Test system for wave guide arc detector circuits

A system for testing a wave guide arc detecting circuit for a microwave system is disclosed. Testing is accomplished by utilizing a pulsed laser source to generate pulses of optical energy which are used to simulate an electrical arc across the wave guide. The laser source is then positioned such that the pulsed output signal impinges upon the wave guide arc detector circuit to be tested. This is most advantageously accomplished by affixing the wave guide arc detector circuit to the wave guide in a fashion similar to normal operation. This enables the arc detecting circuit to view the end of the wave guide through the interior of the wave guide. The laser source is positioned such that the output pulse of the laser source impinges on the open end of the wave guide. The operational status of the wave guide arc detecting circuit is determined by measuring the elapsed time between the generation of the output pulse of the laser and the detection of the pulse by the arc detecting circuit being tested.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to test systems and more specifically, to test 
systems for testing wave-guide arc detection circuits. 
2. Summary of the Prior Art 
Testing wave guide arc detector circuits is difficult due to the complexity 
of generating an arc across a wave guide using pulses of microwave energy 
under controlled conditions. Among the complexities associated with this 
method are uncertainties associated with accurately determining the 
leading edge of the arc and controlling the energy level of the arc. These 
difficulties have resulted in all prior art efforts to test such method 
being relatively inconsistent. 
SUMMARY OF THE INVENTION 
The disclosed invention provides apparatus and a convenient method for 
testing wave guide arc detector circuits utilizing pulsed light source as 
a simulated arc. A pulse generator circuit is utilized to provide an 
electrical pulse which is coupled as a trigger to a laser diode light 
source to generate a pulse of optical energy. The leading edge of the 
pulse of optical energy is detected by measuring the current through the 
laser diode source and assuming that the pulse of optical energy is 
substantially coincident with the leading edge of the current through the 
laser diode. The laser diode output pulse is coupled to a section of wave 
guide such that it impinges on an open end of the wave guide. The arc 
detector circuit to be tested is coupled to view the output pulse of the 
laser diode through a small opening in the outer wall of the wave guide. 
If the wave guide arc detection circuit is operable a signal indicating 
the detection of the output pulse of the laser is generated by the wave 
guide arc detector circuit. An interval timer determines the time interval 
between the leading edge of the output pulse of the laser and the output 
signal of the detector to determine if the operation of the wave guide arc 
detection circuit is acceptable.

DETAILED DESCRIPTION 
FIG. 1 is a block diagram illustrating the use of optical wave guide arc 
detector circuits to protect a radar system. A conventional radar system 
10 is coupled to an antenna 12 through a wave guide 14. The wave guide 14 
includes a coupling section 16 which may be conveniently a 90.degree. 
coupling section permitting the optical wave guide arc detector circuit 18 
to view the interior of the wave guide 14 in a direction toward the radar 
system 10. This capability is conveniently provided by having a small 
opening 20 in the exterior wall portion of the coupling section 16 which 
is approximately 45.degree. with respect to the line of sight toward the 
radar transmitter 10. This is a convenient arrangement because most arcs 
in the wave guide (between adjacent interior walls) either develop near 
the trasmitter 10 or very rapidly migrate to this point. Additionally, the 
arcs are most critical in radar systems utilizing traveling wave tubes 
because the arc can destroy the coupling window isolating the interior of 
the traveling wave tube from the wave guide. In the system illustrated in 
FIG. 1 it is assumed that if the system does utilize a traveling wave tube 
that the optical arc detector 18 will have a clear view of the coupling 
window through the wave guide 14. Arcing across the wave guide 14 between 
the transmitter 10 and the opening 20 will be detected by the optical arc 
detection 18 causing this circuit to generate a disable signal which turns 
off the radar system 10 thereby extinguishing the arc. 
FIG. 2 illustrates the preferred embodiment of the invention. To initiate a 
test of the optical arc detector circuit 18, a start test signal is 
coupled to a pulse generator 30. In response to this signal, the pulse 
generator 30 generates a single trigger pulse which is coupled to a laser 
32 to generate a pulse of optical energy preferably, in the general power 
range of 3 watts. This pulse of optical energy is used to simulate an arc 
in the wave guide. (The laser 32 includes a diode laser and the associated 
drive circuitry, as described in more detail later.) 
The coupling section of wave guide 16 is affixed to the laser 32 such that 
the pulse of optical energy from the laser 32 impinges on the open end of 
the coupling section of wave guide 16. The optical arc detector circuit 18 
has a substantially unobstructed view of the output pulse of the laser 32 
through the coupling section 16. When the optical arc detector circuit 18 
detects the output pulse of the laser 32, an electrical output disable 
signal is generated. The laser 32 also includes a circuit which detects 
the current flowing through the laser diode to produce an electrical 
signal substantially coincident with the optical output pulse of the laser 
32. The output (disable signal) of the optical arc detector circuit 18 and 
the diode current signal from the laser 32 are coupled to an interval 
counter 34 to determine the delay between the simulated arc and its 
detection. Proper operation of the arc detection circuit 18 is indicated 
by this delay being within acceptable limits. Each trigger pulse results 
in a complete test of the optical arc detector circuit 18. 
In the experimental model, the laser 32 was designed to generate a light 
pulse having a selectable power level between 1 and 10 watts with a 
duration not greater than 0.30 microseconds in response to each trigger 
pulse. In practice, the delay between the optical output pulse of the 
laser 32 and the current to the laser diode forming a part of the laser 32 
is in the order of 1 nanosecond with the wave length of the laser diode 
being in the order of 900 micrometers. In the current state of the art, it 
is desired that the optical arc detector 18 detect wave guide arcs within 
a few microseconds of their origin. This being the case, the delay between 
the optical output of the laser 32 and the current to the laser diode can 
be ignored and the assumption made that for test purposes the current 
through the laser diode is coincident with the optical output pulse. 
For a better understanding of the wave guide, a cross section of the 
coupling section 16 along lines III--III is illustrated in FIG. 3. 
Typically the cross section of the wave guide 14 is uniform along its 
entire length. As can be seen, the cross section of the wave-guide is 
rectangular with the inner wall forming a rectangular interior portion. 
Arcs normally develop between the interior of the two longer walls 36 and 
38. 
Typically, the wave guide is a highly conductive metal with polished inner 
surfaces with adjoining sections of the wave guide bolted together with 
flanges. This is further illustrated in FIG. 4 by the end view of the 
coupling section 16. As can be seen, the coupling section 16 includes a 
flange portion 42 whose outer edges extend beyond the outer walls 44 of 
the wave guide section. Included in the surface of the flange is a small 
groove 46 for a suitable gasket so that when adjoining sections can be 
suitably coupled together using bolts as fasteners through holes 48, 50, 
52 and 54, such that the interior of the wave guide is protected from 
environmental contamination. Additionally, the flange 42 may include a 
choke groove (not illustrated). In normal operation a flange of the type 
illustrated in FIG. 4 forms the end portion of coupling section 16 and for 
test purposes is bolted to a complementally flat mating surface which is 
affixed to laser 32. The optical arc detector circuit 18 is secured to the 
coupling section 16 in its normal position and is provided with a suitable 
power source to permit it to operate normally. Testing of the arc 
detection circuit 18 is then initiated, as described above. 
Typically, the sensor utilized in the arc detector circuit 18 is a 
photodiode having a spectral response ranging from 350 to 1150 nanometers. 
This spectral response includes the wavelength of the output of the laser 
32. Any arc detector circuit having a sensitivity and spectral response 
compatible with the laser 32 can be tested. 
In the experimental model, the laser 32 was comprised of a commercial laser 
diode pulser type No. IL30C300P8 manufactured by Power Technology, Inc. 
driving a laser diode type No. SC2007A manufactured by RCA. The pulse 
generator circuit 30 may be any type of circuit generating a digital 
output with a duration not being critical in that the commercial laser 
diode pulse driver includes circuitry to limit the output pulse of the 
laser diode to approximately 0.3 microseconds. Interval counter 34 can 
conveniently be any of a number of commercial devices capable of measuring 
the time interval between two electrical signals, i.e. the laser diode 
current signal and the disable signal. 
Currently, laser diode sources are commercially available for a generating 
peak power outputs in the range of 1 to 300 watts with the laser described 
above being adjustable between one and ten watts. The higher power laser 
sources are suitable for use in the disclosed system if the higher output 
is required or desired. Additionally, should the requirements require the 
light (optical) pulses have a longer duration, it is visualized that Xenon 
flash tubes might be used instead of the laser sources.