Tektronix 585 - Page 46

Circuit Description — Type 585
Unblanking
As mentioned previously, dc-coupled unblanking is accom 
plished by employing separate power supplies for the grid
and cathode of the crt. Unblanking pulses from the Time-
Base Generators control the crt grid through cathode fol 
lowers V183B, V293A and the floating grid supply. (V183B
is diagrammed with the Time-Base Generator A and V293A
is diagrammed with the Time-Base Generator B).
At the fastest sweep rates the stray capacitance of the
floating crt grid supply makes it difficult for the crt grid
to rise fast enough to unblank the crt in the required time.
An isolation network composed of R827, R828 and C828,
C829 isolates that capacitive loading. By this arrangement
the fast leading edge of the unblanking pulse is coupled
through C828 and C829 to the grid of the crt. For short
duration unblanking pulses, at the fastest sweep rates, the
power supply itself is not appreciably affected.
Longer unblanking pulses at slow sweep rates, charge the
stray capacitance in the circuit through R827. This pulls
up the floating supply and holds the crt grid at the un 
blanked potential for the duration of the unblanking pulse.
Amplitude Calibrator
The AMPLITUDE CALIBRATOR is a square-wave generator
producing an output at approximately 1 kc which is avail 
able at the front panel CAL. OUT connector. Multivibrator,
V875 and V885A, is connected to switch the cathode fol 
lower, V885B, between cutoff and conduction.
During the negative portion of the multivibrator output
waveform the grid of V885B is driven well below cutoff
and its cathode rests at ground potential. During the posi 
tive portion of the waveform V875 is cut off and its plate
rests slightly below + 100 volts. The voltage at the plate
of V875 and grid of V885B when V875 is cut off, is deter 
mined by the setting of the CAL. ADJ. control R879.
Cathode-follower V885B has a precision, tapped divider
for its cathode resistor. When the CAL. ADJ. control is
properly adjusted, the cathode of V885B is at +100 volts
when V875 is cut off. By means of the tapped divider R885
through R893 and a second 1000 to 1 divider R896-R897,
output voltage steps are available from 0.2 millivolts to
100 volts. C885, connected between the cathode of V885B
and ground, corrects the output waveforms for a slight
overshoot.
A 0.25 ohm resistor located between the CAL. OUT coax
connector and ground is approximately equal to the resist 
ance of the braid of a 42 inch long RG-58A/U coax cable.
Its purpose is to cancel any coax braid ground current effects
on calibrator voltage accuracy that may exist when the Type
585 AMPLITUDE CALIBRATOR is employed as a signal
source between the oscilloscope and some other instrument
chassis. The ground currents in this case are usually de 
veloped in the ac power line third wire grounding system
when the Type 585 and the other instrument chassis are
supplied from different convenience outlets.
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