Frequency Synthesis
3.1-32 Introduction/Theory of Operation
LOCK DET adds up with signal SQ DET, weighted by resistors R0113, R0114, and is routed to one
of the µP´s ADCs input U0101-43. From the voltage weighted by the resistors the µP determines
whether SQ DET, LOCK DET or both are active.
In order to modulate the PLL the two spot modulation method is utilized. Via pin 8 (MODIN) on
U3701 the audio signal is applied to both the A/D converter (low freq path) as well as the balance
attenuator (high freq path). The A/D converter converts the low frequency analogue 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 (U3701-28) and
connected to the VCO modulation diode D3731 via L3731, C3733.
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