2.2 Measurements…….……………………………….…………….. 3

4.2.2.1 Calibration….……………………………………….…………….. 3

4.2.2.2 System noise (S)…………………………………..…………….. 5

4.2.2.3 Total noise (T)………....…………………………..…………….. 5

4.2.3 Determination of the "microvolts-per-volt-in-a-decade" index.. 6

4.3 Errors…………………………………………………………….. 6

5. DETAILED REQUIREMENTS 8

5.1 Summary…………………………………………..…..…………. 8

5.2. Examination and measurements…………………………....…. 8

5.2.1 Marking resistance to solvents…….……………..……………. 8

5.2.2 Component protective coating, encapsulation material

and sleeve material resistance….……………..………………. 8

6. NOTES 8

6.1 Supersession data………………………………………………. 8

FIGURES PAGE

1. Block diagram of system …..……………………….……………………… 3

TABLES PAGE

1. Standard operating conditions …..………………….……………….…… 4

2. Correction factor for presence of "system noise".………………….…… 7

iii

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MIL-STD-202-308

METHOD 308

CURRENT-NOISE TEST FOR FIXED RESISTORS

1. SCOPE

1.1 Purpose. This resistor noise test method is performed for the purpose of establishing the "noisiness" or "noise

quality" of a resistor in order to determine its suitability for use in electronic circuits having critical noise requirements.

This method is intended as a standard reference for the determination of current noise present in a resistor, for use in

an application with specific current-noise requirements. It is not intended as a general specification requirement.

Interference caused by the generation of spurious noise signals in parts tends to mask the desired output signal, thus

resulting in loss of information. For low-level audio frequency and other low-frequency circuits, where low-noise parts

are used, resistors may become an important source of interfering noise. One source of noise in a resistor is

molecular thermal motion which generates a fluctuation voltage termed "thermal noise". It is not necessary to

determine the magnitude of thermal noise by measurement since the mean-square value of the fluctuation voltage is

predictable from Nyquist's equation, which shows the mean-square value to be proportional to the product of

resistance, temperature, and the pass band of the measuring system. Generally, an increase in fluctuation voltage

appears when direct current (dc) is passed through resistive circuit elements. The increase in fluctuation voltage is

termed "excess noise" or "current noise". The magnitude of current noise is dependent upon many inherent

properties of the resistor such as resistive material and other factors such as processing, fabrication, size and shape

of resistive element, etc. Since there is no apparent functional relationship between current noise and many of these

factors, current noise generally cannot be predicted from physical constants. Therefore, it is necessary to measure

current noise to determine its magnitude. The method employed in this test has been designed to evaluate

accurately the "noisiness" or "noise quality" of individual resistors in terms of a noise-quality index. The noise-quality

index, expressed in decibels (dB), is a measure of the ratio of the root-mean-square (rms) value of current-noise

voltage, in microvolts (µv), to the applied dc voltage, in volts. The pass band associated with the noise-quality index

is one frequency decade, geometrically centered at 1,000 hertz (Hz). This index is termed the "microvolts-per-volt-in-

a-decade" index. In the design of circuits, an added advantage accrues from the definitiveness of the index which

allows the estimation of interference attributable to current noise. Conversely, for a given limit of current-noise

interference in a particular circuit design, a maximum acceptable value of the index may be established. Ordinarily, it

is not necessary to duplicate the operating conditions of the particular circuit design when measuring the current

noise. The noise quality of populations of resistors may be reasonably estimated by measurement of the index of

representative groups of resistors using suitable sampling procedures. Measurements on sample groups tend to

have a normal distribution and once representative parameter values for the distribution have been established (the

mean and standard deviation), such parameter values would serve as norms in judging "noisiness" and product

uniformity insofar as noise is concerned.

1.2 Precautions. Adherence to the ambient temperature specified in 4.2.1 is emphasized as an important

consideration of this method. It is also necessary, in making noise measurements, using the apparatus of this

method, to delay reading the noise meter for a period of time no less than four times the effective time constant of the

detector to allow the meter sufficient time to reach at least 98 percent of the representative average value. The

effective time constant of the apparatus is normally adjusted to a value close to 1 second and therefore, a minimum

time delay of 4 seconds is normally required for the noise meter to indicate a valid average. Immediately after this 4

second delay, the meter should be read even though it continues to fluctuate as the noise signal varies. Normally,

the operator in making a visual reading of the fluctuating meter pointer, should estimate an average for a short

duration, in the order of 1/2 to 1 second.

1.3 Checks for conflicts. When this test is referenced, care should be exercised to assure that conflicting

requirements, as far as the properties of the specified finishes and markings are concerned, are not invoked.

2. APPLICABLE DOCUMENTS

This section not applicable to this standard.

北测（上海）电子科技有限公司

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MIL-STD-202-308

3. DEFINTIONS

This section not applicable to this standard.

4. GENERAL REQUIREMENTS

4.1. Apparatus. Noise measurements should be made on Quan-Tech Laboratories, Inc., Model 315 Resistor-Noise Test

Set, or equal, built in conformance with specifications recommended by the National Bureau of Standards (NBS) and

detailed in a report entitled "A Recommended Standard Resistor-Noise Test System," by G.T. Conrad, Jr., N. Newman, and

A.P. Stansbury published in the IRE Transactions of the Professional Group on Component Parts, Volume CP-7, Number 3,

September 1960. The NBS-test system provides a means for establishing direct current through the resistor under test and

measuring the resulting dc voltage and noise voltage appearing at the terminals of the resistor. These two voltages are

indicated simultaneously on scales calibrated in db. Instrumentation is so arranged that the associated value of the

"microvolts-per-volt-in-a-decade" index may be readily determined in accordance with 4.2.3.

4.1.1 Test system. The test system shall be as shown in the simplified block diagram on figure 1. The dc portion

of the system consists of a variable dc power supply and a dc vacuum-tube voltmeter (VTVM). The alternating-

current (ac) portion of the system consists of a calibration signal source and an indicating amplifier. The

interconnecting leads, as well as the resistor under test, should be adequately shielded.

4.1.1.1 DC measurement considerations. The variable dc power supply furnishes dc loading power through an

isolation resistor to the resistor under test. The isolation resistor prevents noise, appearing at the terminals of the

resistor under test, from being severely attenuated by the very low, parallel impedance presented by the output

terminals of the dc power supply. The isolation resistor must be free of current noise. Quiet wirewound-type resistors

are suitable. One of four values for the isolation resistor, Rm, (1,000 ohms, 10,000 ohms, 100,000 ohms, or 1

megohm (mego)) is selected, depending on the resistance of the resistor under test, RT. The dc voltage appearing

across the resistor under test is indicated by the dc VTVM. The meter has two scales - one showing the dc voltage

across the resistor under test, V, and the other indicating the quantity D-20 log V, in dB. The scale simplifies

computation of the current-noise index. The choice of value of the dc voltage is not critical, however, to avoid

subjecting the resistor under test, and the isolation resistor as well, to excessive dc power dissipation or voltage, or

both, standard nominal values of dc voltage and values for the isolation resistor are given in table 1.

4.1.1.2 AC measurement considerations. Noise voltage appearing at the terminals of the resistor under test is

amplified and its rms magnitude is shown by the ac indicating amplifier. The indicating amplifier consists of a high-

gain, low-noise amplifier, a filter, an rms detector, and an output meter. The filter restricts the frequency response of

the amplifier to a flat-top, 1,000 Hz pass band, geometrically centered at 1,000 Hz. The output-meter scale, like that

of the dc VTVM, is calibrated in dB to simplify calculations.

4.1.1.3 Calibration technique. The calibration technique consists of first applying a predetermined value of 1,000

Hz, sine-wave signal across a 1 ohm resistor located in series with the resistor under test, and then adjusting the gain

of the amplifier, by means of a variable attenuator, until the output meter deflects to the "calibrate" line. This

procedure standardizes the gain of the system and calibrates the indicating amplifier. It should be noted that since

the calibration setting depends upon the impedance at 1,000 Hz of the resistor under test, resistors having the same

dc resistance may not calibrate alike. The resistance of the calibration resistor (1 ohm) is considered negligible

compared to that of any resistor under test (100 ohms to 22 mego); therefore, the effect of the calibration voltage

appearing at the terminals of a zero-impedance generator located in series with the resistor under test. The

magnitude of the calibration voltage is so chosen that the indicated output is equal to that which would be obtained if

the calibration voltage were a noise voltage having an rms value of 1,000 µv in a decade. Such a signal should

produce a reading of 60 dB when the system is properly calibrated; thus, 0 dB means 1 µv in a decade.

4.1.2 Synopsis. To summarize, this apparatus provides a measure of the rms value of the current-noise voltage

generated in the resistor under test and transmitted in a frequency decade. The calibration technique refers the

measured noise voltage to the terminals of an essentially zero-impedance noise-voltage generator located in series

with the resistor under test. The noise voltage so measured, when corrected for the presence of system noise, is the

"open circuit" current-noise voltage of the resistor under test. Since both the current-noise voltage and dc voltage are

expressed in dB, the value of the “microvolts-per-volt-in-a-decade" index is obtained by subtracting the dc reading

from the corrected noise reading. The corrected noise reading is discussed in 4.2.2.3.

北测（上海）电子科技有限公司

联系方式：xuyj@beice-sh.com 13917165676