HP 8560A Service Manual page 368

negative Sampler IF's, the Main Roller 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 IF's. The opposite is true for positive Sampler IF's,
so in these cases, the Main. Roller Oscillator sweeps more conventionally from a lower
frequency to a higher frequency. This implies that the sense of the span ramp sweeping
the Roller Oscillator PLL must change between negative and positive Sampler IF's. This
is accomplished by controlling the polarity of the VCO Sweep Driver. The polarity is
controlled by VCOSGN from A14U425 pin 12. When VCOSGN is high, the VCO Sweep
Driver has positive polarity.
The Main and Offset Roller Oscillators have opposite sense with respect to the VCO
span ramp (VCO RAMP). A positive-going ramp moves the Main Roller Oscillator lower
in frequency (Main Roller spans) while a positive-going ramp moves the Offset Roller
Oscillator higher in frequency (Offset Roller spans). Thus the polarity of the VCO Sweep
Driver depends on whether a Main Roller or Offset Roller LO span is selected.
Table lo-11 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
E of Al4 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.