things never change, and the basic definition of a pulse waveform is
one of them. This article form the May 1967 edition of Electronics World
does a nice job of defining the fundamental characteristics of a pulse,
including rise and fall times, overshoot, pulse width, etc. Ham shacks
and company labs alike are still stocked with the venerable
HP 215A and
HP 213B signal generators. If you're lucky, you can get a good deal
on them from someone on eBay.
May 1967 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
Electronics World articles.
See all the available
Electronics World articles.
Selecting and Using Pulse GeneratorsBy John Lenk
Basically a laboratory version of the
square-wave generator, but with adjustable on-off times, this instrument
has many uses in developing digital circuitry, in checking diode and
transistor switching times, as a klystron modulator, and for impulse
The output of a pulse generator is similar
to that of a square-wave generator. The fundamental difference between
the two concerns the signal duty cycle. Square-wave generators have
equal "on" and "off" periods, equality being retained as the repetition
frequency is varied. On the other hand, the duration of a pulse generator
"on" period is independent of pulse repetition rate. The duty cycle
of a pulse generator can be made quite low that the pulse generator
is usually able to supply more power during the "on" period than a conventional
Pulse generators with fast rise times
are widely used in the development of digital circuitry. Teamed with
suitably fast oscilloscope, these generators enable evaluation of transistor
and diode switching times. Pulse generators can be employed as modulation
sources for klystrons and other r.f. sources to obtain high peak power
while maintaining low average power. Pulse generators are also used
for impulse testing. A very short pulse is rich in harmonics so that
input testing amounts to simultaneous frequency-response testing of
components or systems.
To adequately describe the characteristics of a pulse generator,
it is first necessary to establish uniform terms for pulses. These terms
are illustrated in Fig. 1. When actual pulses are very irregular (with
excessive tilt, over-shoot, or rounding), the definitions may become
ambiguous, requiring a more complete description.
are typical characteristics of a laboratory pulse-generator output:
Fig. 1. Terms used in describing output
Leading Edge Only:
Rise time (Tr): <1.0
nanosecond (ns) (10 to 90%).
Overshoot and ringing: overshoot <5%
peak; ringing <±5% of pulse amplitude.
Corner rounding: occurs
no sooner than 95% of pulse amplitude.
Time to achieve flat top
(Ta): <6 ns.
Trailing Edge Only:
Fall time (Tf): <1.0 ns (10 to 90%).
Rounding: occurs no sooner than 95% of fall.
Time to settle within 2% of baseline (Tb): 10 to 25 ns,
varies with setting.
Baseline shift: < 01
% under all conditions.
Perturbations on flat top: <2% of pulse
Peak voltage: >10 volts into
50 ohms, >20 volts into
Polarity: positive or negative.
Pulse width (between 50% points): continuously
adjustable, zero to 100 ns (zero ns width occurs when 50% points meet,
creating an impulse of one-half the amplitude of wide pulses).
Repetition rate (internal): <100 Hz to >1
MHz in 4 ranges.
The quality of the output pulse is of primary importance in the selection
of a pulse generator. If the displayed pulse is degraded, a high-quality
test pulse will insure that the cause is in the test circuit alone.
Rise and fall times should be significantly faster than the circuits
or systems to be measured. Any overshoot, ringing, or sag in the test
pulse should be known so that these faults will not be confused with
similar results caused by the test circuit.
Hewlett-Packard 215A pulse generator is shown.
HP-213B produces pulse with under 0.1-nanosecond rise time.
The range of pulse-width
control should be wide enough to fully explore the range of operation
of a circuit. Narrow pulse widths are useful in determining the minimum
trigger energy required in some circuits.
Maximum pulse amplitude
is of prime concern if appreciable input power is required by the circuit
under test, such as a magnetic core memory. At the same time, the attenuation
range should be broad enough to prevent over-driving the test circuits
as well as to simulate actual circuit operating conditions.
The range of pulse repetition rates is important if the tested circuits
can operate only within a certain range of pulse rates or if a variation
in the rate is needed. In some systems, methods of external triggering
are also significant. In fast pulse systems, the generator source impedance
is an important consideration because a generator which has a source
impedance that is matched to the connecting cable will absorb reflections
resulting from impedance mismatches in the external system that is used.
1. Use proper types
of cables, terminations, attenuators, and impedance-matching networks.
Always match impedances unless the test circuit specifically calls for
2. Keep ground-return paths short and direct. Use
heavy conductors to provide low impedance in the ground return.
3. Make sure that all connections are tight and that all connectors
are securely assembled.
4. Shield measuring-equipment leads
to prevent undesired coupling to other parts of the circuit. Shielding
is especially required where pulse radiation is a problem and particularly
where high-impedance dividers or circuits are involved.
the effects of secondary parameters in components, such as inductance
in resistors and in capacitor loads.
6. Consider the possible
non-linear behavior of components due to changes in either voltage or
7. Select components which function properly at
the frequencies and rise times expected to be encountered.
Fig. 2. Resistive impedance-matching
network and formulas. (upper)
Fig. 3. Three-way dividing pad circuit
for Z measurement. (lower)
Obviously, the accuracy of rise-time measurements can be no greater
than the rise time of the pulse generator. If a pulse generator with
a 20-nanosecond rise time is used to measure the rise time of a 15-nanosecond
oscilloscope, the measurements would be hopelessly incorrect. Also,
if the same pulse generator and oscilloscope were used to measure the
rise time of another system, the fastest rise time for accurate measurement
would be something greater than 20 nanoseconds.
As a general
rule, if the rise time of the test device is at least ten times as long
as the rise times of the generator, oscilloscope, or cables, the error
introduced will not be more than 1%. If the rise time of the device
under test is less than ten times that of the test equipment, it will
be necessary to calculate the rise time. The most common method involves
finding the square of all rise times associated with the test, adding
these squares together, and then computing the square root of this sum.
For example, using the 20-ns pulse generator and the 15-ns oscilloscope,
the calculation would be: 20 x 20 = 400; 15 x 15 = 225; 400 + 225 =
625. √625 = 25, so 25 nanoseconds is the fastest possible rise time
capable of measurement.
Another rule of thumb applying to rise
times is that if the equipment being measured has a rise time three
times slower than the test equipment, the error is only slightly less
If there are significantly long lengths of coaxial
cable in the signal path, the above method can be used only as an approximation,
since the "skin-effect" losses in coaxial cables do not add properly
with this method.
Connecting Pulse Generators
1. In most measurements involving pulse generators, a complete
d.c. return path must be provided between the device under test and
the pulse-generator output connector.
2. If the pulse is applied
to a load which has a d.c. potential across it, the actual amplitude
of the pulse is equal to the voltage set by the pulse-generator amplitude
control less one-half the d.c. voltage across the load.
example, assume that the pulse-generator output is connected to a load
which has +10 volts across it and that the pulse-generator amplitude
control is set to +1 volt. The actual amplitude is found by substituting
these values as follows: Va = Vs - (Vl/2)
or +1 - (+10/2) = -4 volts where Va is the actual pulse amplitude,
Vs is the voltage setting of the amplitude control, and Vl
is the d.c. voltage applied across the load.
3. If it is impossible
to use an impedance-matching network, one possible solution is to employ
a long coaxial cable between generator and load. This will delay the
load's reflections until after the time of interest.
output can be supplied with an impedance-matching network that will
produce a smooth transition of power (no reflections) with a minimum
attenuation. Such a network is shown in Fig. 2. To match impedances
with the illustrated network, the values of R1 and R2 must be selected
For example, to match a 50-ohm system to a 125-ohm
system, Z1 = 50 ohms and Z2 = 125 ohms. Therefore, R1 = √125(125 - 50)
= 96.8 ohms, and R2 = 50 √25/(125 - 50) = 64.6 ohms.
as seen from one end of the network does not equal that seen from the
other end. Using the equations shown in Fig. 2, it will be noted that
a signal applied from the lower impedance source Z1 encounters a voltage
attenuation A1. Also, a signal applied from the higher impedance source
Z2 will encounter a greater voltage attenuation A2.
with an R1 of 96.8 ohms and an impedance Z2 of 125 ohms, A1 = (96.8/125)
+ 1 = 1.77.
With an R1 of 96.8 ohms, an R2. of 64.6 ohms.
and an impedance Z1 of 50 ohms, A2 = (96.8/64.6) + (96.8/50) + 1 = 4.44.
A pulse generator
can he used to determine impedance of an unknown device by comparing
the reflected pulse with the incident pulse on an oscilloscope. This
can be explained as follows.
As a signal travels down a transmission
line, each time it encounters a mismatch or different impedance, a reflection
is generated and sent back along the line to the source. The amplitude
and polarity of the reflection arc determined by the value of the impedance
encountered in relation to the characteristic impedance of the cable.
If the mismatch impedance is higher than that of the line, the reflection
will be of the same polarity as the applied signal; if it is lower than
that of the line, the reflection will be of opposite polarity.
The reflected signal is added to or subtracted from the amplitude
of the pulse if it returns to the source before the pulse has ended.
Thus, for a cable with an open end (no termination), the impedance is
infinite and the pulse amplitude would be doubled. For a cable with
a shorted end, the impedance is zero and the pulse would be canceled.
The following procedure provides a practical method of determining
impedance with a pulse generator and scope.
the equipment as shown in Fig. 3.
2.Observe the incident and
reflected pulses on the oscilloscope. Using Fig. 4 as a guide, determine
the values of V0 (incident) and Vx (reflected).
(This method is generally limited to the first reflections unless the
deviations are small, due to multiple reflections and reflection losses.)
4. Using the following equation, calculate the unknown impedance.
Z = 50/(2V0/Vx - 1) where Z is the unknown impedance,
V0 is the peak amplitude produced by the 50- ohm reference
impedance, and Vx is the peak amplitude at the time of reflection.
Waveform obtained with 125-ohm cable and 50-ohm system. (upper)
5. Test connections using generator with conventional scope. (lower)
Using Conventional Oscilloscopes
generator is often used with a sampling oscilloscope, and generator
and oscilloscope manuals describe the procedure. However, a pulse generator
can also be used with conventional triggered oscilloscopes. Fig. 5 shows
the test connections.
Internal triggering is convenient since
no external triggering connections are required. However, with external
triggering it is possible to observe the shaping and amplification of
a signal pulse in the circuits of a device under test without resetting
the oscilloscope triggering controls for each observation. If the external
triggering signal is derived from the waveform at the input circuit
of the device under test, the time relationship and phase between the
output and input waveforms may be seen and compared on the oscilloscope
If the signal from the test device is fast-rise non-repetitive
or has a low duty cycle, the oscilloscope used in this setup must have
an internal delay line so that the leading edge of the single waveform
can be readily observed on the scope.
Tektronix Type 109 pulse generator has rise time of under 0.25 nanoseconds.
One of the drawbacks to a conventional oscilloscope is that the
frequency response of the test device may fall outside the bandwidth
limitations of the vertical amplifier system of the oscilloscope. In
some cases, the output signal from a device under test can be observed
by direct connection through coupling capacitors to the vertical deflection
plates of a conventional oscilloscope. Thus, the limited bandwidth of
the oscilloscope vertical amplifier can be bypassed.
factors pertaining to the vertical deflection-plate system must be considered
for pulse measurement: d.c. operating potential of the plates, lead
inductance, deflection-plate capacitance, transit-time limitations,
delay lines, and deflection factor.
A typical circuit for direct
a.c. coupling to the vertical plates is shown in Fig. 6. This circuit
permits the internal vertical amplifier of the oscilloscope to be bypassed
but still allows the normal d.c. operating and positioning voltages
to be applied to the deflection plates from the internal vertical amplifier.
However, when using this circuit, a high-quality external delay line
must be used. This will retard the pulse sufficiently to get it on the
The values of R1 and R2 are found by solving the equation
given in Fig. 6. The resonant frequency (F0) of the leads
and the capacitance of the deflection plates (CD) for use
in the equation may be determined by the following procedure:
1. Turn off the oscilloscope power.
vertical amplifier leads from the CRT neck pins. (A convenient method
of connecting to the deflection-plate pins is to use clips removed from
a miniature tube socket.)
3. Cut a wire loop equal in length
to the total length of C1, C2, R1, R2, R3, and R4.
substitute the wire loop for the components between the vertical deflection-plate
5. Bring a grid-dip meter near the loop and measure the
resonant frequency (F0)
6. Remove the wire loop.
7. With a capacitance meter, measure the total capacitance between
the plates (CD) at the deflection-plate neck pins. (Capacitance
between the plates can also be found by referring to the specifications
of the oscilloscope.)
Tektronix Type R116 pulse generator.
Since the deflection plates are located close to the path of the
electro beam, a small amount of current will flow in the deflection-plate
circuits The values of R3 and R4 must be low enough so that this current
will not produce a large voltage drop at the deflection plates. If the
resistors are too large, distortion, defocusing, or positioning difficulties
may be experienced. Since the deflection-plate current varies non-linearly
with the position of the beam, the effects are most noticeable when
the beam is positioned near the top or bottom of the screen. The approximate
value of 100,000 ohms that is given for R3 and R4 will probably be satisfactory
in most cases.
C1 and C2 should be physically small to minimize
lead inductance. The values of C1 and C2 are selected on the basis of
the required low-frequency response and may be calculated from the equation
given in Fig. 6. (Fc is the low-frequency cut-off.) For example,
if R3 and R4 are 100,000 ohms and if the desired Fc is about
1.6 kHz. C1 and C2 should be 0.001 µF.
The stub cable that connects
to terminating resistor R0 should be long enough so that
if a double-transit reflection appears, it can be easily identified
and corrected by adjustment of the termination.
For making vertical
measurements with the test setup, the deflection factor of the oscilloscope
must be known. This can be measured as follows:
1. While the
leads from the vertical amplifier are connected to the deflection-plate
neck pins, connect a d.c. voltmeter between the pins.
the voltage change as the beam is positioned vertically over the full
height of the graticule.
3. Divide this voltage excursion
by the graticule height in divisions to obtain the deflection factor
(Many of the diagrams and techniques
described above are based on information from Tektronix, Inc. and Hewlett-Packard.
Fig. 6. Circuit for coupling to vertical