Tektronix 1L20, Инструкция по эксплуатации

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Circuit Description— Type 1L20
Fig. 3 -3 . Typical impedance vs. frequency graph of a crystal. Note
the series-and-parallel-resonance points.
to a sharp peak (b) at the parallel-resonance frequency.
Then the impedance drops fairly abruptly because of the
parallel shunt-capacitance of the crystal and its mounting.
If this parallel capacitance is cancelled, the impedance of
the crystal exhibits the impedance-versus-frequency curve
shown in Fig. 3-4.
lightly loaded, the frequency response of the circuit w ill be
much broader than the response of the crystal alone, giving a
wide bandpass (low resolution) display. When the RESOLU­
TION control SW550 is turned counterclockwise (toward
+ 100 volts), the increase in forward bias on D506 increases
the loading on the series resonant circuit, thus lowering the
Q of the L508-C508 tank circuit. As a result, the crystal's
sharp cutoff characteristic becomes the determining factor,
resulting in a high-resolution display.
Y50I
:C508
Fig. 3-4 . Impedance vs. frequency graph of a crystal when para­
llel capacitance is cancelled.
This cancelling of the shunt capacitance of the crystal
and its holder is accomplished by the series capacitance
C504 (see Fig. 3-5). Since the voltage at the bottom end
of the secondary of T464 (Fig. 3-5) is opposite in phase to
the voltage on the top end, the capacitive reactance intro­
duced by C504 will directly subtract from the shunt Xc of
the crystal. Hence, with C504 properly adjusted, Y501
exhibits no parallel resonance and assumes the impedance-
versus-frequency curve of Fig.3-4.
Variable Resolution Filter
The resolution of the spectrum display is determined by
the series resonant circuit of Y501, L508, and C508. When
Fig. 3 -5 . Shunt-capacitance cancelling circuit for Y 501 .
R543 is adjusted to set the resolution a t 100 kHz with the
RESOLUTION control at the clockwise end of its rotation.
The other positions o f the RESOLUTION control are not
calibrated, although the resistance of each step o f the vo lt­
age divider has been chosen so that the resolution o f the
display w ill be near optimum for most displays when the
RESOLUTION control is coupled to the DISPERSION control.
Since the variable resolution circuit cannot be coupled
directly to a low-impedance load, it is followed by emitter
followers Q510 and Q520 to isolate the high impedance o f
the fitter from the relatively low input impedance o f the
following am plifier Q530.
V ariab le Resolution Amplifier
Q530 is a common emitter amplifier which provides the
increased signal amplitude required to operate the LOG
and SQ LAW circuits of the Output Amplifier,
Output Am plifier
5 MHz Am plifier and Detectors
The signal from the Variable Resolution Amplifier is con­
nected to the input section of the Vertical Display switch
SW660.
In the LOG position of SW660, there is no attenuation of
the signal in the input sectjon of the amplifier. This straight-
through coupling of the signal provides the full 40 dB dyna­
mic range required for LOG display.
®
3 -5