Tektronix 1L40, Instruction Manual

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Circuit Description— Type 1L40
so approximately 0.5% of the signal w ill be applied to
V620. W ith a 150 mV signal, the dynamic resistance of the
diode decreases to approximately 5 kfi, so approximately
10% of the signal w ill be applied to V620. The circuit nor­
mally requires about 70 mV of signal for full screen display
so the diodes usually operate along the steep slope of the
dynamic resistance curve.
This non-linear dynamic resistance of the divider produces
a display which emphasizes small signal level differences.
The vertical response of the SQ LAW display is approxi­
mately proportional to the signal power.
The input signal from the VERTICAL DISPLAY selector is
coupled through T610 to the 5 MHz crystal filter. The series
response characteristic of the crystal determines the band­
pass of this filter circuit. C610 is adjusted to shunt or reduce
the parallel response point of the crystal filter. L620 and
C620 are tuned to 5 MHz. The circuit shapes the response of
the 5 MHz IF and attenuates any spurious signals that may
pass through or are generated in the 5 MHz variable resolu­
tion circuit.
V620 is a high gain am plifier driving the emitter follower
Q650. This circuit provides the voltage gain and drive re­
quired by the detectors for both the video and recorder
output. L624 tunes the plate circuit to 5 MHz.
The detector diodes D660 and D661, connected as a volt­
age doubler circuit, provide the 40 dB dynamic range for
the log display. D657 is the detector diode for the RECORD­
ER output.
The log circuit consisting of R664, D664, D665, R665 and
the Log Cal adjustment R666, provides a display that
approaches 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 am pli­
tude further increases, the diode current approaches the
linear region of the voltage-current characteristic curve; how­
ever, this current through R665 develops sufficient voltage
across D665 to turn this diode on, and the two diodes now
operate in series to extend the range of the Log circuit to at
least 40 dB.
Video Filter switch SW661 switches capacitor C661 across
the detector output to restrict the video bandwidth. This pre­
vents high frequency components from distorting the display
and enables easier evaluation of signal modulation when
viewing signals with minimum resolution bandwidth.
The VIDEO position of the VERTICAL DISPLAY selector con­
nects the external video INPUT connector through the G A IN
control to the vertical am plifier input of the plug-in oscillo­
scope. G A IN control R411B, ganged with R411A in the
narrow band amplifier, provides the gain adjustment for all
positions of the VERTICAL DISPLAY switch.
The DC reference level of the signal into the vertical ampli­
fier of the oscilloscope is set by the POS control R672. W ith
the control centered, the output DC level is approximately
67.5 volts.
The LOCK CHECK switch SW889 connects the output signal
and DC reference of the phase lock circuit to the oscilloscope
vertical input. This permits the beat signal display (when
phase lock operation is set) and the DC output level to be
viewed on the CRT screen.
Power Distribution; + 1 0 Volt, — 10 Volt and
Local Oscillator Heater Supplies
+ 10-Volt Supply. This is the reference voltage for the
— 10-volt supply and the positive supply for the semiconduc­
tor circuits in this instrument. A voltage divider circuit
between the -f- 225-volt supply and ground sets the + 10-volt
voltage for the base of Q710. Any change in the q-10-volt
supply is reflected as a bias change to Q710. This is am pli­
fied and applied as an error signal to the base of Q717 to
increase or decrease the current through the transistor to the
- f 10-volt supply.
— 10 -Volt Supply. Voltage divider R720-R721, between
the + 1 0 -v o lt supply and the — 10-volt bus, sets the bias of
Q720. Any deviation in the — 10-volt supply is amplified by
Q720 and applied as a corrective signal to Q727, to in­
crease or decrease the current through the emitter load resis­
tor R727 to the — 150-volt supply and counteract any load
change in the — 10-volt supply.
Local Oscillator Heater Supply
The 10-volt filament or heater supply for the oscillator
tube (V42 see RF diagram) is provided by the +75-volt, 50
mA supply via voltage drops through R49 and the heater of
V620, with the - f 75-volt supply aided by the - f 100-volt sup­
ply furnishing 13 mA.
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