Mixer (U6251); Receiver Back-End; Vhf (136-174 Mhz) Band; Intermediate Frequency (If) Section - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Receiver Back-End

3.10.4.3 Mixer (U6251)

The monolithic, passive mixer IC down-converts the received signal to an Intermediate Frequency
(IF) of 73.35 MHz. The mixer is designed to provide low conversion loss (< 7.0) and high
intermodulation performance. To improve the performance of the mixer in both bands, a shunt 9.1 pF
capacitor (C6297) along with a resistive PI network (R6278, R6280, R6281) is designed at the IF
port (pin 5) of the mixer. The mixer is driven by the receiver injection buffer, a two-stage discrete/IC
design used with the VCO to efficiently drive the mixer over temperature with minimum power
variation. The injection buffer provides 15 dBm to the mixer. The VCO does high-side injection for
the 800 MHz band and low-side Injection for the 700 MHz band.

3.11 Receiver Back-End

This section discusses the receiver back-end (RXBE) components and detailed theory of operation.
The receiver back-end processes the down-converted, filtered IF signal to produce digital data for
final processing by the Patriot microcontroller IC.

3.11.1 VHF (136-174 MHz) Band

The receiver back-end contains the following major components:

• Intermediate frequency (IF) section.

• ABACUS III IC

3.11.1.1 Intermediate Frequency (IF) Section
The XTL 1500 radio uses two leadless, surface-mount, two-pole, third-overtone, quartz crystal filters
(Y3400, Y3401) separated by a 21 dB gain IF amplifier. The filters are centered at 109.65 MHz. This
narrowband bandpass filter contributes to the radio's adjacent-channel and alternate-channel
rejection performance. Components L3401, L3403, L3404, L3405, C3421, C3414, C3409, C3416,
C3420, C3418 and C3415 are used as impedance-matching networks. Components Q3400, R3409,
R3401, R3402, R3405, R3407, and R3413 are used for biasing and stabilizing the transistor Q3400.
Components C3424, C3404 bypass the DC supply. L3400 is RF choke. Diode D3400 and Inductor
L3408 protect the Abacus and the second IF filter from strong In-band signals.
3.11.1.2 ABACUS III IC
The receiver back-end is designed around the ABACUS III (AD9874 IF digitizing subsystem) IC and
its associated circuitry. The AD9874
that digitizes a 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
section in the receiver front-end.
Components C3000, C3038, and L3002 match the input impedance from 50 ohms (IF Filter
terminating impedance) to the ABACUS III IC input IFIN. Formatted SSI 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).
6815854H01-A
(Figure 3-18 on page 3-24) is a general-purpose, IF subsystem
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June 15, 2005