Band B: The oscillator for band B is similar to band A
If the local oscillator is slowly tuned through a frequency
oscillator. The fundamental frequency range 470 MHz to 1100
range, signals above the specified sensitivity level will appear
above the noise when they reach this optimum point. This
MHz and the 2nd harmonic of the oscillator is used for the
ensures optimum search capability, and when a signal is
frequency range 270 MHz to 2000 MHz for scales 2 and 3.
intercepted, the operator then switches to manual tuning
The mixer for this band is a crystal diode. Input RF is
and optimizes the mixer for the given RF center frequency.
applied through a 1 dB isolation pad to the diode. C68, in
series with R68, is tuned for response flatness. An RF choke
Phase Lock Circuit
L67, isolates the IF and provides a DC path for the MIXER
The phase lack circuit synchronizes the local oscillator
frequency with a stable reference frequency. This reduces
Band C: The oscillator for band C is a triode oscillator
oscillator drift and incidental frequency modulation, permit-
connected to tunable transmission lines which are tuned by
ting narrow dispersion settings for signal analysis.
the RF CENTER FREQUENCY control. The oscillator funda-
The phase detector samples the instantaneous RF voltage
mental frequency range is 1.7 GHz to 4.2 GHz. Harmonics
generated by the tunable local oscillator at a rate determined
through the 10th and the fundamental are used to heteradyne
by the reference frequency. The sample voltages are then
with the input RF to provide the input frequency range from
intergrated and applied to a comparator which generates a
1.5 to 40 GHz.
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.
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
Fig. 3-2. Simplified equivalent
of the hybrid directional coupler.
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-
The oscillator output is applied to a hyrid directional
played on the CRT as a vertical displacement of the display.
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
Turning the INT REF FREQ control clockwise closes SW1106
the IF connector.
so collector voltage is applied to Q1100. The crystal con-
trolled 1 MHz oscillator will now operate. The output 1 MHz
The C band frequency range requires the following four
signal from the emitter of Q1110 is applied to the trigger
mixers: One coaxial mixer for the frequency range 1.5 to
generator circuit. Diodes D1122 and D1123 set the quiescent
12.4 GHz, and three wave guide mixers with frequency ranges
current through the tunnel diode D1124 and couple the
of 12.4 to 18 GHz, 18.0 to 26.5 GHz and 26.5 to 40 GHz.
signal to the 1 MHz MARKER OUT connector J1120; or, if an
external reference signal is applied, they couple the signal
The IF output is applied through a 1 dB attenuator pad
to the trigger generator circuit.
and the 265 MHz law-pass filter. DC return for the mixer is
Frequency of the reference oscillator Q1100 is primarily
through the 1 dB attenuator to the mixer peaking circuit.
controlled by the crystal Y1104, inductor L1104, and the
The mixer peaking circuit has two modes of operation, a
capacitance of diodes D1116 and D1117. Diode D1116 is back
search mode and a manual mode. In the search mode, the
biased to act as a voltage-controlled capacitance diode;
sweep voltage from the sweep generator circuit is applied
however, when signal amplitude across crystal Y1104 be-
to the base of Q65 and Q51. This varies the collector-to-
comes excessive, D1116 will conduct on the peak signal
swing. D1117 then becomes back biased and acts as the
current so that optimum mixer peaking is provided at some
point through the sweep scan.