Agilent Technologies 8564EC, Service Manual

Page: 420 / 512

552 Chapter 11
Synthesizer Section
Unlocked YTO PLL
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.
In fractional N spans (LO Spans
≤
2 MHz) the YTO remains locked to
the sweeping fractional N PLL. Thus, the sampler IF must always
equal the fractional N oscillator frequency (conditions for lock). Since
the YTO must always sweep up in frequency, for negative sampler
IFs, the fractional N oscillator must sweep from a higher frequency
to a lower frequency. This is necessary since an increasing YTO
frequency decreases the sampler IF for negative sampler IFs. The
opposite is true for positive sampler IFs, so in these cases, the
fractional N oscillator sweeps more conventionally from a lower
frequency to a higher frequency.
Table 11-5 on page 553 summarizes the amplifier polarities for the
various combinations of sampler IF polarities and LO spans.
The YTO main coil filter is used to improve residual FM in FM
spans. See function block I of A14 frequency control schematic (sheet
2 of 5) in the Component-Level Information binder. Transistors Q304
and Q305 switch the filter (capacitor C36 and resistor R48) into the
circuit. Transistor Q303 and U333 keep C36 charged during main
spans so the frequency does not jump when C36 is switch in.