Tektronix PG502 User manual

Poktronix. Inc.
P-0. BOX 500
Beaverton, Oregon 97005
@/973
070.1598-00
Serial Number

PG 502
Ii,
“B:I@
TABLE
I OF CONTENTS
SECTION 1 OPERATING INSTRUCTIONS
Introduction
Instrument Description
installation and Removal
Operating Considerations
Output Terminations and Connections
Maintaining Pulse Fidelity
Impedance Matching
RisetimE Measurements In Linear Systems
Variable Pulse Delay
Operating Mode?.
Period and Duration Selection
output Levels
External Trigger
External Duration
Eunctions Available at Rear Connector
Definitions Of Pulse Characteristics
Specifications
Performance Conditions
SECTION 2 THEORY OF OPERATION
Introduction
Low Frequency Period Generator
4ns Period Generator
External Trigger Duration Buffer
Manual Trigger Multivibrator
Period and Trigger Out Buffers
Trigger Shaper
Duration Generator
Output Buffer
Output Driver
Output Amplifier
output Level Programmer
Power Supply
SECTION 3 SERVICE INFORMATION
Symbols and Reference Designators
Rear Interface Connector Assignments
Electrical Parts List
Internal Adjustment Procedure
Timing Board Parts Location Grid
Front Panel Controls and Connectors
Block Diagram
Output Board Parts Location Grid
Generators and Trigqer Schematic
Output Schematic
Switch Details and Power Supply Schematic
Mechanical Parts List
Exploded View
Accessories and Repackaging
Page
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1.2
1.2
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I-2
1.4
1.4
1.5
1.5
I-5
1-5
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l-6
1-6
1~8
l-8
2-l
2-l
2-l
2.1
2-I
2-2
2.2
2-2
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2”2
2-2
2.2
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3.1
2-2
3-3
m

PG 502

Section l-F% 502
OPERATING
INSTRUCTIONS
INTRODUCTION
Instrument Description
The PG 502 is a 250 MHz general purpose pulse genera-
tw for use in the TM 500 series power modules. Major
capabilities of this instrument include high repetition rate,
narrow pulse width, fast risetime, and independent pulse
top and bottom level controls. Front panel controls provide
manual trigger, square wave output, and complementary
pulse output for high duty factors.
A selectable 50 11 back termination in the pulse output
circuitry is also provided. All other inputs and outputs are
internallv terminated in 50 fl.
Triggers preceding the output pulse are available at the
fr’ont panel. The pulse output may also be externally
triggered.
The front panel is color coded for easy reference to
controls and their associated functions. Orange denotes
pulse duration controls and settings; green. ‘triggering
functions; and yellow is used for an operating caution note.
Alpha-numerics done in red are the frequency equivalents
for the pulse period settings.
Installation and Removal
The PG 502 is calibrated and ready for use when
received. It operates in any compartment of a TM 500
series power module. See the power module instruction
manual for line voltage requirements and power module
operation. Fig. 1-l shows the installation and removal
procedure. Check that the PG 502 is fully inserted in
the power module. Pull the power switch on the power
module. The POWER light on the PG 502 front panel
should “ow be on. Refer to the Controls and Adjustments
foldout page in Section 3 of this manual, for a complete
description of the front panel controls.

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 .S2back 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

Operating Instructions--PG 502
Therefore:
A,=
and
I R,=z,
For example; to match a 5Oa system to a 125R
system. Z, equals 50 s-1and 2, equals 125 R.
I Therefore:
and
R, = 125(125 - 50) = 96.8 ohms,
R, = 50 f ,;;T50 = 64.6ohms.
Though the network in Fig. 1.2 provides minimum
,“,,
!j
w attenuation, for B purely resistive impedance-matching
device, the attenuation as seen from one does not equal
that wn from the other end. A signal (E, ), applied from
the lower impedance source, encounters a voltage attenua-
tion (A,1 which is greater than 1 and less than 2, as
follows:
A, -Li=El+,
E2 *2
A signal (E2) applied from the higher impedance source
(Z,) encounters a greater voltage attenuation (A,1 which is
greater than 1 and less than 2(2,/Z, ):
In the example of matching 50 Q to 126 .Q,
and
A, 96.8 = 125 + 1 = 1.77
96.8 96.8
A, =64.6+50 + 1 =4.43
The illustrated network ten be modified to provide
different attenuation ratios by adding another resistor (less
than R,) between 2, and the junction of A, and R,.
When constructing such a device, the environment
surrounding the components should also be designed to
provide smooth transition between the impedances. Accept-
able performance can be obtained with discrete eompo-
rents using short lead lengths; however, a full coaxial
environment is preferred.
The characteristic impedance of a coaxial device is
determined by the ratio between the outside diamter of the
inner conductor to the inside diameter of the outer
conductor expressed as:
138
Z, =F log, ,, D/d,
The relative dielectric constant is e (e,,, = l), D is the
inside diameter of the outer conductor and d is the
diameter of the inner conductor.
Further information on attenuator design may be found
in Reference Data For Radio Engineers, Fifth Edition,
Howard W. Sams& Co. Ir~c., New York, N.Y.. Chapt. 10, or
other suitable reference work.
Consider carefully the effects of impedance mismatches
or discontinuities in transmission lines and terminations.
Short lengths of wire exhibit inductance causing pulse
aberrations. Use 50 fi environments or, if this is impossible,
keep ail lead lengths as short as possible (l/4 inch or
shorter).
1-3

If the QG 502 is driving the 1 Ma capacitively-shunted
vertical input of a” oscilloscope, connect a 50 a termina-
tion to the oscilloscope input. Connect the coaxial cable
from the PG 502 to a 50 a 10X attenuator, and connect
the attenuator to the termination. The attenuator isolates
the input capacity, providing a” improved termination for
the cable. Another method is to back terminate the PG 502
by pulling The BACK TERM (PULL) pushbutton on the
front panel, and connecting the coaxial cable to the
oscilloscope input through a 50 fi termination.
Risatima Measurements in Linear Systems
Consider the rise and fallfime of associated equipment
when measuring the rise or falltime of a linear device. If the
risetime of the device tinder tesl is at least ten times slower
than the combined risetimes of the QG 502. the monitoring
oscilloscope, and acsociated cables, the error introduced
will not exceed I%, and usually may be ignored. If the rise
or falltime of the test device is less than ten times slower
than the combined risetimes of the testing sysw”, deter”
mine the actual risetime of the device under test by using
the following formula:
R, equals the overall rise or falltime of the entire
measurement system and R,, R,. R,, etc. are the risetimes
or falltimes of the individual componentc comprising the
system. - rrl
I
I
Variable Pulse Delay
Variable pulse delays may be obtained using another
PG 502, or other suitable pulse generator. For example,
using two PG 502s. push the COMPLEMENT button and,
using a” oscilloscope, set the OUTPUT (VOLTS) LOW
LEVEL control on rhe delay generator for 0 V. Set the
HIGH LEVEL control for +I V. Some fine luning of the
output levels of this generator may be necessary to achieve
250 MHz operation. Connect the OUTPUT from the delay
generator to the +TRIE/DURATION INPUT connector on
the output. generator. Take the pulse output from the I
OUTPUT connection on the output generator, and the
trigger from the + TRIG OUT connector on the delay
generator.
The PERIOD controls on the delay generator now set
the period of the output waveform, and the DURATION
controls set the delay. The output pulse duration and
voltage levels are set by the appropriate controls on the I
R, = YH , + H 2 + R 3 output generator. See Fig. I-3

In the square wave mode, the duration is automatically
set to approximately 50% of the period setting.
Output Levels
The output amplitude and offset are selected by
independent pulse HIGH LEVEL and pulse LOW LEVEL
controls. Use the front panel voltage calibration marks
when the load resistance is 50 ti. and the back termination
is not. used. The output voltage is one half of the voltage
indicated by thedial calibration when the back termination
is used. The OUTPUT (VOLTS) controls are interlocked 50
that it is impossible to set the HIGH LEVEL control more
negative than the low level. It is also impossible fo set the
controls for more than about 5.5 V P-P outpuf amplitude
into 50 Il. Pulse amplitude always equals the pulse high
level minus the pulse low level. Offset may be the high level
or the low level, whichever is used as rhe baseline reference
level. The flexibility of this method of controlling the
output amplitude and offset is useful in certain applications
such as logic testing, i.e.. cither the hiqh or low level can be
varied wifhout disturbing the other.
Instructions-PG 502
OPERATI NG MODES
Period and Duration Selection
The period generator free runs at the rate 5et by the
PERIOD selector and the PERIOW VARIABLE control in
all modes except SO WAVE and EXT DURATION. The
duration of the output pulse is set by the PULSE
DURATION selector and its associated PULSE DURA.
TION VARIABLE control. The PERIOD and PULSE
DURATION selectors are mechanically coupled, so the
duty factor cannot exceed 50% with the VARIABLE
controls in the Xl positions. Under most circumstances.
duty factors far in except of the specified 50% may be
obtained in the NORM mode using the VARIABLE
controls. Excessive duty factor is indicated by any of the
following pulse abnormalities: (1) pulse output period in
multiples of the trigger output period, and (2) alternate
pulses with durations less than the pulse duration setting.
Wuty factors approaching 100% may be obtained by
switching to the complement mode. Set the PULSE
DURATION control for a pulse width equal to thedesired
pulse off time and push the front panel COMPLEMENT (-1
pushbutton.
Use of the Inormal complement function allows inter-
changing the pulse on-off times without varying the voltage
ICVPIS.
External Trigger
The period generator is disabled when the PERIOD
selector is in the EXT TRIG position. An external
positive-going signal applied to the TRIG/DURATION IN
connector, triggers the duration generator. The pulse
duration of the output pulse varies with the front”panel
Pulse DURATION selector and VARIABLE CQtltlOl. The
period of the output waveform is the period of the
triggering signal. See Fig. l”4. The external trigger signal
must remain above the recognition threshold for at lwst
2 ns. It must also remain below the reset threshold for at
least 2 ns to reset the generator for the next trigger.
Entwnal Trigger Signal
A manual trigger is available for single pulse operation.
Disconnect any external trigger input when not in use.
External Duration
The period generator and duration generator are disabled
when the DURATION selector is in the EXT DURATION
position. A voltage exceeding the recognition level applied
to the TRIG/OURATION INPUT connector will activate
the outpuf of the PG 502. The period and duration of the
output will depend on the period and duration of the
externally-applied voltage. See Fig. 1.5. When operating in
this mode, the output of the PG 502 is activated as long as
the MAN TRIG pushbutton is depressed
1-5

Operating Instrucfions-PG 502
Refer to the rear connector assignment illustration in the
Service Section at the rear of this manual for suggested pin
assignments. These connections are not factory wired.
To obtain a trigger out signal complementary to the
front panel trigger out pulse, connect one end of a coaxial
cable to the pads on the Timing Board marked Internal Trig
Out. Connect the other end to appropriate pins as shown in
the illustration. Connections made to the Infernal Trig Out
pads do not +erfere with the front panel f TRIG OUT
signals. A one-half volt signal into 50 fi is available at the
Internal Trig Out pads.
To, obtain the + TRIG OUT signal at the rear interface
connector, disconnect the coaxial cable from the front
panel + TRIG OUT connector and the coaxial connector
labeled Trig, Out on the Timing Board. Replace this cable
with another 500 cable about ten inches long, with a
similiar coaxial ~nnector on one end. Solder the other end
to the holes located as shown in the illustration.
To obtain Trigger/Duration input capabilities at the rear
interface connector, remove the coaxial cable from the
front panel connetior and the coaxial corm&or on the I
Timing Board. Use a new piece of coax about ten inches
long with suitable connection. Solder the free end of this
cable to the rear interface pads located as shown in the I
illustration.
Remember, when planning to use the rear interFace I
connectors, pulse fidelity may be disturbed due to the
impedance mismatch the signals are subjected to in passing
through the connectors. I
A slot between pins 23 and 24 on the rear connector
identifies the PG 502 as a member of the signal source
family. A barrier may be inserted in the corresponding
position of the power module jack to prevent other than
signal source plug-ins from being used in that compartment.
This protects the plug-in should specialized connections be
made fo that compartment. Consult the Building A System
Section of the power malule manual for further informa-
tion.
DEFINITIONS OF PULSE CHARACTERISTICS
The following is a glossary of common pul~l characteris-
tics used in this manual. They are illustrated in Fig. 1.6. Amplitude. The maximum absolute peak value of a
pulse measured from the baseline regardless of sign, and

Operating Instructions-PG 502
t--
Pulre Period
T = ,/Rate
Duly Facto* = Pulse DurationlPulsa Period
excluding unwanted aberrations or overshoot. Measurement
points are at 50% of the pulse duration time (pulse high
IevelJ and on rhe baseline (pulse low level) at 50% of rhe off
time (the pulse period minus the pulse duration).
Aberrations. Unwanted deviations or excursions in the
pulse shape from an ideal ~uare corner and flat top, i.e..
overshoot. undershoot or rounding, ringing, and tilf or
dope.
Baseline. The quiescenf DC voltage reference level of
the pulse waveform,
Complementary Pulse. Normal pulse with high and low
levels inrerchanged. Pulse on-time becomes pulse off-time.
Duty Factor. Sometimes referred to es duty cycle. The
ratio of pulse duration to period, or the product of pulse
duration and pulse repefifion rate. Duty fector in % =
Duration/Period X 100.
Falltime. The time interval. et rhe pulse trailing edge,
for the pulse amplitude to fall from the 90% amplitude
level to the lOPA amplitude level.
Flatness. The absence of long rerm variations to the
pulse top; excluding overshoot. ringing or pulse rounding.
Sometimes referred to a5 tilt or slope.
8
High Level. The most positive value of a pulse, regard-
less of unwanted aberrations or overshoot, measured at a
point that is located at 50% of the pulse duration.
Low Level. The most negative value of a pulse. regard-
less bf unwanted aberrations or overshoot, measured at a
point fhat is at 50% of the off time.
Offset. A DC potential of either polarity applied to the
waveform to bias the baseline to an amplitude other than
zero.
Overshoot. The short term’pulse excursion (0; tran-
si@nT) above the pulse top or below the baseline, which is
simultaneous to the leading or trailing edge of the pulse.
Period. The time interval for B full pulse cycle. inverse
of frequency or repetition rate, or the interval between
corresponding pulse amplitudes of two consecutive un.
delayed or delayed pulses. Generally measured between the
50% amplitude levels of two consecutive pulses.
Preshoot. A transient excursion rhat precedes the Step
function. II may be of the same or opposite polarity as the
pulse.
Pulse Duration. The time interval between the leading
and trailing edge of a pulse at which rhe instantaneous
amplitude reaches 50% of the peak pu,lse amplitude.
1-7