HP 8560E, Service Manual

Page: 540 / 665

Chapter 10 539
Synthesizer Section
Unlocked YTO PLL
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 10-8 on page 539 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 and
fractional N spans. See function block I of A14 frequency control
schematic 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.
Table 10-8 Amplifier Polarities
YTO Error Sign
Amplifier ERRSGN
(A14U313 pin
19)
Fractional N Positive Positive TTL High
Oscillator
Swept Sampler IF
Negative Negative TTL Low
Sampler IF
FM/Main YTO Positive Positive TTL High
Coils Swept Sampler IF
Negative Negative TTL Low
Sampler IF