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G8MNY > TECH 28.07.04 08:29l 92 Lines 5273 Bytes #999 (0) @ WW
BID : 3053_GB7CIP
Read: GUEST OE7FMI
Subj: A Versatile Pulse Tester 1/3
Path: DB0FHN<DB0MRW<OK0PPL<DB0RES<ON0AR<IK1ZNW<GB7CRV<GB7CIP
Sent: 040727/2327Z @:GB7CIP.#32.GBR.EU #:3053 [Caterham] $:3053_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY [BATC's CQTV No 195] (Updated April 04)
This tester based on ideas in magazine articles [1 & 2] and 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)ÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂ470ÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÄ¿ ³
³ ³ ):: ³ ³ ³C6 ³ ³ ³ _
+³ ³ hi )::1mH ³ _³_ === ³ ³ ³ ( )
C1 === ÚÄÄ´ PIV ):: ³ /_\' ³u1 ³ ³ 100K ³ ³14cm of coax
100u³ ³ ÃÄÄ´<ÃÄ´ ³ ³10v ³ ³ ³ ¬W³ ³_³sets pulse width
³ === ³ ³ 10K ÃÄÄÄÄÙ ³ +³16 ³ (.)ÄÄÄÄÄÄÄÄ¿
³C2³ ³ ³BFX84 ³ ³ 10K ÚÄÄÁÄ¿ : ÀÄÄÄ´ ³
³u1³ 10K \³ ³ ³ ÃÄÄ´clk ³ TRIM : ³Avalanche³
³ ³ ³ T1 ÃÄ´<ô ³ ÚÄXTALÄ´10³4040³ 2-30p: ³/transistor³
³ ³ SET e/³ ³ ³ ³ ÃÄÄ1MÄÄ´ ³ Q2ÃÄÄ´ÃÄÄÄÄÂÄ´ T4 ³ Test
³ ³ HT<-¿ ³ ³ ³ ³ ³ ÚÄÄ´ ³ ³7 250 : ³ ³\eÄ22ÄÂÄÄÂÄ@ Cable
³ ³100K ³ ³ ³ === ³ ³ ³/ ³ ³ rstÿ Khz :100 ³ 62 ³ BNC
³ ³ ³ _³_ ³ ³ C3³ ³ ÃÄ´T3 ³ ³Q5 ³³11 : ³ 270 ³ ³
³ ³ 33K /_\ ³ ³ 1n³ ³C4³ ³\e ³ ÀÂÄÂÄÙ³ : ÀÄÄÄÄÄÄÄÄ)ÄÄÁÄ´
³ ³ ³ ³ ³ ³/ ³ ³1n³ 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 : 82 ³ BNC
ÄÄÄÄÁÄÄÁÄÄÁÄÄÄÄÄÄÁÄÄÄÁÄÄÄ)ÄÄÄÁÄÄÁÄÄÄÁÄÄÁÄÄÄ)ÄÁÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÙ
DC-DC HT CONVERTER ³ XTAL OSC ³ DIVIDER NEEDLE PULSE GENERATOR
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ64KHz
NEEDLE PULSE GENERATOR.
The heart of the unit is 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 100ê 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 and pulse quenching
reflection from the length of unterminated coax on 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 @ the far end & produces an
even lower voltage @ the connector which stops (quenches) the conduction. After
a while the voltage will have built up again & the whole cycle repeat.
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 and very short wiring to the output
socket, I found the pulse width was around 3nS wide [1].
A very narrow pulse has wide bandwidth and if repeated with precise timebase
can provide good RF markers up to the 1st null frequency determined by the
pulse width. A 0.3uS wide pulse gives a 1st order null at 333MHz. 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
0.3nS are possible.
CRYSTAL CLOCK.
Using a 1MHz crystal oscillator and 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 and the size of the
trigger pulse.
With 250KHz pulse repetition frequency, cable lengths of up to 500M can be
pulse tested and 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.
Testng & adjustment & How to use it in part 2
/QSL
73 de G8MNY @ GB7CIP
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