Uhf Range 1 (380-470 Mhz) Band; Figure 3-17. Abacus Iii (Ad9874) Ic Functional Block Diagram From Data Sheet (Vhf) - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

Theory of Operation: Receiver Back-End
AD9874
-16dB
IFIN
LNA
FREF
LO
Synth.
LO VCO and
Loop Filter

Figure 3-17. ABACUS III (AD9874) IC Functional Block Diagram from Data Sheet (VHF)

3.5.1.2.1 Second Local Oscillator
The ABACUS III IC local oscillator (LO) synthesizer controls the second LO. Signal FREF is the
16.8 MHz reference from the frequency generation unit (FGU). The second LO frequency is
107.4 MHz by default, or 111.9 MHz in special cases as necessary to avoid radio self-quieters. The
second LO signal mixes with IFIN to produce a 109.65 MHz final IF. The external VCO consists of
transistor Q3000, together with its bias and instability network and tank elements. Darlington
transistor Q3001 along with C3035 and C3017 form an active DC filter. The second-order loop filter
is comprised of C3044, C3005, and R3009.
3.5.1.2.2 Sampling Clock Oscillator
The ABACUS III IC sampling clock synthesizer, at Fclk=18 MHz (IF2=Fclk/8, where Fclk is the clock
rate), utilizes a negative-resistance core that is internal to the ABACUS III IC which, when used in
conjunction with an external LC tank (made up of L3003 and C3039) and a varactor (D3001), serves
as the VCO.
3.5.2

UHF Range 1 (380-470 MHz) Band

The receiver back-end (see
• Intermediate frequency (IF) filter
• ABACUS III IC
6881096C74-B
DAC AGC
Decimation
ADC
f = 13-26MHz
CLK
Sample Clock Voltage
Reference
Synthesizer
CLK VCO and
Loop Filter
Figure 3-14 on page 3-15) contains the following major components:
Formatting/SSI
Filter
Control Logic
SPI
3-21
DOUTA
DOUTB
FS
CLKOUT
MAEPF-27817-O
May 25, 2005