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PulseGen 5242-720px.JPG

Pulse Generator

This video, by W2AEW, inspired us to want to make a Pulse Generator for use with an oscilloscope as a Time Domain Reflectometer (TDR). A variant on the circuit is well described here.

Our First Practical Use

Within a month of first building this board, we had a bad piece of RG-8X coax and it was difficult to diagnose the problem. We used an ohmmeter to verify that the coax was good from a DC perspective and it looked fine while we were testing. It was connected end to end and there were no opens or shorts that we could see with the meter. However, we knew the cable was bad because er swapped it out for another cable and the other cable worked just fine. When we went to use the broken cable, we could jiggle the cable and make it work and then it would flake out but it was far from consistent. Intermittent problems are the worst sort of problems, aren't they? As they say - Murphy's Law is a universal constant.

We decided to hook it up to this Pulse Generator board and the results were impressive to say the least. When weflexed the cable the waveform went from a nice stepped waveform to nearly flat on my oscilloscope (a little bump which went down quickly). That told us that the problem was on the near end of the cable and that it was a short between the center conductor and the shield. In the end the problem was a short at the PL-259 connector.


  • 74AC14 driver(s).
    • Board parallels 5 resistors to match cable impedance.
      • Series resistors are located directly below the IC (tradeoff since requires a feedthrough)
      • Paralleled outputs can drive 50 (Or 75) Ohms load.
      • Paralleling (5) 249 ohm resistors gives 50 Ohms.
      • Paralleling (5) 374 ohm resistors gives 75 Ohms.
    • 74AC14 has typical 2 nS rise time with a max 5 nS rise time.
    • Symmetric high/low drive.
  • 6th inverter in 74AC14 drives low frequency oscillator (it's the edge that matters not the oscillator speed).
    • Hysteresis function makes oscillator easy by adding resistor feedback and cap to ground.
    • Oscillator duty cycle controlled through component selection.
  • Power Supply from 2V to 6V so battery operation is possible eliminating need for power supply, etc.
    • The TI datasheet shows part ratings for 3V, 4.5V, and 5.5V operation.
  • Power LED to indicate unit is ON
  • 5mm Terminal Block for power allows for batteries, etc to be easily connected without soldering
  • Board size is 1.76" x 1.22" (45mmx31mm).
    • Small form factor can hang directly on scope input with very little stress to connector.
  • Connectors are a stuffing options with either BNC or SMA connectors.
    • High quality Amphenol BNC connectors (expensive but well worth the additional cost)
  • Silkscreen marking for "SCOPE" and "CABLE" although either end can be used.
  • Four #4 mounting holes - work with 4-40 screws.
  • Rounded corners fit more easily into plastic enclosures with inside radius at the corners
  • Tests fully tested

Power Consumption

Current Draw with 50 ohm load, 0 ft coax = 17.2 mA


Battery Case with Switch


  • 2 AA batteries (not included)
  • Case
  • Power Switch

BNC Coupler


  • Male-male
  • Allows the Pulse Generator card to be connected to a scope

F Adapter


  • Used to connect to F type cable
    • Typically 75 ohm - Cable TV type of cable


Schematic features include:

  • Two bypass caps
  • One large bulk electrolytic cap


R7 and D1 are optional (not installed on standard unit) and used to adjust pulse width.

PCB Layout (top side)


3D Case Design

Link to case design

Typical Coax Specifications

The velocity factor (VF) of coax characterizes the speed that the wave (edge in this case) will travel down the cable Signal Velocity. The closer the VF gets to 100% the faster. This normally doesn't matter due to the high speed of electricity but when measuring the return time of the reflection from the end of the coax the velocity factor becomes important.

These numbers are provided for reference. Look up the velocity factor of the coax you are using starting with the cable Part Number (usually printed on the jacket of the coax).

Cable loss specifications

RG-58/U, Belden 8240       RG-58C/U, Belden 8259  
Velocity Factor 66%       Velocity Factor 66%  
MHz dB/100 ft Pout   MHz dB/100 ft Pout
50 2.5 28   50 3.7 21
100 3.8 21   100 5.4 14
200 5.6 14   200 8.1 8
400 8.4 7   400 12.4 3
700 11.7 3   700 17.7 1


  • Cable Under Test
    • 25 Ft Cable
    • RG-58C/U
Item Qty units
Measured time between edges 78.4 nS
Velocity Factor 0.66  
Speed of Light 3.00E+08 m/s
Speed of Light in coax 1.98E+08 m/s
in to m conversion 39.14000 in/m
Speed of Light in coax 7.75E+09 in/s
inches to feet 0.08333 ft/in
Speed of Light in coax 6.46E+08 ft/s
s>nS 1E-09 nS
Speed of Light in coax 0.646 ft/ns
Length (roundtrip) 50.632 ft
Half of roundtrip 0.500 ft one way/ft
Length 25.316 ft
error vs cable nominal length 1.26%  


50 Ohm Output


  • Powered by two AA batteries
  • Agilent Oscilloscope Model 54624A, 100 Mhz
  • Short Male to Male adapter between the scope side of the board and the Oscilloscope
Waveform Setup
PG-noCoax.png No coax attached (Open circuit)
PG-42in-unterm.png 42 in coax unterminated end
PG-42in-term.png 42 in coax with 50 ohm termination
PG-25ft-unterm.png 25 ft unterminated (Open circuit)

75 Ohm Output

  • Rigol 1054Z, 50 MHz scope
  • UUT w 5x390 ohm resistors
  • Piece of 75 Ohm coax, RadioShack RG-6 E237761
  • BNC to F connector



Design History/Background

Pulse Generator History/Background

Factory Acceptance Test - Test Station

The automated test station measures the low and high voltage levels with and without termination with the 12 bit A/D inside an Arduino and displays the voltage values on an OLED menu/display MyMenu card. The test is performed by powering the card from the Arduino Nano. Unloaded high voltage is 5V (count of 1023) and unloaded low voltage is 0V (count of 0).

The tester counts the number of samples of high and low and presents those counts. This is typically something like 3-7 counts of high and low for the nominal output frequency of 300 Hz.

The output is then loaded with a 50 or 75 Ohm load and measured. The loaded value is about half the 1023 or around 511. A number off by too far means the wrong termination was used or there is an issue with the driver terminations on the card.

This allows the card to be functionally tested very quickly.


  • Tester
  • UUT
  • Cable
  • 9V wall wart to power Arduino Nano (no terminal is required) or optional USB mini cable


  • Connect power/ground to screw terminals
  • Connect BNC cable


  • Select Test Pulse
  • Results
Low Count=4 or 5
High Count=6 or 7
Low V=0
High V=1000 to 1023
  • Install 50 or 75 Ohm terminator
Low Count=4 or 5
High Count=6 or 7
Low V=0
High V=500 to 520

Factory Acceptance Test - Scope


  • Rigol DS1054A oscilloscope
  • 3V battery pack
  • 50 or 75 Ohm BNC terminator
  • BNC male-to-male


  • Connect power/ground to screw terminals
  • Connect BNC male-to-male to "SCOPE" side of UUT
  • Scope 1uS per div horiz, 1V per div vert


  • Turn on switch
  • Measure
    • Freq 500 Hz +/- 10%
    • Unterminated voltage (take note)


  • Install terminator
  • Voltage should be half the unterminated voltage (above)


  • Turn off switch
  • Mark TESTED box on back for passing test
  • Mark resistance (Rev X5 card and higher)

Assembly Sheet

PulseGen Assembly Sheet