Agilent Technologies 856290216 service manual Amplifier Polarities

In fractional N spans (LO Spans 12 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 11-7 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 jump when C36 is switch in.

Table 1 l-7. Amplifier Polarities

	Fractional N	Positive	Positive	TTL High
Ofrdlator Swept		Sampler IF
		Negative	Negative	TTL Low
		Sampler IF
F:M/Main YTO		Positive	Positive	TTL High
	Coils Swept	Sampler IF
		Negative	Negative	TTL Low
		Sampler IF

11-24 Synthesizer Section