Black, S. H.

English

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The invention disclosed in this document resulted from 
research in aeronautical and space activities performed under 
programs of the National Aeronautics and Space Administration. 
The invention is owned by NASA and is therefore available for 
licensing in accordance with the NASA Patent Licensing Regu- 
lation (14 Code of Federal  Regulations 1245.200). 
To encourage commercial utilization of NASA-owned inven- 
tions, it is NASA policy to grant nonexclusive, royalty-free, 
revocable licenses to any company or  individual desiring to use  
the invention while the patent application is pending in the U. S. 
Patent Office and within a specified period, presently two years, 
after issuance of the patent to NASA. If commercial use of the 
invention does not occur during this period, NASA may grant a 
limited exclusive, royalty-free license thereby adding an incen- 
tive to further encourage commercial development. Any company 
desiring to make, use, o r  sell  this invention is encouraged to 
obtain a royalty-free license from NASA. 
Address inquiries and all requests for licenses to Assistant 
General Counsel for Patent Matters, Code GP-1, National 
Aeronautics and Space Administration, Washington DC 2 0546. 
NASA-HQ 
https://ntrs.nasa.gov/search.jsp?R=19690014952 2020-03-16T18:23:45+00:00Z
Feb. 18, 1969 JAMES E. WEBB 3,4283 10 
ADMINISTRATOR OF THE NATIONAL AERONAUTICS 
A N D  SPACE ADMINISTRATION 
AUTOMATIC GAIN CONTROL SYSTEM 
Filed Dec. 23, 1965 
/# 
R. F. 
0U;rPUT 
CONTROL 
VOL TAG€ 
64 
i 
United States Patent Office patented Feb. 18, 1969 
1 2 
automatic gain control amplifier. Input radio-frequency 
(1R.F.) signals are fed into the attenuator. The attenaated 
R.F. signal is fed into a reflex amplifier together with a 
direct current (D.C.) control voltage from the output slage 
of a receiver system. The reflex amplifier amplifies the 
5 attenuated input R.F. signal and feeds it to the next 
stage of the receiver system. The D.C. control voltage is 
fed into an automatic gain control (A=) amplifier. The 
output of the AGC control amplifier is used to vary the 
amount of attenuation of the electrically operated con- 
tinuous amplifier. 
More particularly, according to a preferred embodiment 
of the invention, input R.F. signals are fed into the elec- 
trically operated continuous attenuator formed of three 
diodes, whose cathodes are joined together and connected 
through a resistor to ground. The anodes of the first 
and second diode are biased by means of a D.C. control 
voltage, which are coupled thereto through an inductor. 
The R.F. signal is coupled to the anode of the first diode 
and the attenuated R.F. signal is taken off the anode of 
the second diode and couuled through a couuling cauaci- 
3,428,910 
AUTOMATIC GAIN CONTROL SYST3?M 
James E. Webb, Administrator of the National Aero- 
nauties and Space Administration with respect to an in- 
vention of Stanley M. Black, Phoenix, Ariz. 
Fded Dee. 23, 1965, Ser. No. 516,154 
@'aims US. C1. 330-29 
h t .  CI. H03g 3/30; H03P 1/30 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; USe 2457). 
The invention relates in general to automatic gain 
control systems and, more partioularly, to a system for 
providing a relatively constant output signal level using 
an electrically controlled attenuator. 
In a receiver system which must be able to handle a wide 
range of input signal levels, a form of automatic gain 
control must be employed to prevent overload of the 
final output stage. Heretofore, a conventional type of 
automatic gain control system utilized a diode detector 
to develop a unidirectional control voltage whose ampli- 
tude was proportional to the receiver system output level. 
This unidirectional control voltage was fed back to one 
or more earlier stages of the receiver system so as to re- 
duce the gain of these earlier stages, thus causing the 
output level to remain substantially constant. When ex- 
tremely large input signals were present, however, dis- 
tortion in the $gain control stage of the automatic gain 
control system caused reduced handling capability. Utiliz- 
ing presently available transistors, the maximum undis- 
torted input signal was limited to no more than 10 to 50 
millivolts, B.M.3. 
In order to overcome the attendant disadvantages of 
prior art automatic gain cootrol systems and to provide an 
automatic gain control system, which is capable of han- 
dling input signals in the order of 1,WO millivolts R.M.§., 
the system of the present invention utilizes an electrically 
operated continuous attenuator, which is controlled by an 
tor to the base of a transGtor of thereflex &pl&er.-The 
ACX: control voltage i s  also connected to the base of 
the reflex amplifier transistor through an input L-C net- 
work. The AGC control voltage and the R.F. signal are 
amplified by the reflex amplifier and the R.F. output is 
taken off of an inductance in an output L-C network con- 
nected to the collector of the reflex amplifier transistor. 
Further, the AGC control voltage is fed back through the 
output lM= network to the base of an AGC control ampli- 
fier transistor. The collector of the control amplifier tran- 
sistor is connected to the anode of the third diode of 
the electrically operated attenuator. As the amplified D.C. 
control voltage is varied, the attenuation is varied thus 
causing the input R.F. si'gnal to the attenuator to be 
The advantage of this invention, both as to its con- 
struction and mode of operation, will be readily appre- 
ciated as the same become better understood by refer- 
ence to the following detailed description when considered 
40 in connection with the accompanying drawings in which 
like reference numerals designate like parts throughout 
the figures, and wherein: 
FIG. 1 is a block diagram of the automatic gain con- 
trol system of this invention; and 
FIG. 2 is a circuit diagram of a preferred embodiment 
Referring now to the drawings, there is shown in FIG. 1 
a block diagram of the automatic gain control system 
wherein 9R.F. input signals, which are to be controlled, 
50 are coupled to the input terminal of an electrically oper- 
ated continuous attenuator 12. The output of the attenu- 
ator is fed to the R.F. input terminal of a reflex ampli- 
fier 14. A unidirectional AGC control voltage, proportional 
to the amplitude of the signal present in the last amplifier 
55 of a receiver system in which the automatic gain control 
system is to be utilized, is fed into the D.C. input ter- 
minal of the reflex amplifier 14. The R.F. output signal 
is taken from the R.F. output terminal of the reflex 
amplifier. The amplified D.C. signal is coupled from the 
D.C. output terminal of the reflex amplifier to an AGC 
control amplifier 16. The amplified D.C. control voltage 
is fed from the control amplifier 16 back to the attenuator 
12. The system operates in such a manner as to keep R.F. 
output voltage of the last stage relaii.ve constant by caus- 
35 corrected. 
45 of the block diagram system of FIG. 1. 
3,428,910 
3 4 
ing more or less attenuation to be inserted by the elec- sistor 80 which results in current flow through the diode 
trically operated continuous attenuator 12. 32. As current flows through the diode 32, the voltage 
Referring now to FIG. 2, there is shown a preferred drop across resistor 34 increases to the point where it ex- 
embodiment of a circuit utilized in the automatic gain ceeds the bias voltage applied to terminal 35. This con- 
control system of FIG. 1. R.F. input signals, which are to dition results in effectively baok-biasing diodes 26 and B. 
be controlled, are applied to an input terminal 22 which Therefore, with a strong input &.F. si,gnal condition, the 
is coupled to one side of a coupling capacitor 24 of the diodes 26 and 28 are turned off and diode 32 turned on, 
electrically operated continuous attenuator 12. The elec- resulting in a maximum attenuation. As can be readily 
trically operated continuous attenuator is iormed of a first seen, the system operates in such a manner as to keep the 
diode 26, a second diode 2.8, and a third diode 32. The R.F. output voltage of the last stage of the receiver sys- 
other side of the capacitor 24 is coupled to the anode tem relative constant by causing more or less attenuation 
of diode 26. The cathodes of each of the diodes are cou- to be inserted by the electrically operated continuous at- 
pled through a resistor 34 to ground. The diodes 26 and tenuator 12. 
28 are biased by means of a positive D.C. control volt- Additional gain control variation is also provided by 
age connected to a terminal 35. The terminal 35 is cou- 15 the system because the R.F. gain of the reflex amplifier 14 
pled to the anodes of diodes 26 and 28 through a pair of varies with the D.C. control voltage applied to the ter- 
inductors 36, 318, respectively, which form R.F. chokes. minal 66. This additional gain control variation is ob- 
The attenuated B.F. signal is coupled from the anode tained because the voltage drop which results from the 
of diode 28 to one side of a coupling capacitor 42. The AGC voltage across resistor 76 causes a reduction of the 
other side of the capacitor 42 is connected to a transistor 2o available c 
44 of the reflex amplifier 14 at the base 46 of the tran- 
sistor 44. The transistor 44 further comvrises an emitter 
. 
48 and a collector 52. The emitter 48 is cbupled to ground 
through a first path comprising a biasing resistor 54 and 
a second path comprising an R.F. by-pass capacitor 54. 2! 
The collector 52 of the transistor 44 is connected to one 
side of an output L-C circuit formed of a capacitor 58 
and an inductor 62 connected in parallel. Output R.F. 
signals are taken off of a terminal 64, which is connected 
Input AGC control voltage to the reflex amplifier are 
coupled from a terminal 46 to the base 46 of transistor 
44 through an input L-C circuit formed by an inductor 68 
and a capacitor 72 connected in parallel. The terminal 
66 is further connected through an R.F. shunting capacitor 
74 to ground. 
The other side of the output L-C circuit is connected 
to a source of positive reference voltage through a D.C. 
output resistor 76. Further, a feed-throu'gh capacitor 78 
is connected to the lead at the other side of the output 
L-C circuit so as to prevent R.F. signals from passing to 
the resistor 76. The junction of the output L-C circuit 
and the resistor 76 is connected to a control transistor 
8.0 of the AGC control amplifier 16 at a base 82 of the 
transistor 180. The control transistor 80 further comprises 
an emitter 84 and a collector 86. The emitter 04 is con- 45 
nected through a biasing resistor 88 to a source of positive 
reference voltage and the collector 86 is connected to 
the anode of the third diode 32. 
With the foregoing in mind, operation of the cirouit of 
FIG. 2 is as follows: 
In the no-sigiial condition, the AGC control voltage 
applied to the terminal 66 is of such value as to allow 
only a small amount of current to flow in the collector 
circuit of transistor 44. The resulting voltage drop across 
the D.C. output resistor 76 is, therefore, not sufficient to 55 
turn on control transistor 80. Thus, the diode 32 is effec- 
tively an open circuit. In addition, a positive bias voltage 
is applied to the terminal 35, forward-biasin*g the diode 26 
and the diode 28. This causes a current flow through both 
diodes 26 and 28 through the resistor 34. Therefore, it can 60 
be seen that in the no-signal condition, diodes 26 and 2.8 
are turned on while diode 32 is effectively turned off, re- 
sulting in minimum attenuation. 
When an input R.F. signal is applied to the terminal 22 
and coupled to the attenuator 12 at the anode of diode 26, 65 
the attenuated 'R.F. signal is coupled from the anode of 
diode 28 to the base 46 of transistor 44 where it is ampli- 
fied. This amplified R.F. signal is fed to the output L-C 
circuit. The amplified R.F. signals are taken off the in- 
ductor 62 at the terminal 64, the terminal 64 being con- 70 
nected to the next stage of a receiver system. As stronger 
W.F. signals are passed through the system, the positive 
AGC voltage applied to terminal 56 increases the collector 
current through transistor 44. The resulting voltage drop 
across resistor 74 eventually succeeds in turning on tran- 75 
to the inductor 62. 31 
50 
an ourpur termmai, means coupring b a a  U.L. OUL~PUL 
terminal of said reflex amplifier to said input terminal 
of said AGC control amplifier, and means coupling 
said output terminal of said control amplifier to said 
attenuation control terminal of said attenuator net- 
work for varying the attenuation level of said attenu- 
ator network, said attenuator network comprising: 
a first diode, a second diode, and a third diode, 
biasing means coupled to the anode of said 
first diode and the anode of said second diode, 
means coupling said R.F. input signal to said 
anode of said first diode, means coupling the 
anode of said second diode to said R.F. input 
terminal of said reflex amplifier; and means cou- 
pling said anode of said third diode to said out- 
put terminal of said control amplifier. 
2. An automatic gain control system in accordance with 
claim 1 wherein said AGC control amplifier comprises: 
a transistor having a base, an emitter, and a collector, 
means coupling said D.C. output terminal of said re- 
flex amplifier to the base of said control amplifier 
transistor, means coupling the emitter of said con- 
trol amplifier transistor to a source of reference 
potential, and means coupling the collector of said 
control amplifier transistor to said anode of said 
third diode. 
3. An automatic gain control system in accordance 
a transistor having a base, emitter, and collector, means 
coupling the anode of said second diode to the base 
of said reflex amplifier transistor znd an 1R.F. output 
L-C circuit comprising an inductor and capacitor con- 
nected in parallel, means coupling the collector of 
with claim 2 wherein said reflex amplifier comprises: 
3,428,910 
5 6 
said reflex amplifier transistor to one side of said 3,111,631 11/1963 Bettinger _ _ _ _ _ _ _ _ _ _ _  330-133 
parallel L-C network, and 'means coupling said A.F. 3,304,505 2/1967 pricer _ _ _ _ _ _ _ _ _ _ _ _  330-29 x 
output terminal to said indsuctor of said L-C net- 
work. ROY LAKE, Prfmary Examiner. References Cited 
UNITED STATES PATENTS JAMES B. MULLINS, Assistant Examiner. 
2,774,866 12/1956 Burger _ _ _ _ _ _ _ _ _ _ _ _  330-29 X U.S. C1. X.R. 
2,971,164 2/1961 Saari _ _ _ _ _ _ _ _ _ _ _ _ _ _  3 3 6 1 4 5  330-145 
5- a 
AWARDS ABSTRACT NASA C a s e  
AUTOMATIC GAIN CONTROL SYSTEM 
The i n s t a n t  invent ion  is d i r e c t e d  t o  an automatic g a i n  
con t ro l  system having an e l e c t r i c a l l y  operated continuous a t t e n u a t o r  
for  varying an R , F ,  i npu t  s i g n a l  i n  accordance with t h e  output of 
t h e  l a s t  stage of a r ece ive r  system. The system provides a simple 
means f o r  c o n t r o l l i n g  t h e  a t t enua t ion  of a diode network so t h a t  
t h e  R,F, s i g n a l  w i l l  be a t t enua ted  and a r e l a t i v e  cons tan t  
amplitude provided a t  t h e  output stage of t h e  r ece ive r  system. 
The system shown i n  block diagram format i n  F I G .  1 comprises 
an e l e c t r i c a l l y  operated continuous cont inua tor  1 2  i n t o  which R , F ,  
i npu t  s i g n a l s ,  which are t o  be con t ro l l ed ,  are coupled. The 
a t t enua ted  R,F, ou tput  s i g n a l  is f ed  i n t o  a reflex ampl i f i e r  and 
t h e  amplif ied R,F, s i g n a l  i s  coupled from t h e  r e f l e x  ampl i f i e r  t o  
t h e  next stage of t h e  R,F, system. Fur ther ,  a n  AGC con t ro l  vol tage 
f r o m  t h e  las t  s t a g e  of a rece iver  system i s  a l s o  f e d  i n t o  the 
r e f l e x  ampl i f i e r .  The amplif ied AGC vol tage  i s  coupled t o  t h e  
inpu t  of an AGC c o n t r o l  ampl i f i e r ,  and t h e  output  thereof  fed  t o  
t h e  a t t enua to r  1 2  t o  vary t h e  a t t enua t ion  the reo f .  
The novel ty  of the--i.rwention appears--to--be i n  providing a n  
AGC c o n t r o l  a m p l i f i e r ,  which v a r i e s  t h e  a t t e n u a t i o n  of an a t t e n u a t o r  
network so t h a t  t h e  inpu t  R,F. s i g n a l  may be reduced t o  the  e x t e n t  
necessary t o  keep t h e  output  s i g n a l  of t h e  system r e l a t i v e l y  
cons tan t  
Inventor : Stanley  H. Black 
Employer: Sperry Gyroscope Company 


Automatic gain control system

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