Anritsu 680 C Series, Руководство по обслуживанию

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A phase detector in the phase-lock loop circuit com-
pares the two inputs and determines whether the
variable input waveform is leading or lagging the
reference. The phase detector generates a correction
signal that (depending on polarity) causes the VCO
frequency to increase or decrease to reduce any
phase difference. When the two inputs match, the
loop is said to be locked. The variable input from the
VCO then equals the reference input in phase, fre-
quency, accuracy, and stability.
In practical applications a frequency divider is
placed between the output of the variable oscillator
and the variable input to the phase-lock loop. The
circuit can then be used to control a frequency that
is an exact multiple of the reference frequency. In
this way, the variable oscillator acquires the stabil-
ity of the reference without equaling its frequency.
In the A3 Reference Loop, the 100 MHz oven-
controlled crystal oscillator (OCXO) can be con-
trolled by the phase-lock loop using a 10 MHz refer-
ence. This is because a divide-by-ten circuit is
between the OCXO's output and the variable input
to the phase-lock loop. Both inputs to the phase de-
tector will be 10 MHz when the loop is locked.
If a programmable frequency divider is used, a
number of frequencies can be phase-locked to the
same reference. The limitation is that all must be
exact multiples of the reference. The A4 Coarse
Loop and A5 Fine Loop both use programmable fre-
quency dividers.
Overall
Operation The YIG-tuned oscillator generates a high-power RF
output signal that has low broadband noise and low
spurious content. The frequency of the YIG-tuned
oscillator is controlled by means of (1) its main tun-
ing coil and (2) its FM (fine tuning) coil. Main tun-
ing coil current from the YIG Driver PCB coarsely
tunes the YIG-tuned oscillator to within a few
megahertz of the final output frequency. The YIG
phase-lock loop is then used to fine tune the YIG-
tuned oscillator to the exact output frequency and to
reduce FM noise close to the carrier.
2-10 690XXB/691XXB MM
FUNCTIONAL FREQUENCY
DESCRIPTION SYNTHESIS