| |
G8MNY > TECH 10.04.04 09:31l 120 Lines 6816 Bytes #999 (0) @ WW
BID : 52202_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: A Versatile Pulse Tester 1/2
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 040410/0726Z @:GB7CIP.#32.GBR.EU #:52202 [Caterham] $:52202_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.
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.
How to use it in part 2
/QSL
73 de G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
