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G8MNY > TECH 26.03.24 09:10l 331 Lines 17660 Bytes #999 (0) @ WW
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Subj: A Versatile Pulse Tester
Path: IZ3LSV<I0OJJ<GB7CIP
Sent: 240326/0806Z @:GB7CIP.#32.GBR.EURO #:4096 [Caterham Surrey GBR] $:4096_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (BATC's CQTV No 195, Updated Oct 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This tester based on ideas in magazine articles [1 & 2] & has been developed to
have several useful functions.
Coax or Balanced Cable Fault Locator.
Coax or Balanced Cable Impedance tester.
Wideband Crystal Calibrator.
Spectrum Analyser Calibrator.
Filter Plotting (like a Tracking Generator).
THE CIRCUIT
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
+12V ³ HT 25-100V ³
Ä´>ÃÂÄÄÄÄÄ)ÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂ470RÂÄÄÄÄÂÄÄÄÂÄÄÄÄÄ¿ ³ _
³ ³ ):: ³ ³ ³C6 ³ ³ ³ ( )
+³ ³ hi )::1mH ³ _³_ === ³ ³ ³ ³ ³14cm of coax
C1 === ÚÄÄ´ PIV ):: ³ /_\' ³u1 ³ ³ 100K ³_³sets pulse width
100u³ ³ ÃÄÄ´<ÃÄ´ ³ ³10V ³ ³ ³ ¬W³ (.)ÄÄÄÄÄÄÄÄ¿
³ ³ 10K ³ 10K ÚÄÁÄÄÄÄÙ 10K +³16 ÀÄÄÄ´ ³
³C2³ ³ ³BFX84 ³ ³ ³ ÚÄÄÁÄ¿ : ³ UHF ³
³ === ³ \³ ³ ³ 1MHz ÃÄÄ´clk ³7 TRIM: T4³/ Avalanche³
³u1³ ³ T1 ÃÄ´<ô ³ ÚÄXTALÄ´10³ Q2ÃÄÄ´ÃÄÄÄÄÂÄ´ transistor³
³ ³ SET e/³ ³ ³ ³ ÃÄÄ1MÄÄ´ ³4040³ 2-30p: ³ ³\e ³ Test
³ ³ HT<Ä¿ ³ ³ ³ ³ ³ ÚÄÄ´ ³ ³ 250 : ³ ÀÄ22RÂÄÄÂÄ@ Cable
³ ³100K ³ ³ ³ === ³ ³ ³/ ³ ³ rstÿ kHz :100 ³ 62R³ BNC
³ ³ ³ _³_ ³ ³ C3³ ³ ÃÄ´ T3 ³ ³Q5 ³³11 : ³ 270R ³ ³
³ ³ 33K /_\'³ ³ 1n³ ³ C4³ ³\e ³ ÀÂÄÂÄÙ³ : ÀÄÄÄÄÄÄÄÄ)ÄÄÁÄ´
³ ³ ³ ³ ³ ³/ ³ ³ 1n³ ³ === ³5³8 ³ : all short³ ³ Monitor
³ ³ ³24VÀÄÄ(Ä´ T2 ³ ³ === ³ ³C5 ³ ³ ³ : leads ÃÄÄÄÄ@ Scope
³ ³ ³ ³ ³\e ³ ³ ³ ³ ³15p³ ³ ³ : 82R ³ BNC
ÄÄÄÄÁÄÄÁÄÄÁÄÄÄÄÄÄÁÄÄÄÁÄÄÄ)ÄÄÁÄÄÄÁÄÄÄÁÄÄÁÄÄÄ)ÄÁÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÙ
DC-DC HT CONVERTER ³ XTAL OSC ³ DIVIDER NEEDLE PULSE GENERATOR
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ64kHz
NEEDLE PULSE GENERATOR
The heart of the unit is a UHF NPN transistor T4 using its avalanche
characteristics, which is actually just an ordinary high speed low voltage
switching transistor run well over its voltage. A 100R base resistor keeps the
transistor off, but with high voltage applied to the collector the transistor
will suddenly conduct in avalanche mode. But with a transmission line
capacitance that gives a pulse quenching reflection from the length of
unterminated coax onto the collector & a high value charging resistor, the
transistor can be made to generate a stream of very narrow pulses on its own.
This is because when it suddenly conducts as lower voltage is sent up the coax
line gets reflected at the far end & produces an even lower voltage at the
collector which stops (quenches) the conduction. After a while the voltage will
have built up again & the whole cycle repeat.
Collector Transistor
Voltage Avalanche
_³ _| _ ___
Avalanche ³ .ú' ³ .ú' ³ Reflected
³ .ú' ³ .ú' ³ coax
_³_.ú' ³ _.ú' ³ __pulse
Quench ³ ³³\Coax ³³ height
³ ³³inverted ³³
³ ³³quenching ³³
³ ÀÙpulse ÀÙ
Emitter Ú¿ Ú¿
Voltage ÀÄÄÄÄÄÄÄÄÄÄÄÙÀÄÄÄÄÄÄÄÄÄÄÄÙÀÄÄÄÄ
These can be used as they are, for coax cable time domain reflection testing
with just a suitable T4 emitter network & a good oscilloscope. With a 14cm
length of 50 ohm coax as the capacitor & very short wiring to the output
socket, I found the pulse width was around 3nS wide [1].
A very narrow pulse has wide bandwidth & if repeated with precise timebase can
provide good RF markers up to the 1st null frequency determined by the pulse
width. A 3nS wide pulse gives a 1st order null at 330MHz. For very fast pulse
you need short coax line & a uWave transistor, & no leads just the surface
mount components around a socket. Then pulse widths of less than 0.5nS are
possible (2GHz 1st null).
CRYSTAL CLOCK
Using a 1MHz crystal oscillator & divide chain to obtain clocks of interest
(other crystals & divide options can be used) to trigger the avalanche
transistor T4, enables the output to be of more use than a free running
circuit. As the transistor can be made to free run as a pulse generator with
just high voltage, only low energy pulses are needed to start the avalanche
effect so a low power CMOS 4040 divider IC can be used as a driver. Trigger
sensitivity is a function of both the supply voltage & the size of the trigger
pulse.
With 250kHz pulse repetition frequency, cable lengths of up to 500m can be
pulse tested & with a wide 300kHz IF filter in a spectrum analyser a reasonable
smooth graphs could be drawn of VHF filters etc.
HIGH VOLTAGE
A stable voltage from a 30-100V is required for the avalanche effect this comes
from a voltage controlled DC-DC converter driven by a medium speed clock. A
64kHz clock output feeds narrow edge pulses through a 1nF to a 10k pull up
provide light bias. Then via a diode to stop problems with the negative going
pulse, on to the base of a BFX84 T1 this has a small choke of around 1mH as the
collector load. When T1 turns off high back emf from the choke goes through a
high PIV diode to a 0.1uF to store the positive HT volts. A 100k pot samples
some of this voltage, which is applied via a 24V zener to the base of T2 a NPN
transistor that shorts out the base drive of T1. The result is that the drive
pulse length is shortened giving simple but very efficient voltage control.
TESTING & ADJUSTMENT
The project takes only 10mA when working correctly. So with a current limited
supply, check the 1MHz 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 & a fast or maximum timebase frequency. If there are no
pulses, check the HT is present at the transistor & 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 re-appear, but at 250kHz (4uS)
period. Adjust the HT voltage & 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 (e.g. Breaks) & negative pulse
reflection for a low impedance fault (e.g. 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 & 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 Ú¿
³ ³ /\ Circuit ³³ Ú¿
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 & the fault
pulse, multiplied by the cable velocity, times 2 (there & back). Cables have
velocity factors of between 0.66 of the speed of light (300M/uS) for solid
coax, & 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 & scope timebase accuracy.
Coiled up cable & 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 & spreads
out. _
Pulse³ Cable
³ _ Loss e.g. 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 & wire wounds are too
inductive. I took it apart & 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 o)ÄÄÄ¿
Test BNC ³ POT<Ä¿
Coax ÀÄÄÄÁÄÄÄÙ
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 dBsÿ Load to High
³ / \ -- __ ÃÄÄÄÄÄÄÄÄÄ ³ \ -- __
³/ \ / \ / \ /~ ³ ³ \ / \ / \ /~
³ V V V ³ ³ V V V
ÀÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄ Freq ÀÄÄÄÄÄÄÄÄÄ Freq ÀÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ 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 75ohm cables for video etc!
BALANCED LINE TESTING
This needs a small pulse transformer in an add on box to isolate & 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 cosine shaped envelope of the markers to the 1st null frequency Fn,
then much weaker repeating « sine wave shape envelopes to infinity.
_ _ Due to the vertical sides
Pulse ^ ³~³ ³~³ of the pulse, there must be
Voltage ³ ³ ³ ³ a spectrum null in the
v ___³ ³______________________³ ³__ Repetition pulse frequency
| | harmonics at a frequency of
>T< 1/T, as a sine wave ~ could
- Repetition Frequency - not fit inside the pulse.
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 & pulse data, then graphs can be
plotted of the ideal spectrum for comparison. This assumes perfect terminations
& ideal pulse shape etc. but a good starting point.
For UHF-SHF use, surface mount components "chip Rs & 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 0.2nS
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 & the filter performance easily adjusted. This is normally only
possible with a tracking generator or a high power noise source.
dB Loss dB Loss
0dB_³ _._ _._ 0dB_³ _
³ /' `-' `\ ³ ,'`-' `-'`,
10dB_³ : : 10dB_³ : :
³ | Critically | ³ | Coupled |
20dB_³ | Coupled | 20dB_³ : 3 pole :
³ | 2 Pole | ³ | VHF Filter |
30dB_³ ,' VHF Filter `. 30dB_³ : :
³_./ \._ ³ _./ \._
40dB ÀÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄ>F 40dB ÀÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄ>F
144 145 146 MHz 144 145 146 MHz
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 & Tx Mon, by G4COL, Radcom June 1998.
[2] Cable Fault Locator, D.Huddart, Electronics World March 2001.
See Tech buls on "Coax Faulting", "Coax Feeder Tests", "Cable Tester", "RF
Noise Bridge for LF, MF, & HF". "RF Directional Coupler" & "Analyser SWIRES
RESEARCH SA87"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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