Protective system prevents damage or destruction of high-power RF amplifiers by arcs which may occur in output transmission line. Advantages of system are listed

Chernoff, R. C.

English

NASA Technical Reports Server

1972	
1.	 B72-10099 
NASA TECH BRIEF 
NASA Pasadena Office	 I G 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Arc Protection System for High-Power RF Amplifiers 
The problem: 
To prevent damage or destruction of a high-power 
RF amplifier by arcs which may occur in the output 
transmission line. The available systems which use 
light or reflected RF wave energy to trigger protec-
tion circuits are too complex and do not afford reli-
able arc protection for the output windows of Kly-
strons, traveling-wave tubes, and the like. 
Output 
Transmission 
Fast RF
	
Line 
Switch 
High XCouPler 
Input 
Transmission
Power
 RF 
Line
Flip- 
Flop
Amplifier
Band- 
Reject I
I 
Filter
PulseA Wideband 
Amplifier / \	 I Receiver 
The solution: 
Trigger a protective system by the BF noise burst 
that accompanies an arc. 
How it's done: 
An RF arc anywhere in the output transmission 
line causes a burst of RF noise that propagates away 
from the arc in both directions. The noise burst is 
coupled out of the transmission line by a loose, non-
directive coupler through a band-reject filter. This 
filter is required to prevent the wideband receiver
and the pulse amplifier indicated in the figure from 
responding to normal signal modulation. The energy 
passed by the band-reject filter is detected by the 
wideba.nd receiver, and the detected envelope of the 
arc-generated RF burst is amplified by the pulse 
amplifier. The output of the pulse amplifier triggers 
a flip-flop which provides a bias signal that turns off 
a fast RF switch in the input line to the high-power 
RF amplifier. Because the fast-acting RF switch con-
trols the drive energy from the input transmission 
line to the power amplifier, the are in the output 
transmission line will be extinguished when the out-
put RF power level falls in response to drive cutoff. 
The response time of the system (from the start 
of an arc to its extinction due to drive cutoff) is the 
sum of the following delays: 1) propagation time of 
RF noise from arc location to coupler, 2) the response 
time of the wideband receiver, 3) the response time 
of the pulse amplifier and flip-flop, 4) the response 
time of the fast RF switch, 5) propagation time of 
an RF step through the tube, 6) propagation time 
of RF step from tube output to location of the arc. 
With careful design, the sum of these delays (ex-
clusive of the propagation times of the output trans-
mission line) can be held to less than 150 nanoseconds. 
The maximum propagation times for a transmission 
line cannot be predetermined precisely, for they are 
established by the length of the particular trans-
mission line. 
Specific advantages of the system over prior de-
signs based on detection of light from the arc or the 
reflection of RF waves are: 
1. The entire transmission line can be monitored 
from a single noise burst detection port (the loose, 
nondirective coupler) regardless of line length or 
(continued overleaf) 
NEW
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned.rights. 
https://ntrs.nasa.gov/search.jsp?R=19720000099 2020-03-17T03:25:04+00:00Z
2.
3.
4.
shape. In contrast, systems using photodetection	 Note: 
can only react when an arc occurs within line of 	 Requests for further information may be di-
sight. Systems using reflected RF waves cannot	 rected to: 
detect arcs unless they occur further from the 	 Technology Utilization Officer 
source (amplifier tube) than the directional cou- 	 NASA Pasadena Office 
pier; an arc at or near the tube window will not	 4800 Oak Grove Drive 
be detected.	 Pasadena, California 91103 
The frequency spectrum of the RF noise is very 	 Reference: B72-10099 
wide. Therefore, signal loss in the band-reject 
filter is small.	 Patent status: 
The risetirne of the wideband RF receiver is much 	 No patent action is contemplated by NASA. 
shorter than that of any type of photodetector in 	 Source: Ralph C. Chernoff of 
general use.	 Caltech/JPL 
The normal level of out-of-band noise in the trans- 	 under contract to 
mission line is very much lower than that which 	 NASA Pasadena Office

would be produced by an arc. Sensitivity adjust-
ment and maintenance of the described system 	 (NPO-11560)
are, therefore, not as critical as with systems using 
reflected RF waves for arc detection.
Category 02


Arc protection system for high-power RF amplifiers

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