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G8MNY > TECH 11.04.04 11:03l 173 Lines 8906 Bytes #999 (0) @ WW
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Subj: A Versatile Pulse Tester 2/2
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Sent: 040410/2303Z @:GB7CIP.#32.GBR.EU #:52330 [Caterham] $:52330_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated April 04)
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 will flattening
out at termination matching point.
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 TESTING.
This needs a small pulse transformer in an add on box to isolate and match to a
balanced line. A turns ratio of 1:2 will drive the line with about 200ê
floating source. This is quite correct as most balanced lines are around 140ê
at HF, even if used as 600ê at AF. For the transformer I used a small 2 hole
ferrite core testing it with 1/2/3 turns to see which did not reduce the
overall height of the DC pulse (not saturating) & did not cause too much
negative pulse response (L too low), 3 turns was what I ended up with for the
primary & therefore 6 turns for the secondary in slightly thinner enamelled
copper wire. Avoid too many turns as this will reduce the magnetic coupling
increase winding capacitance & cause unwanted ringing etc. The same variable
termination as above can be used to find the Z of any balanced line.
_____ _______
BNC 3 ):( Balanced
Turns):( 6 Turns
______):(_______ Line
Small
2 hole core
You may be surprised just how good ordinary twisted wire is, once it is
properly matched.
CRYSTAL CALIBRATOR.
As the narrow pulse has very high harmonic content strong signals can be heard
into the GHz bands. The use of 250KHz as the pulse rate give 4 markers per MHz.
Do not put the pulse output into a radiating aerial, as it will cause wideband
QRM locally!
SPECTRUM ANALYSER CALIBRATION.
In theory the pulse spectrum is in the form of (sine x)/x [1] this gives an
overall cosign shaped envelope of the markers to the 1st null frequency Fn,
then much weaker repeating « sine wave shape envelopes to infinity.
If the terminated DC pulse height is measured with peak detector (fast diode &
cap, mine was 8V which is 1.28 Watts of peak pulse power!) the true power level
of any of the RF markers is then..
³ Fr
/³\ ³ ³ 2Fr V x T x Fr Sine(Fh/Fn)
³ ³ ³ 3Fr Harmonic Voltage = ---------- x -----------
Harmonic³ ³ ³ ³ 0.707 Fh/Fn
³ ³ ³ ³ 4Fr
Voltages³ ³ ³ ³ ³ Fh = Freq of a Harmonic
³ ³ ³ ³ ³ Fn = Freq Null = 1/T
³ ³ ³ ³ ³ Fr = Repetition Frequency
³ ³ ³ ³ ³ T = Pulse width
³ ³ ³ ³ ³ ³ V = Pulse height
³ ³ ³ ³ ³ ³ . | | .
³ ³ ³ ³ ³ ³ | ³ ³ ³ ³ | . | | .
³ ³ ³ ³ ³ ³ ³ Fh³ ³ ³ ³ ³ ³ ³ ³ . ³ ³ ³ ³ ³ ³ .
³ ³ ³ ³ ³ ³ ³ | ³ ³ ³ ³ ³ ³ ³ ³ | | ³ ³ ³ ³ ³ ³ ³ ³ | .
ÀÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÁÄÁ
Frequency -> Fn 2Fn 3Fn 4Fn 5Fn
A spreadsheet can be loaded with this formula and pulse data, then graphs can
be plotted of the ideal spectrum for comparison. This assumes perfect
terminations and ideal pulse shape etc. but a good starting point.
For UHF-SHF use, surface mount components "chip Rs and Transistor" are soldered
directly across threaded BNC sockets with no wires or tags, this does give
better results but with a lower pulse size of around 4V peak with a sub 0.3 nS
pulse length with 5cm of coax, with no visible spectrum Fn null up to 2GHz.
FILTER PLOTTING.
When this fairly broadband signal source is put via a VHF/UHF filter into a
spectrum analyser with a 300KHz wide IF, the shape of the filter can be seen
immediately and the filter performance easily adjusted. This is normally only
possible with a tracking generator or a high power noise source.
Although this noise source should not be put into a effective radiating aerial,
handheld ¬ wave & helical whips do immediately give their frequency band away
where they match the RF.
Be aware that this broadband pulse can easily overload wideband equipment
(often only 2 tone calibrated), so a band limiting filter should be used before
testing response of preamps etc.
References:- [1] Rx Calibrator and Tx Mon, by G4COL, Radcom June 1998.
[2] Cable Fault Locator, D.Huddart, Electronics World March 2001.
/QSL
73 de G8MNY @ GB7CIP
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