Tektronix 1L10, Инструкция по эксплуатации

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Theory of Operation— Type 1 LI 0
The sawtooth voltage from the voltage divider is applied
to the base o f emitter follow er Q340. The emitter follower
has a high input impedance and thus, does not significantly
load the voltage divider. Output of the emitter follow er is
coupled to the base of Q341. Q341 forms one half of a
difference amplifier. The purpose of the difference am pli­
fier w ill be described a little later. For now, consider that
the output of the difference amplifier is developed across
R354 and applied to the voltage variable capacitance diode
D362 through R355.
The sawtooth voltage appearing across D362 causes its
capacitance to change in an amount proportional to the
sawtooth voltage. D362 forms part of the capacitance of
the tank circuit (L364) o f the Sweep Frequency O scillator
Q360. The amplitude o f the sawtooth voltage applied to
the capacitance diode D362 determines the frequency
change of the Sweep Frequency Oscillator. Output of the
Sweep frequency O scillator passes through C398 and mixes
with the 10.7-mc signal from the W ideband Amplifier. Center
frequency of the Sweep Frequency Oscillator is 11.5 me. This
gives a beat frequency o f 800 kc.
Part of the output o f the Sweep Frequency O scillator is
also coupled back through a closed loop circuit that in­
cludes an amplifier, discriminator and the other half of
the difference amplifier. This closed-loop system corrects
for the inherent non-linearity of the voltage variable capaci­
tance diode D362. This is accomplished as follows: The
sweep frequency is amplified by the R-F am plifier Q370
and coupled to the Discriminator circuit Y380. The Dis­
criminator converts the sweep frequency back into a saw­
tooth signal. The sawtooth signal derived by the Discrimina­
tor contains any non-linearity that was introduced by the
voltage variable capacitance diode D362. This sawtooth
signal is coupled back to the difference amplifier through
the emitter follow er Q350. Assuming that the oscilloscope
sawtooth voltage on the other side of the difference am pli­
fier is linear, only the non-linear portion of the sawtooth
will be amplified and coupled to the voltage variable ca­
pacitance diode. The result is that a non-linear driving
voltage appears on D362. This non-linearity is such that
it is equal and opposite to non-linear characteristics of the
voltage variable capacitance diode. This, in turn, forces the
Sweep Frequency O scillator to produce a linear sweep fre­
quency.
The mixed output of the Swept l-F Oscillator and the
W ideband Am plifier is coupled through an 800-kc filter
and then to an amplifier stage (Variable Resolution A m pli­
fier schematic).
Variable Resolution Amplifier
The Variable Resolution Am plifier schematic contains an
800-kc filter, a feedback stabilized amplifier (Q500 and
Q510), a 900-kc oscillator and a variable bandwidth cir­
cuit.
The filter circuit (L405, L410, L415, etc.) allows only the
passage of the 800-kc signal. Output of the filter is coupled
to a feedback stabilized amplifier consisting of Q500 and
Q510. The 800-kc output of the amplifier is mixed with
the output of the 900-kc oscillator giving a beat frequency
of 100 kc.
The 900-kc oscillator is a stable crystal-controlled os­
cillator. Amplitude of the oscillator is peaked with L454.
In the SEARCH position of the COUPLED RESOLUTION
switch the Variable Resolution circuit (Q520 through Q560)
is bypassed. In all other positions of the switch the 100-kc
signal passes through the Variable Resolution Amplifier.
The first stage (Q520) of the Variable Resolution Am pli­
fier is a conventional amplifier with the output from the
collector applied to the base of Q530 through coupling ca­
pacitor C524.
Fig. 3 -2 . Typical impedance versus frequency graph of a crystal.
Note the series- and parallel-resonance points.
Q530 and Q540 form a bandwidth limiting circuit. Band­
w idth o f the circuit is set by the amount of forward bias
on D548. To understand how this circuit operates, first con­
sider the impedance characteristics o f a crystal (such as
Y530 in the collector circuit of Q530). Fig. 3-2 shows a
typical impedance versus frequency curve o f a crystal. In
examining the curve, from left to right, we first encounter
a very low impedance point at the series-resonant frequency
point. At some higher frequency, the impedance increases—
this is the parallel-resonance point. W ith a still higher fre­
quency, the impedance drops fairly abruptly because of the
inherent parallel capacitance o f the crystal mounting.
Fig. 3-3 . Impedance versus frequency curve of a crystal when the
parallel capacitance is effectively cancelled.
3 -3