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Operating Instructions-PG 502
I.
OPERAT 'ING CONSIDERATIONS
Output
Terminations
arid Connections
reflections.
For maximum
fidelity.
use ths special
three
The output
of the PG 502 operates as a 100 mA current
foot
long 50 .Q coaxial
cable with special BNC connectors
I
source. It is designad to operate
into an external
50 $I load.
supplid
%s % standard
'accessory
(Tektronix
Part
No.
An unterminated
or improperly
terminated
output
causes
012-0482-00).
Us% the internal
back termination
whenever
aberrations
on the output
pulse (see Impedance
Matching).
oossible.
Loads
less than
5Ofi
reduce the pulse amplitude.
Loads
I
greater than 50 $1 increaae the amplitude.
An external
50 $2
When
signal comparison
tne%sur%ments
or time differ.
load also urovides
a DC return
path for the output
current.
ence determinations
are made, the two signals from the test
device should
travel
through
coaxial
cables with
identical
I
A selectable
50 .S2 back termination
is provided
[pull the
10s~ and time delay characteristics.
button
tabled
BACK
TERM
(PULL)
on the front
panel].
The back termination
also helps to absorb
reflections.
The
I
output
v&age
is divided
by two
when
using the back
Make certain
the attenuators
and terminations
used can
termination.
The back termination
provides
the DC return
safely
handle
the
maximum
PG 502
power
output
of
path
for
the
outp,ut
when
driving
high
impedance
or
0.5
Watts.
capacitively-coupled
loads.
If the output
of the PG 502
I
drives a high impedance
load using the back termination,
the output
voltage is limited
to approximately
f5 V.
When
making
connections
that
%re not
in a 5Ofi
environment,
keep all lead lengths short,
l/4 inch or lass.
Accessory
filters to increase risetimes
and reduce the need
I
A DC current
in the 50 51 output
load CBUSS the output
for high quality
attenuators
and terminations
are'available.
pulse to be offset.. Do not apply
voltages greater than plus
See your
Tektronix
Representative
for more information.
or minus 5 V to the output
of the PG 502. If the load has %
I
DC voltage
across it greater
than the maximum
allowed,
connect
a blocking
capacitor
in series with
the OUTPUT
connector
and
the
load.
Us% the back
termination
to
Impedance
Matching
provide
a PC return
path for the output
current.
Make
A mismatch,
or different
impedance
in a transmission
el
certain
the time constant
of the capacitor
and the load is
large
enough
TO, maintain
pulse
flatness.
The
output
line, generates a reflection
back along the line to the source.
circuitry
of the
PG 502
is fully
protected
against
any
The amplitude
and polarity
of the reflection
ar% determined
voltage transients
in the output
resulting
from passive loads.
by the
load
impedance
in relation
to the characteristic
I
impedance
of the cable.
If the load impedance
is higher
than the characteristic
impedance
of the line, the reflection
Under
cenain
conditions,
it is possible
to operate
the
will be of the wna
polarity
as the applid
signal. If it is
PG 502
into
a high
impedance
load
without
using the
lower,
the reflection
will
be of opposite
polarity.
These
I
internal
termination.
Pulse amplitudes
up to about
18 V
reflections
add or subtract
from
the amplitude
of the
(-9 V to +9 VI can be obtained
in this manner
with
load
incident
pulse causing distortion
and irregular
pulse shapes.
impedances
in excess of 180 .Q. The PG 502 is not specified
I
when operating
in this mode. To we the instrument
in this
manner,
view
the
output
with
an oscilloscope
while
A simple
resistive
minimum
attenuation
impdsncs-
matching
network
that can be used to match the PG 502
adjusting
the OUTPUT
(VOLTS)
control,s
for the desired
waveform.
output
into relatively
low impedances
is shown in Fig. 1-2.
.I
To
match
impedances
with
the illustrated
network,
the
following
conditions
must exist:
Maintaining
Pulse Fideky
Due to the extremely
fast pulse r$etimes
obtained
from
I
the PG 502, special consideration
must be given to prewva-
tion of pulse fidelity.
Even et low repetition
rates, 1 GHz
(R, +q R,
frequency
components
are pr%s%nt in the output
waveform.
Rl+z,*R=
rm5t equal 2,
I
Use high quality
coaxial
cables, %tt%nuators.
and termina-
tions.
and
I
RG 58 type coaxial
cable and typical
BNC connectors
a
exhibit
impedance
tolerances
which
may
cause
visible
,I
I-2
1,
Q
I
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