Tektronix 1L30, Instruction Manual

Page: 31 / 129

Circuit Description— Type 1L30
|
Input strobe
pulses
Ci Di
Fig. 3-3. Simplified phase detector circuit.
j| are of equal and opposite polarity, the resultant voltage
will approximately equal the instantaneous (or sampled)
4 oscillator voltage. The capacitor C3 (Fig. 3-3) charges to
| the sampled instantaneous voltage. This voltage is applied to
I a DC amplifier then to the varactor to give correctional
I control.
1 . As the local oscillator frequency approaches a harmonic of
I the reference frequency, an AC voltage or beat frequency is
| developed at the detector output. This AC signal is amplified
I by Q860, and when the LOCK CHECK button is depressed,
■| it is applied to the vertical amplifier so the operator can ob-.
I serve these beat indications. At the zero beat null, the output
;; signal amplitude snaps to a minimum trace on the screen to
indicate to the operator that a phase lock condition exists.
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
I FREQ control so that it no longer has an effect on the oscil-
? lator frequency. If the control is moved toward its extreme
positions, the circuit will lose control. The resultant jump
3 in frequency is easily seen at dispersions of 50 kHz/cm or
i less.
A
1
Sweep Circuit
] The sweeper circuit provides a constant amplitude swept
1 frequency band, centered, at 275 MHz, to the wide band
amplifier mixer section. The frequency deviation of the
I
swept frequency output can be varied from approximately
0 to 10 M Hz/cm (100 MHz total). A block diagram of the
sweeper circuit is shown in Fig. 3-4.
3 - 4
■4
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 a selector
switch SW201 on the back panel of the instrument switches
in additional attenuation so the amplitude of the voltage
to the comparator Q 230-Q 240 is approximately the same for
either the 100 V or 150 V input sawtooth amplitude. This
sawtooth voltage is applied to the attenuation network of
the DISPERSION switch SW220.
Sweep Center adjustment R204 sets the dynamic operating
ra/ige of the comparator Q 230-Q 240 and shifts the DC. level
of the output current ramp to the swept oscillator. DISPER­
SIO N CAL adjustment R208 calibrates the dispersion for the
10 M Hz/cm position of the DISPERSION selector. It adjusts
the output amplitude of the current ramp at the collector of
Q240. W ith the 10 M H z/cm position calibrated, the re­
maining positions of the DISPERSION selector wiil be within
the calibration specifications listed in the Characteristics
section.
Two dispersion ranges (M H z /C M and kH z/C M ) are pro­
vided by the DISPERSION RANGE selector SW230.
Sweep Comparator. The sweep comparator Q230-Q240
output voltage is dependent on the differential amplitude be­
tween the ramp signal applied to the base of Q230 and the
ramp signal applied to the base of Q240. The ramp signal
that is applied to Q240 is the resultant voltage output from
a frequency discriminator that is sampling the sweep oscil­
lator output frequency.. The signal voltage from the sweep
comparator is applied as a bias signal to the capacitance
diode D314, changing the capacitance o f the circuit to tune
the oscillator through the frequency dispersion range in
synchronization with the horizontal sawtooth voltage that is
applied to Q230.
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