Motorola SSETM 5000 service manual Transmitter, Abacus III AD9874 Functional Block Diagram

Theory of Operation: Transceiver Board 2-11

		MXOP MXON	IF2P IF2N	GCP GCN
	AD9874					DAC AGC
	-16dB
IFIN	LNA			M	ADC	Decimation
						Filter
FREF					......=13-26MHz
	LO			Samp. Clock		Voltage/
						Current
	Synth.			Synthesizer
						Reference
	IOUTL	LOP LON	IOUTC	CLKP	CLKN	VREFP RREF VREFN
	LO VCO and		CLK VCO and
	Loop Filter			Loop Filter

Formatting/SSI DOUTA

DOUTB

FS

CLKOUT

Control Logic
	SPI
PC	PD	PE	SYNCB

MAEPF-27412-O

Figure 2-5. Abacus III (AD9874) Functional Block Diagram

Input signal RXIF is 73.35 MHz IF from crystal filter FL400 in the receiver front-end. Components L547 and C542 match the input impedance to 50 ohms. Formatted SSI data is output to the VOCON board on ports FS, DOUTA, and CLKOUT.

2.4.2.2.2 Second Local Oscillator

The second LO is controlled by the Abacus LO synthesizer, which mixes with IFIN to produce a 2.25 MHz final IF. The external VCO consists of Q502 and its bias network and frequency-determining elements. Signal FREF is the 16.8 MHz reference from the FGU. Darlington transistor Q501 with C550 and R501 form an active power-line filter.

The second LO frequency is 71.1 MHz by default or 75.6 MHz in special cases as needed to avoid radio self-quieters. The loop filter is composed of R551, C558, C559, R552, and C512.

2.4.2.2.3 Sampling Clock Oscillator

The Abacus sampling clock synthesizer operates at 18 MHz = 8 x 2.25 MHz. The VCO uses an internal transistor and external resonator. The resonator is composed of L503, C535, C929, and D501.

The loop filter is composed of R512, C536, R514, C570, and C571.

2.4.3Transmitter

NOTE: Refer to Figure 2-6for the transmitter block diagram and Figure 12-4for the transmitter schematic.

The transmitter takes modulated RF from the FGU and amplifies it to the radio’s rated output power to produce the modulated transmitter carrier at the antenna.

The transmitter consists of an RF driver IC that receives its input signal from the voltage-controlled oscillator (VCO) and a high-power output transistor. Transmitter power is controlled by a power-

6881094C12-A November 11, 2004