Tektronix 1L30, Instruction Manual

Page: 37 / 129

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Circuit Description— Type 1L30
the LOG display. D657 is the detector diode for the RE­
CORDER output. The emitter follower Q650, is longtailed
through R653 to the — 150 volt supply, and provides a con­
stant DC output level, to the vertical amplifier for the plug-in
oscilloscope. This maintains minimum baseline shift when
the VERTICAL DISPLAY selector is switched between its
positions.
The log circuit consisting of R664, D664, D665, R665 and
the Log Cal adjustment R666, provides a display that ap ­
proaches a logarithmic curve when the VERTICAL DISPLAY
selector is in the LOG position.
Low amplitude video signal voltages appear across D664
with little or no attenuation. As the signal amplitude in­
creases, the current through the diode becomes an exponen­
tial function of the voltage across the diode. R664 becomes
the current source for the diode, so the voltage output of
the
circuit becomes a logarithmic function. As the signal
amplitude further increases, the diode current approaches
the
linear region of the voltage-current characteristic curve;
however, this current through R665 develops sufficient volt­
age across D665 to turn this diode on, and the two diodes
now operate in series to extend the range of the Log cir­
cuit to at least 40 dB.
Video Filter switch SW661 switches capacitor C661 across
the detector output to restrict the video bandwidth. This
prevents high frequency components from distorting the dis­
play and enables easier evaluation of signal modulation
when viewing signals, with minimum resolution bandwidth.
The VIDEO position of the VERTICAL DISPLAY selector
connects the external Video INPUT connector through the
G A IN control to the vertical amplifier input of the plug-in
oscilloscope. The G A IN control R411B ganged with R411A
in the narrow band amplifier, provides one control to change
the gain for all positions of the VERTICAL DISPLAY switch.
The DC reference level of the signal into the vertical
amplifier of the oscilloscope is set by the POS control R672.
With the control centered, the output DC level is approxi­
mately 67.5 volts.
The LOCK CHECK switch SW889 connects the output sig­
nal and DC reference of the phase lock circuit to the ver­
tical input so the beat signal display when phase, lock opera­
tion is set, and the DC output level may be viewed on the
CRT screen.
Power Distribution; + 1 0 Volt and — 10 Volt
Supplies
+ 10 Volt.
This is the reference voltage for the — 10
volt supply, the voltage source for the local oscillator heat­
ers and the positive voltage supply for the semiconductor
circuits in the Type 1L30. Reference voltage for Q710 is
set by the voltage divider R710-R711, between the regulated
+ 225 volt- supply and ground. The collector voltage of
Q710 controls the current through the series regulator Q717
to the + 1 0 0 volt supply from the oscilloscope.
— 10 Volt.
The — 10 volt supply is referenced to the
+ 10 volts through the divider R720-R721, which sets the
forward bias of Q720. Q 720 controls the forward bios of
the series regulator Q 727 which regulates the current through
R727 to the — 150 volt source of the oscilloscope.
instruments prior to serial number 670 used - f 350 as the reference
voltage.
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