Optical-capacitive coupling is used for isolation of control voltages, such as the high-voltage level of a klystron control electrode that is not referenced to ground, to serve as error voltages referenced to system ground so that the magnitude and sense of correction may be transferred

Kolbly, R. B.

English

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October 1969	 Brief 69-10569 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Automatic Frequency Control of Voltage-Controlled Oscillators 
.
C
Fig. 1. Simplified Schematic of a	 To Power 
Circuit for an S-Band Reflex Klystron	
Supply 
The problem: 
Development of a method of use of an error voltage, 
referenced to ground, for simple and inexpensive 
control of the voltage on an electrode that is far off 
ground potential. 
The solution: 
Use of optical-capacitive coupling for isolation of 
control voltages, such as the high-voltage level of a
klystron's control electrode that is not referenced to 
ground, to serve as error voltages that are referenced 
to system ground so that the magnitude and sense of 
the correction voltage may be transferred thereto. 
How it's done: 
The electrode voltage is controlled by a photo-
resistor voltage divider whose value approximates 
the potential of the control electrode. The photo-
(continued overleall 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
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https://ntrs.nasa.gov/search.jsp?R=19690000532 2020-03-16T18:08:07+00:00Z
Oscillator Reflex 
Klystron — _ Output 
AFC Sample 
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;- -
Q3	 CD 
33 
CD	 CD 
-4
Mixer i er
I Harmonic 
FM Input	 I Generator 
Klystron
 
Power	 Reference I	 I Supply 	 Oscillator 
Fig. 2. Connection of a Circuit (Fig. 1) to a System
. 
resistor's resistance is controlled by a lamp isolated 
from it electrically. Increase in the current through 
the lamp decreases the resistance of the photoresistor 
and so changes the voltage of the control electrode. 
Because the lamp has a relatively long time constant, 
a coupling capacitor is used to transfer the fast com-
ponents of the error signal. 
Figure 1 shows a circuit built to stabilize the fre-
quency of an S-band reflex klystron. The error input 
was derived from a discriminator that compared the 
klystron frequency with a harmonic of a stable crystal 
oscillator. The error signal is applied to a stabilizing 
network (lead lag). It is then applied to a summing 
junction of an integrated-circuit operational amplifier 
(such as type-709). Transistors Ql and Q2 form an 
emitter-coupled differential pair for inversion of phase 
without shifting of the base operating point of Q3; 
Q3 is an emitter-follower driving the lamp L  and the 
coupling capacitor C 1. Meter M indicates lamp current 
and acts as an error indicator. 
Figure 2 shows how this automatic-frequency-con-
trol circuit is connected to a system. Available photo-
resistor-lamp combinations have various character-
istics of both lamp and photoresistor. In this particular 
circuit a type-CK 1104 was used as an optical coupler.
This circuit can be used for control of frequency of 
laboratory oscillators and signal-generators; and it 
would be useful in many system applications such as 
microwave carrier systems, and for stabilizing the 
pump frequencies of parametric amplifiers. 
Notes: 
I. Designers of microwave equipment may be inter-
ested. 
2. Requests for further information maybe directed to:

Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP69- 10569 
Patent status: 
This invention is owned by NASA, and a patent

application has been filed. Royalty-free, nonexclusive

licenses for its commercial use will be granted by

NASA. Inquiries concerning license rights should be

made to NASA, Code GP, Washington, D.C. 20546.

Source: R. B. KoIbly of

Caltech/JPL
under contract to

NASA Pasadena Office

(N PO- 1 1064)
. 
[1 
Brief 69-10569	 Category 01


Automatic frequency control of voltage-controlled oscillators

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