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G8MNY > TECH 25.10.06 11:50l 133 Lines 6468 Bytes #999 (0) @ WW
BID : 52593_GB7CIP
Read: DL1LCA DJ9UI GUEST DL8FBH
Subj: Coax Tester
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 061025/0029Z @:GB7CIP.#32.GBR.EU #:52593 [Caterham] $:52593_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
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.
E
---------------O-------/\/\/\----O----<<---
| | R2 270 | (COAX UNDER TEST)
--------- | | -<<--- (SHORT FAR END)
| | | | |
------ < R1 ----- | | F
| 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 and 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 and 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, and 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, and 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 and triggers the silicon controlled rectifier and 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 and 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 and
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 and 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 and the probe leads are
connected as shown by dashed lines in the lower portion of the figure (points
C and D). In this case the silicon controlled rectifier is not part of the
circuit, and 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´ AFÃÄÂÄ¿/³Audible
³Pure 1KHz³ Cable under test ³ Bridge Notch ³ ³AMP³ ÀÄÙ\³Crackles
³eg. 10mV ³ 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. 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. eg. AF Osc, AF Distortion meter, & an AF amp & LS.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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