Tektronix 1L40, Инструкция по эксплуатации

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Circuit Description— Type 1L40
FREQ control provides approximately 0.1 % (or 1 kHz) shift
in the 1 MHz reference frequency and pulls the local
oscillator fundamental frequency by the same percentage.
If the input signal should shift off screen, it can be returned
to the screen by adjusting the INT REF FREQ control.
The phase detector (Fig. 3-4) consists of a two diode
gate and a low-pass filter. The combination of the local
oscillator signal and the very narrow pulses applied to
the opposite electrodes of the diodes, gates the diodes
on and thereby samples the amplitude and phase of the
oscillator signal.
The voltage at the junction of the two resistors w ill be
the sum of the strobe pulse and the instantaneous value of
the oscillator signal voltage.
However, since the strobe
pulses are of equal and opposite polarity, the resultant
voltage is approximately equal to the instantaneous signal
voltage. Capacitor C3 (Fig. 3-4) charges to the sampled
instantaneous voltage and applies this potential to the input
of a DC amplifier.
If the input signal from the local oscillator is not a
harmonic of the reference frequency, the output voltage of
the phase detector w ill be approximately zero. As the
local oscillator frequency approaches a harmonic of the
reference frequency, an AC voltage or beat frequency is
developed at the detector output. This AC signal is am pli­
fied by Q880, and when the LOCK CHECK button is
depressed, it is applied to the vertical am plifier so the
operator can observe these beat indications. A t the zero
beat null, the output signal amplitude snaps to a minimum
on the screen to indicate to the operator that a phase lock
condition is set.
The FINE RF CENTER FREQ control R862 tunes the local
oscillator by changing the DC output level of Q870. When
a phase lock condition exists, the phase lock circuit counter­
acts any DC voltage shift applied by the FINE RF CENTER
FREQ control so that it no longer has an effect on the
oscillator frequency. If the control is moved toward its
extreme positions, the circuit w ill lose control. The resultant
jump in frequency is easily seen at dispersion of 500 kHz/cm
or less.
The sample of the oscillator input signal is not instantan­
eous. The strobe pulses have some finite w idth or duration.
The limits of the strobe width are determined by the
highest input frequency from the local oscillators. The phase
detector operates on the positive or negative slope of the
input signal. The slope ( + or — ) depends on the total
phase inversion between the phase detector output and
the voltage input of the oscillator. The width, therefore,
must be less than '/2 the period of the maximum input
frequency. In the Type 1L40, the maximum input frequency
is 4.2 GHz (period ~ 0.24 ns), which means the pulse width
must be less than 0.12 ns.
This fast-rise, narrow pulse is obtained by differentiating
a snap-off diode recovery step. The crystal oscillator or
reference signal drives a pulse generator which drives the
snap-off diode. The snap-off diode supplies pulses to an
etched circuit transmission line transformer. This transformer
couples the single ended differentiated pulse to the phase
detector as a push-pull differential signal to gate the diode
phase detector on and sample the oscillator signal.
The pulse generator consists of tunnel diode D826, transis­
tors Q820 and Q840 and associated circuit components.
The quiescent current of tunnel diode D826 is approximately
2.5 mA. The positive-going portion of the input signal
switches the TD to its high state and a fast-rise positive
pulse is generated at the base of Q820. The pulse is am pli­
fied and differentiated by the short RC time constant in
the emitter of Q820, then transformer coupled through T820
to Q840.
The quiescent voltage of Q840 is set by the Avalanche
Volts adjustment R831 in the base voltage divider circuit
of Q830. The positive portion of the pulse from trans­
former T820 triggers Q840 into avalanche. The resulting
collector current of Q840 sweeps out the stored charge
of the snap-off diode D846. When the charge has dissi­
pated, the recovery pulse of the diode generates a fast
negative-going step . This negative-going pulse is differen­
tiated and coupled through C844 and C847 to the etched
circuit transmission line transformer T856.
Transformer T856 provides a 2:1 voltage step-up and
converts the single-ended input signal to a push-pull
balanced output signal which is applied to phase detector
diodes D856 and D857. Refer to the swept oscillator
description for the discussion on the transformer operation.
Sweeper Circuit
The sweeper circuit provides a constant amplitude swept
frequency band, centered at 275 MHz, to the wide band
am plifier mixer section. The frequency deviation of the
swept frequency output can be varied from approximately
0 to lO M H z/cm (100MHz total). A block diagram of the
sweeper circuit is shown in Fig. 3-5.
Dispersion Circuitry. The sawtooth voltage from the
oscilloscope is connected to the analyzer SWEEP INPUT
connector by an external jumper cable. If the sawtooth
amplitude is 150 V, selector switch SW201 on the back panel
of the instrument switches in additional attenuation so the
amplitude of the voltage to the comparator Q230-Q240 is
approximately the same for either the 100 V or 150 V input
sawtooth amplitudes.
This sawtooth voltage is applied
through the attenuation network of DISPERSION switch
SW220.
The Sweep Center adjustment, R204, sets the DC reference
level at the base of Q230 which sets the quiescent output
level at the collector of Q240. The output level of Q240 is
applied to D314, the frequency determining component of
the swept oscillator circuit. The input sawtooth voltage to
the sweep circuit is a positive voltage ramp which sweeps
the base of Q230 above its DC reference level. The am pli­
tude of this voltage ramp is a function of the DISPERSION
selector position. The DISP CAL adjustment R208 calibrates
the dispersion for the lO M H z/cm position of the DISPER­
SION selector. The DISPERSION RANGE selector SW230
adds or deletes selected attenuation steps for the input saw­
tooth voltage to the comparator. These attenuation steps
are changed by a factor of 10 by the DISPERSION RANGE
switch to provide M H z/C M and kHz/CM dispersion selections.
Sweep Comparator. Q230 and Q240 are connected as
a differential or comparator am plifier with their emitters
connected to the common resistor R236 which connects to
3-5