r-f filtering. The most troublesome problem in output noise ratio measurements has been to separate the mixer noise from the intermediatefrequency amplifier noise. Previous approaches to this problem have required either (1) a resistor to replace the diode (or a resistorcapacitor combination to replace the entire mixer), where the resistor conductance is the same as the mixer output conductance for each diode to be tested, or (2) a 1/8-wavelength coupling circuit to sufficiently reduce the dependency of amplifier noise upon mixer output conductance. Both approaches require good amplifier gain stability. To avoid these requirements, and to provide greatly increased measurement accuracy, the following procedure was originated, and successfully used, by the author at the Naval Applied Science Laboratory. The measurement requires a temperature-limited thermionic noise diode (such as JAN-5722) connected directly across (and thereby defining) the junction between the mixer and the amplifier, as previously described for the measurement of Fi (circuit layout is critical). For accuracy and convenience, a calibrated (and padded) fixed attenuator of about 3.01 dB (Lo= 2) that may be easily switched in or out between the preamplifier and postamplifier (main amplifier) should be used. A further refinement is the addition to the attenuator of a switch to control the anode voltage of the noise diode, isolated electrically from the attenuator but mechanically connected so as to apply the anode voltage simultaneously as the attenuation is inserted. For this arrangement, the operating procedure is as follows: 1. Replace the mixer diode with a resistor in a diode package or replace the entire mixer with a shielded resistor-capacitor combination, keeping the susceptance the same as with the normal configuration. 2. With the attenuator out of the circuit (and the noise diode off), adjust the amplifier gain to provide a convenient and precise second-detector (d-c) output reading (reference value). 3. Insert the attenuator (applying the noise-diode anode voltage) and adjust the noise-diode filament temperature to restore the seconddetector reference reading. 4. Note the average anode current, I, of the noise diode. 5. Check on system drift as follows: the circuit (turning off the noise diode). detector reference reading has not drifted. through 5. 6. Switch the attenuator out of Repeat steps 1 through 5 for a number of different simulated mixer conductance values covering the conductance range obtained for normal mixer operation using the diodes under test. 7. Plot either I or F1 [calculated from eq (E6)] as a function of G (or 1/G). i 8. Insert a diode to be tested in the holder (normal mixer operation) and repeat the measurement procedure (steps 2 through 5) used with the resistors. 9. From a measurement of the mixer output conductance with this diode, use the plot obtained in step 7 to obtain a corresponding value of I or F: i. Distinguish the average anode current of the noise diode obtained in step 8 from the current obtained from the plot for a corresponding conductance value by using the symbol Ia for the former and Ig for the latter. 6. Calculate output noise ratio, Ny, using the Io vs. G plot as g Note that the first equation (E7) holds when Lo is identical for all relevant insertions of the attenuator, as should be the case for a switchable fixed attenuator with good switch contacts. (E7) (E8) Fine control and stability of noise diode filament temperature is greatly facilitated by a filament supply which is servo-controlled by the anode current. NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE WASHINGTON, DC 20234 2.Govt. Accession No. 3. Recipient's Accession No. 12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP) National Bureau of Standards Department of Commerce 20234 Washington, DC 15. SUPPLEMENTARY NOTES Naval Electronics Systems Library of Congress Catalog Card Number: 79-600161 Document describes a computer program; SF-185, FIPS Software Summary, is attached. 5. Publication Date Performing Organization Code 8. Performing Organ. Report No. 10. Project/Task/Work Unit No. 11. Contract/Grant No. 13. Type of Report & Period Covered 1/1/70 to 3/31/74 16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) The measurement of mixer conversion loss using periodic or incremental modulation of the local oscillator, and the evaluation and minimization of the associated systematic and random uncertainties, are discussed in terms of an X-band mixer measurement system constructed at NBS. It is shown that the systematic uncertainty in the incremental modulation method of measuring conversion loss results largely from the uncertainties in the calibration of microwave attenuation and power. It is also shown that the "modulation" (periodic modulation) and "incremental" Several improvements in the periodic and incremental modulation techniques are introduced. Novel circuits for measuring intermediate-frequency output conductance and local-oscillator return loss are described which may also be useful for other immittance measurements. 7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Conversion loss; diode measurement; intermediate-frequency (i-f) output conductance; measurement uncertainties; microwave mixer diodes; modulation; point-contact diodes; reflectometer; return loss; Schottky-barrier diodes; semiconductor diodes; standing-wave ratio (SWR). 8. AVAILABILITY Unlimited For Official Distribution. Do Not Release to NTIS Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC Order From National Technical Information Service (NTIS), Springfield, NBS TECHNICAL PUBLICATIONS PERIODICALS OURNAL OF RESEARCH-The Journal of Research of the ational Bureau of Standards reports NBS research and developent in those disciplines of the physical and engineering sciences in nich the Bureau is active. These include physics, chemistry, gineering, mathematics, and computer sciences. 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