Tektronix 1L20, Instruction Manual

Page: 22 / 117

Circuit Description— Type 1L20
In the LIN position of the switch, the R606-R607 voltage
divider attenuates the input signal by approximately 3 to 1
so that a signal giving a full-screen display in LIN position
will continue to give a full-screen display when the switch
is moved to one of the other positions.
In the SQ LAW position, the signal is coupled through
two germanium diodes (D603 and D604) connected back
to back and in series with R610 to form a voltage divider.
To illustrate the action of the SQ LAW divider, Fig. 3-6A
shows the current-versus-voltage characteristic curve for the
back to back diodes. Fig. 3-6B shows the diodes dynamic
resistance curve derived from Fig. 3-6A. Note that for very
low m illivolt signals, the diode resistance exceeds 100 kC2.
As a result, the divider ratio of 100:1 at 60 millivolts will
pass about 1% of the signal; at 160 millivolts the divider
ratio w ill be about 2:1 and 50% of the signal will pass. Since
the circuit w ill normally operate with signal levels about 70
millivolts for a full-screen display, the diodes w ill usually be
operating along the steep portion of the resistance curve.
The non-linear dynamic resistance of the diodes results in
a display that emphasizes small differences between similar
signals. The vertical response of the SQ LAW display will
be approximately proportional to the signal power.
In the VIDEO position of SW660, the spectrum display
signal is grounded to prevent it from interfering with a
signal connected to the VIDEO INPUT connector.
The signal from the VERTICAL DISPLAY switch is coupled
through T610 to the 5 MHz crystal filter Y610, which is
similar in action to Y501 with C610 cancelling out the shunt
capacitance around Y610. However, unlike Y501, Y610
operates with a fixed bandpass. Filter C620-L620 helps shape
the 100 kHz response and suppresses spurious responses
generated by Y610.
V620 and Q650 further amplify and isolate the 5 MHz
signal before it is detected by D657 for the TO RECORDER
output and by D660-D661 for the CRT display.
Video Filter
The VIDEO FILTER switch SW661 places C661 across the
output of D660 to limit the high frequency response o f the
detector circuit.
Vertical Display Switching
In the LIN and SQ LAW positions of the VERTICAL DIS­
PLAY switch, the signal is attenuated 2:1 by R662 and R663
before being connected through Pin 1 o f the output con­
nector P ll to the Vertical Amplifier circuits of the oscillo­
scope.
In the LOG position, the signal also passes through an
attenuator before going to the output connector. The LOG
attenuator is made up of R664 in series with silicon diodes
D664 and D665. The silicon diodes in this circuit behave
close to the ideal that voltage differences are proportional
to the logarithm of the current applied. R664 converts the
detector input voltage to current for this circuit. R665 helps
suppress the small current-voltage offset o f the silicon diodes
to make the attenuator more nearly logarithmic. R666 (Log
Cal) sets the maximum amplitude of the LOG display to
slightly exceed the graticule limits.
In the VIDEO position, the VERTICAL DISPLAY switch
connects the VIDEO INPUT connector to the output connector
P ll through C668 and G A IN control R411B.
& # & ^ ^
VOLTAGE IN M.V.
n<£
VOLTAGE IN M.V.
Fig. 3 -6 . Characteristic Curves for 1N 64 Diodes:
(A .) Voltage vs. Current;
(B.) Voltage vs. Dynamic Resistance.
Vertical Positioning
The spectrum display and the VIDEO INPUT circuits are
isolated from DC ground by the coupling capacitors C660
and C668. This allows the POSITION control network to
set the DC reference level of the output circuits.
Lock Check Circuit
The LOCK CHECK switch SW889 allows the output of the
phase-lock circuitry to connected to the input of the
vertical amplifier so the beat frequency signal may be viewed
directly on the CRT.
3 -6
®