Tektronix 511A-AD, Instruction Manual, Page: 70 / 75

SECTION VII
TEKTRONIX TYPE 1-AD-25
AND TYPE 1 -D-25 *
DELAY NETWORK
GENERAL DESCRIPTION
The
Type l-AD-25
Video
Delay Network is
designed
for use in the video
amplifier
of the Tektronix Type 511-A
Oscilloscope. It provides a signal delay of .25 micro ­
second, thus permitting the CRT to be unblanked and
the sweep to be operating linearly, before the initiating
signal reaches the vertical deflection plates. By this
method, random pulses may be observed.
The Type 511-AD Oscilloscope includes a factory in­
stalled Type l-AD-25 Delay Network.
The Type l-AD-25 consists of 23 M-derived sections.
By using M=1.27, very uniform time delay is obtained
for frequencies well above the cut-off of the Type 511-A
output amplifier. To obtain smooth impedance match­
ing between sections of the network, trimming capacitors
are provided. These are adjusted at the factory and
should not require resetting in the field.
The output
termination
consists of
the
Vert. Ampl.
Atten. potentiometer, shunted by R2
to match the char ­
acteristic impedance of the network. To make this termi­
nation less critical, the network is partially matched at
the input by the 270 ohm resistor R1 in series with the
internal impedance of the cathode follower VI 7. This
input matching produces an insertion loss of 3 db., but
is necessary since the impedance of the output termina­
tion varies slightly
with the
setting of
the
Vert. Ampl.
Atten. potentiometer.
When the delay network is not needed, it may be re­
moved from the circuit by means of a rotary switch
operated from the front panel.
ADJUSTMENT AND MAINTENANCE
If the Vert. Ampl. Atten. potentiometer R107 or the
limiting resistor R108 are replaced, it may be necessary
to change the delay network shunt resistance so that the
correct termination is maintained. To check for proper
termination,
set the sweep
speed at
3
to
10
microseconds
and observe the response to a clean, sharp, square
wave or pulse having a rise time of .03 microseconds or
less. The Tektronix Type 104 Square Wave Generator
operating at 100 kc. provides such a signal.
If the termination is incorrect, the first .5 microsecond
of the pulse will be lower or higher in amplitude than the
rest of the flat portion. If the first part is higher, shunt
the high side of R1 07 to ground with a resistor of such
value that a level top on the square wave or pulse is
obtained.
In case the first portion is lower, it will be necessary to
increase the value of the shunt resistor. Remove the
delay network cover and install a ½ watt resistor of
suitable value (1,000 to 10,000 ohms), in series with R2.
CAUTION! Do not disturb the adjustment of the variable
capacitors.
Fig. 56. The same 0.2 miscrosecond pulse with the Type
l-AD-25 delay network switched in, delaying
the appearance of the pulse until the CRT is un ­
blanked and the sweep operating linearly.
Fig. 55. 0.2 miscrosecond pulse with the delay network
switched out. Note absence of leading edge.
*The Type l-D-25 Video Delay Network designed for use in the Type 511 Oscilloscope is the same as the Type
l-AD-25 except that switch SI is replaced by relay RL1.
SECTION 7, PAGE 1