National Bureau of Standards

Library, N.W. Bldg

NOV 7 1962

^ecUnlcai rlote 160

EMISSION STABILIZATION OF

THERMIONIC DIODE NOISE SOURCES

M. W. RANDALL AND M. G. ARTHUR

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

technical ^lote

160

SEPTEMBER 1962

EMISSION STABILIZATION OF

THERMIONIC DIODE NOISE SOURCES

M. W. Randall and M. G. Arthur

NBS Boulder Laboratories

Boulder, Colorado

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.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 15 cents

CONTENTS

Page

ABSTRACT IV

1

.

Introduction 1

2. Method 1

3 . Equipment 2

4. Results 4

5. Conclusions 5

6. References 5

7. Appendix 8

LIST OF FIGURES

1. Functional diagram of stabilizer 2

2. Schematic diagram of stabilizer circuit 3

3. Photograph of stabilizer chassis 6

4. Error voltage recording, I = 10. ma 7

5. Error voltage recording, I = 60.4 ma 7

III
ABSTRACT

An apparatus is described which is capable of stabilizing the

d-c plate current of a temperature-limited thermionic diode noise

source to better than 0. 02 per cent, which corresponds to a noise

power stability of better than 0. 001 db throughout the current range

of 1 ma to 100 ma.

IV
EMISSION STABILIZATION OF THERMIONIC DIODE NOISE SOURCES

by

M. W. Randall and M. G. Arthur

1. Introduction

The theoretical mean square noise current per unit bandwidth

generated by a temperature -limited thermionic diode is related to the

d-c plate current by the relation [van der Ziel, 19541

~2~

= 2 el (1)

where

2

i is the mean square noise current per

unit bandwidth,

e is the charge of an electron

I is the d-c plate current.

When such a diode is to be used as a stable source of noise power,

variations in the d-c plate current must be held to less than a prescribed value. This paper describes an apparatus which is capable

of stabilizing the d-c plate current to better than 0. 02 per cent for all

values of plate current in the operating range of a typical noise diode.

2. Method

A block diagram of the apparatus used to stabilize the d-c

plate current is shown in Figure 1. Plate current, I, passes through

a resistor, R, to develop a voltage E = IR. A reference voltage, E ,

is connected in opposition to E such that an error voltage, E - E, is

produced. The error voltage is amplified by a high-gain d-c amplifier

whose output controls a current regulator in the diode filament circuit.

- 1 -
-2-

This servo loop is made degenerative, and the time variations in the

d-c plate current are thereby reduced to acceptable magnitudes.

^

DC

AMPLIFIER

CURRENT

REGULATOR

*|l|F r

Functional Diagram of Stabilizer

Figure 1

3. Equipment

Figure 2 shows a diagram of the circuit used in the stabilizer.

The resistor, R, is made up of three multi-turn precision rheostats

which permit coarse, medium, and fine adjustment of plate current.

A 1.34 volt mercury cell provides the reference voltage. The d-c

amplifier is a stable, wide-band unit which has a voltage gain of 2000.

Since the gain control in the amplifier is a step control, an auxiliary

gain control, R , has been incorporated to permit continuous adjustment of the open loop gain.

The current regulator is a three^ stage transistor amplifier

with the final- stage power transistors connected in series with the

diode filament. This regulator has a manually-operated current

control, R , which, with the servo loop open, allows the filament

current to be adjusted so that the diode plate current can be set to

some value in the range from less than 1 ma to greater than 100 ma.

When the servo loop is closed, the plate current is controlled by the

error voltage and the loop gain. R then provides a means of adjusting loop conditions so that the error voltage may be set to zero. To
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operate the diode at some plate current value, the multi-turn rheostats

are adjusted to bring the d-c plate current to the desired value, and

R is adjusted simultaneously to hold the error voltage close to zero.

Three meter- relays have been incorporated into the circuit to

prevent damage to the diode or to the current regulator in the event

of component or power failure. In conjunction with conventional relays

not shown in Figure 2, the plate and filament current meter- relays

remove supply voltages and open the servo loop when either of these

currents reaches a preset maximum. A third meter-relay, with

both high and low limit contacts, prevents damage to the current

regulator by any excessive voltage from the d-c amplifier.

Figure 3 is a photograph of the main chassis of the stabilizer.

The meters, power supplies, and other ancillary apparatus are

mounted on separate chassis.

4. Results

As stated in the Appendix, the design objective of this equipment is to stabilize the d-c plate current of a temperature-limited

diode noise generator to such an extent that the available noise power

does not vary more than 0. 001 db from a prescribed value during the

period of operation. If resistor R and reference cell E are assumed

o stable, and equation (1) is assumed true, it can be shown that, for

E = 1. 34 volts, a change of 0. 31 mv in error voltage represents a

0. 001 db change in available noise power.

In operation, the servo action maintains the error voltage within the . 31 mv limit, as shown in Figures 4 and 5. Critical components,

such as resistor R and reference cell E have been allowed to reach

o thermal equilibrium. Figure 4 shows a recording of the error voltage

over a period of 9 hours with a d-c plate current of 10. ma. Figure

5 shows a similar recording over a period of 13 hours with a d-c
-5-

plate current of 60. 4 ma. In both cases, variations in the error

voltage were much less than 0. 31 mv for the entire period, the

variations being about one order of magnitude smaller for short

periods of time.

5. Conclusions

The stabilizer described in this note has been used to stabilize

the d-c plate current of noise diodes used in the precision measurement

of noise figure of amplifiers and receivers, It has been used with a variety of diodes requiring a maximum filament current of 1. 8 amperes

at 1. 6 volts to 2. 6 amperes at 3. 4 volts and plate currents of 1 to 100

milliamperes at voltages of 100 to 300 volts.

Although the apparatus described here has functioned satisfactorily in its intended use, refinements leading to circuit simplifications

and improved operational convenience are being developed.

6. References

1. van der Ziel, A., Noise, Prentice -Hall, Inc., 1954, page 91.
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7. Appendix

The design objective of this equipment is to stabilize the d-c

plate current of a temperature-limited diode noise generator to such

an extent that the available noise power does not vary more than

0. 001 db from a prescribed value during the period of operation.

The theoretical available noise power per unit bandwidth of a

diode noise generator is

, /„~

P = 1/4 i R t a L

= 1/2 e IR_ (2)

where

P is the available noise power

a per unit bandwidth

R is the real part of the conjugatematch load impedance.

The other symbols are defined under (1), page 2.

A variation of 0. 001 db in available noise power is expressed

by the relation

P P

0. 001 db = 10 log

a

(3) a where A P is the change in available noise power. From (2) it

SL follows that

0. 001 db = 10 log

I +AI

(4)

where AI is the change in d-c plate current which produces a 0. 001 db

change in available noise power. Evaluating AI gives

AI = 0. 0002303 1. (5)
- 9 -

If resistor R and the reference cell of Figure 1 have fixed values,

AI will produce a change in error voltage given by

AE = 0. 0002303 E (6)

where AE is the change in error voltage corresponding to a 0. 001 db

change in available noise power.

When resistor R is adjusted to produce zero error voltage

with the desired value of d-c plate current, the voltage E equals the

value

E = E = 1. 34 volts (7) o

'

and AE has the value

AE = 0. 31 millivolts. (8)

Therefore, under the operating conditions specified above, a change

in error voltage of less than 0. 31 millivolts can be interpreted as

indicating a change in available noise power of less than 0. 001 db.

* U.S. GOVERNMENT PRINTING OFFICE : 1962 0—659228

U. 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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