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T Y P E 1398-A P U L S E G E N E R A T O R
be given t o the equipment s e t u p and interconnections.
Bandwidth considerations are fundamental and oscillo-
s c o p e s with the desired frequency response must be
chosen a s indicators.
3.5.2 LONG PULSES- LOW-FREQUENCY RESPONSE.
An oscilloscope with a frequency response t o dc
i s necessary t o observe very long p u l s e s . The low-
frequency cutoff of most oscilloscopes that do not have
d c amplifiers is about
5
or 10 c / s , and t h e s e oscillo-
s c o p e s will exhibit
"
ramp-off
effects with pulse dura-
tions over 10 milliseconds. (Ramp-off i s the slope on
the flat top and bottom.) Almost any indicator h a s
adequate high-frequency response for long-duration
p u l s e s becuase the " f l a t s " are usually of more interest
than are the rapid r i s e and fallvoltage transitions. The
Type 1398-A u s e s a direct-coupled output system and
will not c a u s e ramp-off a t any duration.
3.6 RISE AND F A L L TIMES.
The Type 1398-A has very short r i s e and fall
times (typically 5 n s ) of output current into the internal
1-kilohm loads and their associated stray capacitances.
T h e internal stray capacitances are about 40 p F , which
results in open-circuit rise and fall times of about 90 n s
a c r o s s the internal 1-kilohm load. The rise and fall
times increase linearly with external capacitance and
and d e c r e a s e linearly with external resistance
-
the
final transition time i s about 2.2 RC. T h e intrinsic r i s e
time c a n therefore be observed only if a r e s i s t a n c e of
100 ohms or l e s s i s connected across the output binding
posts. With a n open-circuit connection, the Type 1398-A
output circuit is capacitance-limited, the voltage tran-
sition varies exponentially with time, and no overshoot
i s possible. Because of this important feature, the
Type 1398-A c a n be used t o check almost any amplifier
system for overshoot
-
including any oscilloscope whose
input impedance i s over 1 kilohm.
3.5.3 SHORT PULSES-HIGH-FREQUENCY RESPONSE.
For further information on rise and fall times,
Faithful reproduction of very short pulses or of
refer
paragraph
3.77
the rapidly changing voltage of the leading or trailing
edge of s u c h a pulse requires wide-bandwidth amplifier
3.7 EXTERNAL LOAD CONSIDERATIONS.
and indicator systems. For example, when a pulse with
The output circuits of the pulse generator are a s
a rise time of 0.05 p s is displayed
on
a n oscilloscope
stable a s possible for an instrument of such simplicity.
whose amplifier h a s a r i s e time of 0.05 fis, t h e indi-
Some imDortant
Doints
t o
remember
are:
cated rise time will be 0.07 p s . For a system with n
(1)
The output tubes act a s current sources that
individual components of specified rise time, the equa-
produce 60-rnA pulses into
a
parallel combination of
tion for over-all rise time1 is
the AMPLITUDE control resistors and whatever ex-
ternal load i s c0nnecte.d t o the instrument.
(1)
(2) T h e pulses are direct-coupled to the OUTPUT
PULSE binding posts and therefore contain a nega-
T h e rise time of a n amplifier system, T I , i s related t o
tive d c component of 60 mA.
the 3-dB bandwidth, B, by the equation (2), where the
factor of 0.35 should be used if the overshoot i s l e s s
than 10 percent.'
With very short p u l s e s , it i s necessary t o take care in
the wiring of system components. Short, direct wires
should be used for both signal and ground paths if open
wiring is used, and c o a x i a l c a b l e s should be terminated
properly. A common sign of an improperly connected
ground or of a n inductive loop in the wiring i s the pres-
ence of high-frequency ringing (damped oscillation) on
the pulse transitions.
'
V a l l e y , G. E., a n d Wallrnan, H e n r y , " V a c u u m T u b e Ampli-
fiers,
I'
Radiatiolz Laboraiory
Series,
Vol
18,
McGraw-Hill,
1948, p 77.
lbid,
p
80.
OUTPUT
F i g u r e 3-3.
E q u i v a l e n t c i r c u i t for T y p e 1398-A
output system feeding high load impedance.
Figure 3-3 shows an equivalent circuit for the
Type 1398-A output system a s it appears when feeding
a high external load impedance (e.g., a 12-pF, 10-
megohm oscilloscope probe). T h e pulses from t h i s cir-
cuit will be capacitance-limited by the 50-11s RC time
constant, and a r i s e time of about 100 n s results
( F i g u r e 3-4c). T h e appearance of a brief pulse a t out-
put settings of 0.6 volt and
6
volts i s shown in Figure
3-4 a and b, respectively.
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