/3.*/l ^ecltnical ^lote 160 EMISSION STABILIZATION OF THERMIONIC DIODE NOISE SOURCES [k ■ ThY. ■ ' UN1VERSJTY PARK,. , , /ANiA M. W. RANDALL AND M. G. ARTHUR U. S. DEPARTMENT OF COMMERCE NATIONAL BUREAU OF STANDARDS THE NATIONAL BUREAU OF STANDARDS Functions and Activities The functions of the National Bureau of Standards are set forth in the Act of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950. 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A complete listing of the Bureau's publications can be found in National Bureau of Standards Circular 460, Publications of the National Bureau of Standards, 1901 to June 1947 ($1.25), and the Supplement to Na- tional Bureau of Standards Circular 460, July 1947 to June 1957 ($1.50), and Miscellaneous Publication 240, July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) ($2.25); avail- able from the Superintendent of Documents, Government Printing Office, Washington 25, D. C. NATIONAL BUREAU OF STANDARDS technical ^iote 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 Bu- reau'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 Digitized by the Internet Archive in 2012 with funding from LYRASIS Members and Sloan Foundation http://www.archive.org/details/emissionstabilizOOrand CONTENTS Page IV 1 1 2 4 5 5 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 ABSTRACT 1. Introduction 2. Method 3. Equipment 4. Results 5. Conclusions 6. References 7. Appendix 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, 1954] = 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 pre- scribed 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 o 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 +lili= H 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 adjust- ment 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 adjust- ing loop conditions so that the error voltage may be set to zero. To UJ Q g - 5 - ^S> co II- +' ' I UJ > C3 Ul _J < I- Q.P < CL _l J30 0- CO> • VSc\ $V^ ■\\(V— ♦AAAr-f- O <-, m 4 n-O otj O o o o o cvj cos oQ Wi- lt UJ R2 CO Z3 fH O (VI or CO o CO tr CO UJ 5 M _j UJ _l 00 UJ £ a: CO >- UJ Q Q < cc UJ _l o t- < UJ £ > U- X H Oh v > < or o UJ X u en CM a> -4- 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 equip- ment 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 o 0. 001 db change in available noise power. In operation, the servo action maintains the error voltage with- in 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 satisfac- torily 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 - 6 - CO CO a U N ■s -M cn m O A a, o o Oh M DO - 7 - 'SEE DISCUSSION IN SECTION 4, RESULTS - 8 - 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 2 R 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 conjugate- match 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 - a (3) a where A P is the change in available noise power. From (2) it follows that 0. 001 db = 10 log + (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 v ' 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-S59228 U. S. DEPARTMENT OF COMMERCE Luther H. Hodges, Secretary NATIONAL BUREAU OF STANDARDS A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work. In general, each section carries cut specialized research, development, and engineering in the field indicated by its title. A brief description of the activities, and of the resultant publications, appears on the inside of the front cover. WASHINGTON, D. C. Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. 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