High impedance alternating current sensing transformer device between two bolometers for measuring insertion loss of test componen

Finnie, C. J.

Schuster, D. M.

English

NASA Technical Reports Server

REPLY TO 
ATTN OF: 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON. D.C. 20546 
FROM : 
October 15, 1970 
~~~/~cientific & Technical Information Division 
Attention: Miss Winnie M. Morgan 
~~/0ffice of Assistant General 
Counsel for Patent Matters 
SUBJECT: Announcement of NASA-Owned 
U.S. Patents in STAR 
In accordance with the procedures contained in the Code GP 
to Code US1 memorandum on this subject, dated June 8, 1970, 
the attached NASA-owned U.S. patent is being forwarded for 
abstracting and announcement in NASA STAR. 
The following information is provided: 
U. S. Patent No. 
Corporate Source 
Supplementary 
Corporate Source 
: Calif, Institute of Technology 
Jet Propulsion Laboratory 
NASA Patent Case No.: XNP-01193 
Please note that this patent covers an invention made by an 
employee of a NASA contractor. Pursuant to Section 305(a) of 
the National Aeronautics and Space Act, the name of the 
Administrator of NASA appears on the first page of the patent; 
however, the name of the actual inventor (author) appears at 
the heading of Column No. 1 of the the 
words ". . . with respect to an 
Gayle Parker 
Enclosure: N7 
Copy of Patent 
(NASA CR OR TMX O R  AD NUMBf R) 
Y 
https://ntrs.nasa.gov/search.jsp?R=19710006582 2020-03-17T02:42:59+00:00Z
966 JAMES E. WEBB 3,277,366 
A B M l N I S T R A T O R  OF T H E  NAPLONAL 
AERONAUTICS AND SPACE ADl\4ll 'dlSTRAIIBN 
J~SCRT!ON LOSS MEASURING APPARhTU5 liAVING TRANSFORMER 
MEANS CONNECTEO ACRO~S A PAlK OF BOLOMETERS 
Fzled May 6, 1964 
AMPLIF IER 
- 3 6  
- /  
B O L O M E T E R  O8 fi BOLOMETER 
INVENTORS, 
~ - ~ / c : i . . o / s o  J F,JNN/E 
DAN VEP- SCH US TEE 
B Y  
3,27&,366 United States Patent Ofice Patented Oct. 4, 1966 
3,277,366 
INSERTION LOSS MEASURING APPARATUS HAV- 
ING TRANSFORMER M E A N S CONNECTED 
ACROSS A PAIR OF BOLOMETERS 
James E. Webb, Administrator of the National Aerouau- 
tics and Space Administration, with respect to an inven- 
tion of Clifford J. Fiunie and Danver Schuster 
Filed May 6, 1964, Ser. No. 366,226 
4 Claims. (CI. 324-57) 
This invention relates generally to testing apparatus 
and more particularly to apparatus for measuring the in- 
sertion loss of a component or network. 
In a communication transmission system, the magnitude 
of the loss caused by the insertion into the system of a 
new component or network must often be known. This 
loss is determined by several factors, such as the degree 
of impedance match at  the input terminals, the internal 
transmission loss in the colnponent to be inserted, and 
the degree of impedance match at the output terminals. 
The "insertion loss" at a given frequency is actually the 
ratio of the powers at  that frequency delivered to that 
part of a system beyond the point of component insertion 
before and after the insertion. 
A common way of measuring the insertion loss of a 
component is to send a radio frequency signal modulated 
by an audio frequency signal through the component, plac- 
ing radio frequency bolometers both before and after the 
component to be tested, amplifying the audio frequency 
output of each of the bolometers and conlparing the two 
amplified signals in a null detector. A potentiometer 
placed in series with one of the audio amplifiers can be 
varied until a null is achieved and by noting the attenua- 
tion required to achieve a null with and without the conl- 
ponent in the circuit, the power loss due to the insertion 
of that component can be determined. 
The principal difficulty encountered with prior art ap- 
paratus of the foregoing type is that audio amplifiers near- 
ly always have a certain amount of drift or gain variation 
at different points in time. Thus, if the insertion loss of 
the component is very small so that the audio frequency 
signals derived from the bolometers are almost equal, then 
the difference in the bolometer output signals is also quite 
small and any drift in either one of the audio amplifiers 
can effectively mask the difference between the bolometer 
output signals. Even when very expensive and highly ac- 
curate amplifiers having small drifts are utilized, the drift 
is nevertheless a limiting factor in the accuracy of the test 
apparatus. 
In view of the foregoing, it is an object of the present 
invention to provide an improved insertion loss measuring 
apparatus which is more accurate than known prior art 
apparatus. 
It is a more particular object of the present invention 
to provide an improved insertion loss measuring appara- 
tus whose accuracy is not limited by audio amplifier char- 
acteristics. Moreover, it is an object of the present inven- 
tion to provide an insertion loss measuring apparatus 
which is extremely accurate, reliable, small in size and 
weight and can be inexpensively asembled from commer- 
cially available 1o.t~ cost parts. 
Briefly, in accordance with the invention, a high im- 
pedance alternating current sensing device is connected be- 
tween the output of a pair of bolometers respectively re- 
sponsive to the signal power inserted into and derived 
from the component under test. The sensing device can 
comprise a transformer primary winding coupled to a 
secondary winding which in turn can be connected through 
an amplifier to a sensitivity meter. Prior to connecting 
the bolometers to the input and output terminals of the 
component, the inputs to the bolometers are connected 
2 
togeiher and an accurate voltage divider connected be- 
tween one bolometer output and the current sensing de- 
vice is adjusted to establish a null current condition in the 
sensing device. The bolometer inputs are then respec- 
tively connected to the input and output terminals of the 
component and the tap on the voltage divider is adjusted 
to again establish a current null in the sensing device. 
The change in position of the voltage divider tap in- 
dicates the insertion loss of the component. 
The novel features that are considered characteristic of 
this invention are set forth with particularity in the ap- 
pended claims. The invention itself, both as to its or- 
ganization and method of operation, as well as additional 
objects and advantages thereof, will best be understood 
from the following description when read in connection 
with the accompanying drawing which comprises a sche- 
matic diagram of a preferred embodinlent of the inven- 
tion. 
Attention is now called to the figue which illustrates 
a component 10 whose insertion loss is to be measured. 
In order to measure the insertion loss, or in other words 
the power loss at a particular frequency, a generator 12 
capable of providing a signal at that frequency is pro- 
vided. The output of the generator 12 is connected to 
the input of a mixer 14. The output of an audio fre- 
quency generator 16 is also connected to the input of the 
mixer 14 for the purpose of modulating the radio fre- 
quency signal provided by the generator 12. The output 
of the mixer 14 is connected to a directional coupler 17 
which is connected to the input of a conventional radio 
frequency bolometer 1s. 
The output of the bolometer 18 is connected to a first 
terminal of a voltage divider 20, preferably comprising a 
ratio transformer, whose second terminal is connected to 
a reference potential, e.g. ground. The ratio transformer 
20 is provided with a movable tap 22 connected to one 
terminal of a current sensing device comprising a high 
impedance transformer primary winding 24. 
A second directional coupler 26 is connected to the 
input of a radio frequency bolometer 28 and to a resistive 
load termination device 31. The output of the bolometer 
28 is connected through a phase shifting device 30 whose 
output in turn is coupled to the second terminal of the 
transformer primary winding 24. The phase shifting 
device 31 can comprise a variable capacitor. The first 
terminal of the ratio transformer 20 is connected through 
a variable resistor 32 to ground. The resistor 32 is pro- 
vided with a movable tap 34 which is connected to an 
electrostatic shield 36 surrounding the transformer pri- 
mary winding 24. A resistor 38 connects the second 
terminal of the transformer primary winding 24 to ground. 
The transformer primary winding 24 is coupled to a 
secondary winding 40 whose terminals are connected 
to the input of an amplifier 42. The output of the ampli- 
fier 42 is connected to the input of a sensitivity meter 
44. 
In the use of the apparatus illustrated, the generator 
12 should provide a high frequency output signal, for 
example in the microwave range. The generator 16 
should provide an audio frequericy signal, e.g. one kilo- 
cycle which modulates the microwave frequency signal 
in the mixer 14. The bolometer 18 yields at its output 
terminal a one kilocycle signal whose amplitude is pro- 
portional to the signal stlength or power of the micro- 
wave signal applied to the input thereof. Conventional 
bolonleters are well known in the art and operate essen- 
tially by converting the signal power into heat which in 
turn controls the resistance of an internal element and 
thus the voltage available at its output terminal. 
Prior to actually measuring the insertion loss of com- 
ponent 10, the apparatus is balanced by initially connect- 
3,s 
3 
ing tlre directional coaplerc. 17 and 26 together without 
the component 10 being in the circuit. The tap 22 of the 
ratto transformer 20 is then adjusted to establish a null 
current condition in the transformer coll 24, as indi- 
cated on the meter 44 and the position of the tap 22 
is noted. 
Subsequently, the directional couplers 17 and 26 are 
respectively connected to the input and output terminals 
of the component 10. The tap 8 is then readjusted to 
again obtain a current null in the winding 24. The 
change in position of the ratio transformer tap 22 in- 
dicates the insertion loss of the con~ponent 10. The ratio 
transformer of course can be calibrated with components 
having known insertion losses at certain frequencies to 
thereby enable the insertion loss of other components 
to be determined. 
Inasmuch as a small phase shift may be introduced 
by the component 10, the phase shifting network 30 is 
connected between the output of the bolometer 28 and 
the transformer winding 24 to thereby permit the effect 
of the introduced phase shift to  be compensated for. In  
use, once a null condition is nearly established, by ad- 
justment of the ratio transformer tap 22, the phase shift 
network 30 is adjusted to more precisely develop a null 
condition. 
In order to prevent stray currents from being induced 
in the transformer primary winding 24, the electrostatic 
shield 36 is provided. The shield can be maintained at  
approximately the same potential as the winding 24 by 
proper adjustment of the tap 34 of the variable resistor 
32. The resistor 38 connected between ground and one 
side of the transformer winding 34 balances the in?- 
pedance on the opposite sides of the winding 34. 
From the foregoing, it should be appreciated that an 
apparatus for determining the insertion loss of a com- 
ponent has been disclosed herein in which no amplifica- 
tion of the signals detected at the input and output termi- 
nals of the components under test is introduced. In- 
stead, a sensitive sensing device including a transformer 
primary winding is connected between two bolometer 
outputs so that any differential voltage develops a current 
in the winding. Thus, the system accuracy is not at all 
dependent upon amplifier characteristics which are often 
somewhat unstable. Instability of the amplifier 42 used 
in the present apparatus does not affect the accuracy of 
the invention inasmuch as it is introduced only after the 
signal reoresentatins! the difference between the bolometer 
r77 ,3BB 
q 
ance with the present invention can be considerably more 
accurate than heretofore known apparatus. 
What is claimed is: 
I. Apparatus for determining the insertion loss of a 
5 component at a particular frequency. said apparatus com- 
prising an oscillator for providing an output signal at 
said particular frequency; an oscillator for providing an 
audio frequency output signal; means for modulating said 
particular frequency signal with said audio frequency 
10 signal and for providing a modulated output signal; means 
for applying said modulated output signal to the input 
terminal of said component; means connecting the out- 
put terminal of said component to a load circuit; a first 
bolometer connected to said component input terminal 
15 and responsive to the signal power delivered thereto for 
developing a proportional output voltage a t  a first bo- 
lometer output terminal; a second bolometer connected 
to said component output terminal and responsive to the 
signal power delivered thereat for developing a propor- 
20 tional output voltage at a second bolometer output ter- 
minal; a ratio transformer connected directly between 
said first bolometer output terminal and a source of ref- 
erence potential, said ratio transformer having a vari- 
able voltage tap; and passive circuit means connecting a 
25 current sensing device between said variable voltage tap 
and said second bolometer output terminal. 
2. The combination of claim 1 wherein said current 
sensing device comprises a high impedance transformer 
primary winding; a transformer secondary winding cou- 
30 pled to said transformer primary winding; and indicator 
means connected to said transformer secondary winding 
and responsive to currents induced therein. 
3. The combination of claim 2 wherein said passive 
circuit means includes a variable phase shifting circuit 
35 connected in series with said current sensing device. 
4. The apparatus of claim 2 including an electrostatic 
shield enclosing said transformer primary winding and 
including means for applying a variable voltage thereto. 
40 References Cited by the Examiner 
UNITED STATES PATENTS 
1/1948 Korman ---------- 324-58 X 
5/1952 Dickie ---------- 324---I06 X 
2/1962 Turner ----------- 324-95 X 
4/1962 Calvert ------------- 324-57 
FOREIGN PATENTS 
outputs is developed. Thus, it should be appreciated that 607,656 10/1960 Canada. 
the accuracy of apparatus constructed in accordance with 
the invention is dependent only upon the accuracy of the OTHER REFERENCES 
ratio transformer 20, assuming the characteristics of the Engineering Bulletin No. 4 (Ratio Trans.), Gertsch 
two bolometers to be identical. Inasmuch as highly ac- Products, Inc., May 1956. 
curate ratio transformers are readily available com- 
n~ercially and since differences in bolometer character- 55 WALTER L. CARLSON, Prin'nry Exami'zer. 
istics are negligible, an apparatus constnicted in accord- E. E. KUBASIEWICZ, Assistarzt Examiner. 


Insertion loss measuring apparatus having transformer means connected across a pair of bolometers Patent

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