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G8MNY > TECH 20.07.05 17:46l 126 Lines 6465 Bytes #999 (0) @ WW
BID : 55026_GB7CIP
Read: GUEST OE7FMI
Subj: A Versatile Pulse Tester 2/3
Path: DB0FHN<DB0FOR<DB0MRW<DB0RGB<OK0PPL<DB0RES<ON0AR<ZL2BAU<GB7YKS<GB7YFS<
GB7CIP
Sent: 050720/0903Z @:GB7CIP.#32.GBR.EU #:55026 [Caterham] $:55026_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Jul 04)
TESTING & ADJUSTMENT.
The project takes only 10mA when working correctly. So with a current limited
supply, check the osc is running with a scope, then the divider IC. The BFX84
should have high voltage pulses on it to give an adjustable avalanche DC HT
from 25-100V. With the trigger drive trimmer set to minimum connect the
oscilloscope probe to the test coax port (not directly on the avalanche
transistor's emitter).
Adjust the HT (40-80V) to make the narrow pulses start up, the scope should be
set for 5V pulses and a fast or maximum timebase frequency. If there are no
pulses, check the HT is present at the transistor and coax capacitor. If it is
still not firing up then change the transistor for another fast switching NPN
one. Adjusting the voltage higher should increase the free run repetition rate.
Now turn down the HT until the pulses just stop (30-40V), turn up the clock
drive trigger pulse trimmer, the pulses should reappear, but at 250KHz (4uS)
period. Adjust the HT voltage and drive trimmer for best pulse reliability.
If the spectrum analyser/scanner shows any "in-between" frequencies [2] (narrow
analyser filter needed) or a properly locked scope pulse display has other
pulses faintly present then there is some false triggering, or the oscillator
is being affected by the HT DC-DC converter etc.
The C4 100pF can be made up of a trimmer & fixed C for accurately setting the
marker frequency. Align the crystal trimmer so that the RF marker frequency
zero beat with a know RF source or measure the pulse frequency on a good
counter.
CABLE FAULT LOCATION.
Using the pulse source faults can be seen on the monitor scope as positive
pulse reflection for high impedance fault (eg Breaks) and negative pulse
reflection for a low impedance fault (eg Shorts). To minimise false echoes the
scope should either have a good 1:10 probe connected to the monitor port or be
connected with a terminated cable teed to the scope input.
How well the fault pulse echo can be seen and time measured will depend on your
scope pulse performance, small height display can be more accurate, but
generally a 20MHz scope can see down to about 2 metres, a 100MHz 20cms etc.
On a 20MHz scope On a 100MHz scope
Pulse /\ Echo of Open Pulse ³³
³ ³ /\ Circuit ³³ ³³
³ ³ ³ ³ Cable ³³ ³³
/' `\.___./' `\._ /'`\._____./'`\.__
Wide curvy pulses often The thinner pulses give
sharper if displayed smaller. greater time accuracy.
The location is the time difference between the initial pulse and the fault
pulse, multiplied by the cable velocity, times 2 (there and back). Cables have
velocity factors of between 0.66 of the speed of light (300M/uS) for solid
coax, and 0.78 for semi air spaced types. Open balanced line velocity factor
can be as high as 0.95.
Pulse³ Open ³Pulse Pulse³
³ ³Circuit ³ ³ Cable Z
³ ³Cable ³ ³ Mismatches
ÀÄÄÄÁÄÄÄÄÄÄÄÄÄÄ ÀÄÄÄÄÂÄÄÄÄ ÀÄÄÄÄÁÄÄÂÄÄ
Time = Distance ³Shorted Time = Distance
³Cable
If the fault is intermittent or not extreme (not O/C or S/C), or an identical
cable length is available, then a calibration of the scope can be done with the
far end open & shorted representing 100% cable length. Then the fault location
can me measured off as a % of that length, this can be more accurate than
unknown velocity factor and scope timebase accuracy.
Coiled up cable and odd drum lengths up to 3uS (500M) can be measured, but only
if the cable loss is not too great as the reflection pulse weakens and spreads
out. _
Pulse³ Cable
³ _ Loss eg 50% Height = 6dB there & back loss
³ ³ or 3dB over the length.
³___³__________
Open Circuit Cable
VARIABLE COAX TERMINATION.
This is needed to measure coax impedance. The requirement is for a Zero to Open
circuit variable load that is good to VHF, this is not that straight forward. I
used a small 470ê carbon tracked pot, large ones and wire wounds are too
inductive. I took it apart and modified the start of the track with Silver
conductive paint to give a good Zero Ohms & about 75ê half way around, I also
slashed across the far end track with a sharp knife several times to make the
high resistance end more resistive (about 2K). The pot is mounted in a tin box,
& wired up with short leads to a BNC socket, with the low resistance track end
connected to the BNC centre & rotating wiper to ground. Then it is a simple
matter to DC calibrate the knob with an Ohms scale.
End of ÄÄÄÄ¿
Test BNC POT<Ä¿
Coax ÄÄÄÄÁÄÄÄÙ
With this variable load it is easy to check the impedance of any coax cable by
either making the reflected pulse disappear on a scope trace for long cables.
On a SCOPE..
Pulse ³ Pulse ³ Pulse ³
³ Load ³ Load = ³ Load to
³ to low ³ cable Z ³ high
ÀÄÄÄÄÂÄÄÄ Time ÀÄÄÄÄÄÄÄÄÄ Time ÀÄÄÄÄÁÄÄÄ Time
On a SPECTRUM ANALYSER...
dBs³ _ Load too low dBs³ Load = Cable Z dBsÃÄ. Load to High
³ / \ /\ /\ / ÃÄÄÄÄÄÄÄÄÄ ³ `\ /\ /\ /
Ã/ ³³ ³³ ³³ ³ ³ ³³ ³³ ³³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Freq ÀÄÄÄÄÄÄÄÄÄ Freq ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Freq
For short cables where the close in pulses merge, then the uneven spectrum
ripple due to end mismatch echo on an spectrum analyser/scanner is then the
best way to see matching, as it flattening out the frequency ripple when the
termination is correct.
Cable impedance is then the value of the termination, it is nearly always
resistive unless you have a lapped screen audio cable! With a cable made of
mixed impedance coaxes no null is possible. This will stop you using bits of
non 75ê cables for video etc!
Balanced line, Xtal markers, & use with a Sprctrum Analyser in part 3
Why don't U send an interesting bul?
73 de G8MNY @ GB7CIP
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