Intermediate Frequency (If) Filter; Abacus Iii Ic (U5002); Figure 3-19. Abacus Iii (Ad9874) Ic Functional Block Diagram From Data Sheet (Uhf Range 1) - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Receiver Back-End

3.11.2.1 Intermediate Frequency (IF) Filter

The XTL 1500 radio uses two leadless, surface-mount, two-pole, third-overtone, quartz crystal filters
(Y5400, Y5401) separated by a 20 dB gain IF amplifier. The filter is centered at 109.65 MHz. This
narrow-bandpass filter gives the radio part of its adjacent-channel and alternate-channel rejection
performance. Impedance-matching networks are located at the input and output of each crystal. The
IF amplifier is made with Q5401. The 10 dB attenuator (U5400) located after the second crystal filter
is controlled by the software to limit the signal gain in front of the ABACUS III IC.

3.11.2.2 ABACUS III IC (U5002)

The receiver back-end is designed around the ABACUS III (AD9874 IF digitizing subsystem) IC and
its associated circuitry. The AD9874
low-level, 10-300 MHz IF input with a bandwidth up to 270 kHz. The signal chain of the AD9874
consists of a variable gain, low-noise amplifier, a mixer; a bandpass, sigma-delta, A/D converter; and
a decimation filter with programmable decimation factor. An automatic gain control (AGC) circuit
provides the AD9874 with 12 dB of continuous gain adjustment. The high dynamic range and
inherent anti-aliasing provided by the bandpass, sigma-delta converter allow the AD9874 to cope
with blocking signals 80 dB stronger than the desired signal. Auxiliary blocks include clock and LO
synthesizers, as well as an SPI port. Input signal RXIF is the 109.65 MHz IF from the IF filter.
Components C5002, C5007, and L5002 match the input impedance from 50 ohms (IF Filter
terminating impedance) to the ABACUS III IC input IFIN. Formatted SSI (synchronous serial
interface) data is output to the Patriot microcontroller IC for DSP processing on ports FS, DOUTA,
and CLKOUT. Control logic is sent to the ABACUS III IC from the Patriot microcontroller via the SPI
lines (PC, PD, PE).
AD9874
-16dB
IFIN
LNA
FREF
LO
Synth.
LO VCO and
Loop Filter

Figure 3-19. ABACUS III (AD9874) IC Functional Block Diagram from Data Sheet (UHF Range 1)

6815854H01-A
(Figure 3-19) is a general-purpose, IF subsystem that digitizes a
DAC AGC
Decimation
ADC
f = 13-26MHz
CLK
Sample Clock Voltage
Reference
Synthesizer
CLK VCO and
Loop Filter
Formatting/SSI
Filter
Control Logic
SPI
3-25
DOUTA
DOUTB
FS
CLKOUT
MAEPF 27817 O
June 15, 2005