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Unlocked YTO PLL
Operation
The All 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 above, either the FM or main coil of the
YTO is swept directly. For LO spans of 2 MHz and below, the fractional N oscillator is swept.
The sampling oscillator remains fixed-tuned during all sweeps.
The output of All YTO feeds through the A7 switched LO distribution amplifier to the
A15UlOO
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:
FIF
=
FYTO
-(N
X
FSAMP)
Where:
FIF
is the sampler IF
FYTO
is the YTO frequency
N is the desired sampling oscillator harmonic
FSAMP
is the sampling oscillator frequency
Notice that FIF can be positive or negative depending upon whether the sampling oscillator
harmonic used is below or above 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
a,
MORE
1
OF
2,
FEEQ
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.
Notice that the polarity of the YTO loop error voltage (YTO ERROR) out of the YTO
loop phase/frequency detector changes as a function of the polarity of the sampler IF.
That is, for positive sampler
IFS,
an increasing YTO frequency results in an increasing
YTO ERROR signal. For negative sampler IFS, an increasing YTO frequency results in a
decreasing YTO ERROR signal. This implies that to maintain lock in both cases, the sense
of YTO ERROR must be reversed such that, with a negative sampler IF, an increasing
YTO ERROR results in an increasing YTO frequency. This is accomplished with error-sign
amplifier, A14U328B. This amplifier can be firmware-controlled to operate as either an
inverting or non-inverting amplifier. Digital control line ERRSGN (from A14U313 pin 19)
controls the polarity of this amplifier. When ERRSGN is high (positive sampler IF), the
amplifier has a positive polarity.
Synthesizer Section 11-23