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G8MNY > TECH 01.09.21 09:11l 137 Lines 6596 Bytes #999 (0) @ WW
BID : 45485_GB7CIP
Read: GUEST
Subj: Cable Tester
Path: IW8PGT<IZ3LSV<IR1UAW<IW2OHX<UA6ADV<I0OJJ<GB7CIP
Sent: 210901/0806Z @:GB7CIP.#32.GBR.EURO #:45485 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
From Brian G3YRH @GB7CAM.#12.GBR.EU (Additions Jun 06)
Co-ax cable tester
==================
This tester detects steady short or intermittent open circuits. Momentary open
circuits or steady short circuits trigger a buzzer.
Schematic : Detection of intermittent discontinuities or steady shorts.
---------------O-------/\/\/\----O----<<---E
| | R2 270 | (COAX UNDER TEST)
--------- | | -<<--- (SHORT FAR END)
| | | | | F
------ < R1 ----- | |
| BZ | > 680 --- 9 V | |
------ < ----- | |
| | --- | |
---------- | | |
C | | | |
O-------- | | |
| | | |
----- | | |
\ / | | |
\ / 2N1777 | | |
----- | | |
| \ | HIGH SIDE | |
| \___________|_________________|___|_________
O |nc |nc
\ B | | SEE NOTE 1
D \____________|_____________________|_________
O------O
A S1 LOW SIDE
NOTE 1: Alternative: Single wire under test or jumper to test internal
continuity of tester.
BZ = Buzzer
S1 = Switch
2N1777 = Silicon Controlled Rectifier
nc = No Connection
RESISTANCES ARE IN OHMS.
To test coaxial cable for intermittent discontinuities or steady shorts, place
cable at points E & F, then short far end of coax.
Details
A simple, portable, lightweight testing circuit sounds a long duration alarm
when it detects a steady short circuit or a momentary open circuit in a coaxial
cable or other two conductor transmission line. The tester is sensitive to
discontinuities that last 10uS or longer. Previously, there was no simple,
portable instrument to detect momentary shorts or discontinuities. Such
conventional instruments as ohmmeters & lamp or buzzer type continuity checkers
give visible or audible indications of steady open or closed circuits only.
To detect an intermittent open circuit in a coaxial cable, the far end of the
cable is shorted by a jumper, & the tester is connected as shown in the upper
portion of the figure, with the switch in position B. If the cable is in good
condition, the high side terminal remains ungrounded, the silicon controlled
rectifier remains off, & the buzzer does not sound. If an opening occurs in the
centre conductor or shield of the coaxial cable, the current from the high side
terminal to the low side terminal that would be otherwise shunted by the cable
flows into & triggers the silicon controlled rectifier & thereby turns on the
buzzer. Even if the coaxial cable starts to conducts again, the buzzer remains
on until the silicon controlled rectifier is reset by turning the switch to
position A. An intermittent discontinuity in a single wire (or an internal
discontinuity in the tester) can be detected in this manner if no coaxial cable
is connected & the wire or jumper is connected as shown in dashed lines.
(See Note 1).
To detect a steady short circuit, the jumper (At note 1) is removed. If the
buzzer does not sound, there is a steady short circuit between the inner &
outer conductors. If the buzzer does sound, then the cable is either good or
open.
The tester is used extensively for detecting intermittent open shorts in
accelerometer & extensometer cables. The tester can also be used as an
ordinary buzzer type continuity checker to detect steady or open circuits.
For this purpose, the switch is set a position A & the probe leads are
connected as shown by dashed lines in the lower portion of the figure (points
C & D). In this case the silicon controlled rectifier is not part of the
circuit, & the buzzer remains on only so long as the circuit under test#
provides a conducting path.
This work was done by Bobby L. Anderson of Rockwell International Corp. for
Marshall Space Flight Centre. No further documentation is available.
NASA Tech Briefs January 1990.
rgds Brian g3yrh@gb7cam
-----------------------------------------------------------------------------
By G8MNY
One method I have seen & used for RF/AF cables, was to use an AF tone null
system..
ÚÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄ¿
³Low LevelÃÄÄo----------------oÄÄ´Deep 1kHz WayenÃÄÄ´ AFÃÄ¿/³Audible
³Pure 1kHz³ Cable under test ³ Bridge Notch ³ ³AMP³ ÀÙ\³Crackles
³e.g. 10mV³ being flexed ÀÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÙ ÀÄÂÄÙ & Pings
ÀÄÄÄÄÄÄÄÄÄÙ Null Phase Gain
This system can find these annoying intermittent faults before they are
actually a hard fault somewhere in the cable. Faulting using a low level AC
rather than DC, does not cause the intermittent fault to weld up the cable
wires, so it does not hide the fault on testing.
HOW IT WORKS
The OSC applies a low level AF signal to the cable (10mV?). The far end of the
cable is attached to the AF deep notch filter that can be adjusted with null
depth & phase controls to completely null out the original signal.
SENT RX FROM CABLE TO LS AFTER NOTCH
Level³ ³ ³ ³ Tone + low level AM ³ No
³ ³ ³ ³ crackle sidebands ³ Tone
³ ³ -40dB´ /³\ & harmonics -40dB´ /||\ Just fault noises
ÀÄÄÄÁÄÄÄÄÄ>f ÀÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄ>f ÀÄÄÁÁÄÄÄÁÄÄÄÁÄÄÄÁÄ>f
1kHz 1kHz Harmonics 1kHz
Any discontinuity in the cable signal will change the level slightly & that
will produce low level AM sidebands & even harmonics. The AF amp boosts these
so they can be clearly used while flexing the cable to locate the faulty part
(end) of the cable. Replacement/repairs can then be done (shorten cable) if
appropriate. [Try this out on lap screened Phono AF leads!! :-) ]
Although this was a dedicated tester, it can be made up out of testgear you may
have. e.g. AF Osc, AF Distortion meter, AF amp & LS.
See my TECH buls on "Coax Faulting" "Coax Feeder Tests" & "A Versatile Pulse
Tester".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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