HP 8562E, Service Manual

Page: 398 / 482

In fractional N spans (LO Spans 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
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 The opposite is true for positive sampler so in these cases,
the fractional N oscillator sweeps more conventionally from a lower frequency to a higher
frequency.
Table 11-7 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 Al4 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
when C36 is switch in.
Of
F:
Table 1 Amplifier Polarities
Fractional N
Positive Positive
TTL High
Swept
Sampler IF
Negative
Negative TTL Low
Sampler IF
M/Main YTO
Coils Swept
Positive
Sampler IF
Negative
Sampler IF
Positive
Negative
TTL High
TTL Low
11-24 Synthesizer Section