Theory of Operation: Main Board 3-19
3.1.4.5 Modulation
To support many voice, data, and signaling protocols, APX 2000/ APX 4000/ APX 4000Li/
APX 1000 (900 MHz) radios must modulate the transmitter carrier frequency over a wide audio
frequency range, from less than 10 Hz up to more than 6 kHz. The Trident IC supports audio
frequencies down to zero Hz by using dual-port modulation. The digital audio signal at pin F11 (TXD)
is transferred to the Trident baseband circuitry via the TX Serial Synchronous Interface (SSI) bus.
The data is then internally divided into high and low-frequency components, which modify both the
synthesizer dividers and the external VCOs through a signal on HP_MOD_OUT (pin L9). The DSP
scaling is adjusted to achieve a flat modulation frequency response during the transmitter
modulation balance calibration.
3.1.4.6 Voltage Multiplier and Superfilter
Pins H10 (VMULT2) and H11 (VMULT1) together with diode arrays D722 and D723 and their
associated capacitors form the voltage multiplier. The voltage multiplier generates 10.625 Vdc to
supply the phase detector and charge-pump output stage at pin F1 (MN_CP_VCC). The superfilter is
an active filter that provides a low-noise supply for the VCOs. The input is a regulated 5 Vdc from
DC_LIN_5V at pin K4 (SF_SPLY). The output is a superfiltered voltage at pin J5 (SF_OUT).
3.1.4.7 Loop Filter
The components connected to pins G3 (MN_CP) and G2 (MN_ADAPT_CP) form a 4th-order, RC
low-pass filter. Current from the charge-pump output, MN_CP, is transformed to voltage
VCO_VCTRL, which modulates the VCOs. Extra current is supplied by MN_ADAPT_CP for rapid
phase-lock acquisition during frequency changes. The lock detector output pin B4
(TEST1_LCKDET) goes to a logic “1” to indicate when the phase-lock loop is in lock.
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