b. Circuit Analysis.
(1) Pulse Driver.
The VFO buffer output is applied to the input
switching network (figure 4) consisting of CR1, CR2, and CR3.
When the
range switch is in the three highest ranges for frequencies from 200 MHz to
18 GHz, the VFO signal is switched directly to the driver amplifier. When
the range switch is set to 50 to 200 MHz, the prescaled VFO signal is
switched to the driver amplifier and the VFO signal is shorted to ground.
Transistors Q1-Q6 form the driver amplifier which outputs steep sloped
square waves for driving the stripline pulse generator.
(2) Pulse Generator.
CR1 in a stripline assembly is driven by the
pulse amplifier through C1. CR1 is a step recovery diode which conducts for
only a few nanoseconds after the driving signal reverse biases the diode
junction and then turns off sharply.
This characteristic generates the
pulses for sampler drive.
Figure 4.
Simplified pulse driver and pulse generator circuits.
(3) Sampler. The sampler (figure 5) is designed for harmonic mixing
of very high frequencies. The input signal to the transfer oscillator goes
through a 6 dB pad. The pulses from the sampler (harmonics of VFO) turn on
CR1; C1 charges to some fraction of the input, depending on the phase
relation of the input to the sampling pulse. CR2 switches out of phase with
CR1 and charges in the opposite direction. Between pulses, when the sampler
is off, C1 and C2 hold the charges of the inputs. Phase detection results
when the two charges (outputs) are combined at the input to the automatic
phase control assembly. A shorted stub is used at the input from the pulse
generator to provide a proper impedance match and eliminate standing waves.
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