Message:19050 In: TECH.WW

From: G8MNY
Date: Wed, 18 Mar 26 10:22:00 Z
Newsgroups: TECH.WW
Subject: A Versatile Pulse Tester
Message-ID: <54772_GB7CIP>
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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] 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
��Ŀ
+12V � HT 25-100V �
Ĵ>��)��470R��Ŀ � _
� � ):: � � �C6 � � � ( )
+� � hi )::1mH � _�_ === � � � � �14cm of coax
C1 === ��Ĵ PIV ):: � /_\' �u1 � � 100K �_�sets pulse width
100u� � ��Ĵ<�Ĵ � �10V � � � �W� (.)��Ŀ
� � 10K � 10K �� 10K +�16 ��Ĵ �
�C2� � �BFX84 � � � ��Ŀ : � UHF �
� === � \� � � 1MHz ��Ĵclk �7 TRIM: T4�/ Avalanche�
�u1� � T1 ��´<ô � ��XTALĴ10� Q2��Ĵ��Ĵ transistor�
� � SET e/� � � � ��1M�Ĵ �4040� 2-30p: � �\e � Test
� � HT<Ŀ � � � � � ��Ĵ � � 250 : � ��22R��@ Cable
� �100K � � � === � � �/ � � rstÿ kHz :100 � 62R� BNC
� � � _�_ � � C3� � �Ĵ T3 � �Q5 ��11 : � 270R � �
� � 33K /_\'� � 1n� � C4� �\e � ��ٳ : ��)��Ĵ
� � � � � �/ � � 1n� � === �5�8 � : all short� � Monitor
� � �24V��(Ĵ T2 � � === � �C5 � � � : leads ��@ Scope
� � � � �\e � � � � �15p� � � : 82R � BNC
��)��)��
DC-DC HT CONVERTER � XTAL OSC � DIVIDER NEEDLE PULSE GENERATOR
��64kHz

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 and 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 and produces an even lower voltage at the
collector which stops (quenches) the conduction. After a while the voltage will
have built up again and the whole cycle repeat.

Collector Transistor
Voltage Avalanche
_� _| _ ___
Avalanche � .�' � .�' � Reflected
� .�' � .�' � coax
_�_.�' � _.�' � __pulse
Quench � ��\Coax �� height
� ��inverted ��
� ��quenching ��
� ��pulse ��
Emitter ڿ ڿ
Voltage ��

These can be used as they are, for coax cable time domain reflection testing
with just a suitable T4 emitter network and 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 3nS wide pulse gives a 1st order null at 330MHz. For very fast
pulse you need short coax line and a uWave transistor, and 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 and divide chain to obtain clocks of interest
(other crystals and 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 and with a wide 300kHz IF filter in a spectrum analyser a
reasonable smooth graphs could be drawn of VHF filters etsub 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 and 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_� _._ _._ 0dB_� _
� /' `-' `\ � ,'`-' `-'`,
10dB_� : : 10dB_� : :
� | Critically | � | Coupled |
20dB_� | Coupled | 20dB_� : 3 pole :
� | 2 Pole | � | VHF Filter |
30dB_� ,' VHF Filter `. 30dB_� : :
�_./ \._ � _./ \._
40dB ��>F 40dB ��>F
144 145 146 MHz 144 145 146 MHz

Although this noise source should not be put into a effective radiating aerial,
handheld 1/4 wave and 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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