Chapter 11 551
Synthesizer Section
Unlocked YTO PLL
Unlocked YTO PLL
Operation
The A11 YTO is locked to two other oscillators, the fractional N
oscillator and the offset PLL sampling oscillator. For LO spans of
2.01 MHz and greater, either the FM or main coil of the YTO is swept
directly. For LO spans less than or equal to 2 MHz, the fractional N
oscillator is swept. The sampling oscillator remains fixed-tuned during
all sweeps.
The output of A11 YTO feeds through the A7 LO multiplier/amplifier
(LOMA) to the A15U100 sampler. The offset PLL sampling oscillator,
which drives the sampler, oscillates between 285 and 297.222 MHz. The
sampler generates harmonics of the sampling oscillator and one of
these harmonics mixes with the YTO frequency to generate the sampler
IF frequency. As a result, the frequency of the sampler IF is determined
by the following equation:
Where:
—F
IF
is the sampler IF
—F
YTO
is the YTO frequency
— N is the desired sampling oscillator harmonic
—F
SAMP
is the sampling oscillator frequency
Notice that F
IF
can be positive or negative depending upon whether
the sampling oscillator harmonic used is less than or greater than
the YTO frequency. The actual sampler IF is always positive, but the
sign is carried along as a "bookkeeping" function which determines
which way to sweep the fractional N oscillator (up or down) and
what polarity the YTO error voltage should have (positive or
negative) to maintain lock.
To check if a negative sampler IF is selected, press
CAL, MORE 1 OF
2, FREQ DIAGNOSE, FRAC N FREQ. If the fractional N oscillator
frequency is positive, the sampler IF is also positive. A negative
fractional N frequency indicates that the sampler IF is negative.
F
IF
F
YTO
N F
SAMP
×()–=
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