Motorola 6864115B62-C Detailed Service Manual page 365

Low Band Frequency Synthesis 2-11
only by the resistors connected to BIAS 1, BIAS 2, or the internal current source. A settled
synthesizer loop is indicated by a high level of signal LOCK (U1201-4).
In order to modulate the PLL the two spot modulation method is utilized. Via pin 10 (MODIN) on
U1201, the audio signal is applied to both the A/D converter (low frequency path) and the balanced
attenuator (high frequency path). The A/D converter converts the low frequency analog modulating
signal into a digital code that is applied to the loop divider, thereby causing the carrier to deviate.
The balance attenuator is used to adjust the VCO's deviation sensitivity to high frequency
modulating signals. The output of the balance attenuator is present at the MODOUT port (U1201-
41) and superimposed on the VCO steering line voltage by a divider consisting of C1215, C1208
and C1212.