Tektronix 491, Maintenance Manual

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Band B:
The oscillator for band B is similar to band A
oscillator. The fundamental frequency range 470 MHz to 1100
MHz and the 2nd harmonic of the oscillator is used for the
frequency range 270 MHz to 2000 MHz for scales 2 and 3.
The mixer for this band is a crystal diode. Input RF is
applied through a 1 dB isolation pad to the diode. C68, in
series with R68, is tuned for response flatness. An RF choke
L67, isolates the IF and provides a DC path for the MIXER
PEAKING circuit.
Band C: The oscillator for band C is a triode oscillator
connected to tunable transmission lines which are tuned by
the RF CENTER FREQUENCY control. The oscillator funda-
mental frequency range is 1.7 GHz to 4.2 GHz. Harmonics
through the 10th and the fundamental are used to heteradyne
with the input RF to provide the input frequency range from
1.5 to 40 GHz.
Fig. 3-2. Simplified equivalent of the hybrid directional coupler.
The oscillator output is applied to a hyrid directional
coupler or diplexer; see Fig. 3-2. The diplexer couples the
LO signal to the mixer port (OUT] and the mixer output to
the IF port. The mixer action is therefore in an externally
connected mixer, which may be either coaxial or waveguide,
and the 200 MHz IF is then coupled through the diplexer to
the IF connector.
The C band frequency range requires the following four
mixers: One coaxial mixer for the frequency range 1.5 to
12.4 GHz, and three wave guide mixers with frequency ranges
of 12.4 to 18 GHz, 18.0 to 26.5 GHz and 26.5 to 40 GHz.
The IF output is applied through a 1 dB attenuator pad
and the 265 MHz law-pass filter. DC return for the mixer is
through the 1 dB attenuator to the mixer peaking circuit.
The mixer peaking circuit has two modes of operation, a
search mode and a manual mode. In the search mode, the
sweep voltage from the sweep generator circuit is applied
to the base of Q65 and Q51. This varies the collector-to-
emitter resistance and establishes a variable mixer diode
current so that optimum mixer peaking is provided at some
point through the sweep scan.
If the local oscillator is slowly tuned through a frequency
range, signals above the specified sensitivity level will appear
above the noise when they reach this optimum point. This
ensures optimum search capability, and when a signal is
intercepted, the operator then switches to manual tuning
and optimizes the mixer for the given RF center frequency.
Phase Lock Circuit
The phase lack circuit synchronizes the local oscillator
frequency with a stable reference frequency. This reduces
oscillator drift and incidental frequency modulation, permit-
ting narrow dispersion settings for signal analysis.
The phase detector samples the instantaneous RF voltage
generated by the tunable local oscillator at a rate determined
by the reference frequency. The sample voltages are then
intergrated and applied to a comparator which generates a
corrective voltage to feed back to the local oscillator.
When the local oscillator frequency is an exact multiple
of the reference frequency, the phase detector output is a DC
voltage that is proportional to the instantaneous potential
of the sampled oscillator voltage. If the local oscillator
phase drifts, the phase detector output changes. This change
is amplified through Q1170-Q1180 and applied as a cor-
rective voltage to a voltage-controlled capacitance diode in
the oscillator tuned circuit. This shifts the phase of the oscil-
lator so it remains lacked with the reference frequency.
See Fig. 3-3.
The corrective signal from the comparator and amplifier
is also applied to the vertical circuit when the LOCK CHECK
button SW889 is depressed. This provides a beat frequency
signal indication on the CRT so the operator can locate a
lock point. Beat frequency displays appear on the CRT screen
as the local oscillator is tuned (see Operating section). A
reference voltage related to the position of the FINE RF
CENTER FREQ control is also applied to the vertical de-
flection circuit and is used to center the error signal within
the dynamic operating range of the comparator amplifier
Q1170-Q1180. Phase lock operation should be set within
the dynamic range of the amplifier, preferably in the center
of the dynamic range. This dynamic range is visually dis-
played on the CRT as a vertical displacement of the display.
Circuit Analysis
Turning the INT REF FREQ control clockwise closes SW1106
so collector voltage is applied to Q1100. The crystal con-
trolled 1 MHz oscillator will now operate. The output 1 MHz
signal from the emitter of Q1110 is applied to the trigger
generator circuit. Diodes D1122 and D1123 set the quiescent
current through the tunnel diode D1124 and couple the
signal to the 1 MHz MARKER OUT connector J1120; or, if an
external reference signal is applied, they couple the signal
to the trigger generator circuit.
Frequency of the reference oscillator Q1100 is primarily
controlled by the crystal Y1104, inductor L1104, and the
capacitance of diodes D1116 and D1117. Diode D1116 is back
biased to act as a voltage-controlled capacitance diode;
however, when signal amplitude across crystal Y1104 be-
comes excessive, D1116 will conduct on the peak signal
swing. D1117 then becomes back biased and acts as the
capacitance diode.
3-3