VHF (136-174 MHz) Frequency Synthesis 2-9
In order to modulate the PLL, the two spot modulation method is utilized. Via U3201 pin 10 (MODIN),
the audio signal is applied to both the A/D converter (low frequency path) as well as the balance
attenuator (high frequency path). The A/D converter changes 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 (U3201-41) and
connected to the VCO modulation diode D3362 via R3364.
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