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MC2800EVK

FLEX Paging RF/IF Receiver Board

Motorola => Freescale 

Freescale MSCe2m80ic0EoVnKductor, Inc.

APPLICATION NOTES

This FLEX paging receiver board uses Motorola’s NPN

silicon low–noise high–frequency transistors MMBR941 to

construct the functional RF blocks, including a LNA, an

oscillator and a mixer. The M–ary FSK IF receiver MC2800

performs the second down–conversion and demodulates the

received FSK signal into two digital output bits.

The LNA employs cascode architecture (Q1 and Q2) with

its output being matched to 230 Ω, the input impedance of the

284 MHz SAW filter. The input matching network should be

changed when an antenna precedes the LNA. Special care

must be taken in characterizing the LNA and antenna to

acheive a good interface match.

The local oscillator (Q4) uses a Colpitts structure. Its

output is bandpass filtered to obtain the 4th harmonic. The

values of C12 and C13 can be altered to fine tune the LO

frequency. L9 is added in this capacitor network to increase

the oscillator gain.

A common emitter circuit (Q3) is used as the mixer with

both LO and RF being ac–coupled to its base. Its output is

dc–coupled to the 21.4 MHz crystal filter. To minimize the

distortion of the downconverted frequency spectrum, the filter

output must be matched to MIX_IN of the MC2800.

For the characteristics of the MC2800 as well as its control

(EN & CL pins) and bit–rate filter (R1 & R2 pins) setup,

please refer to the MC2800 data sheet. It is recommended

that a single ceramic filter configuration be used. That is, a

455 kHz 6–pole filter (FILTER3 in Figure 2) is inserted

between the MIX_OUT and IF1_IN, and a 1.0 µF multi–layer

ceramic chip capacitor (C73 in Figure 2) is connected

between the IF1_OUT and IF2_IN. Although it is noted in the

data sheet of MC2800 that 3.0 to 4.0 dB performance

degradation is observed in the single ceramic filter

application, this performance degradation does not occur in

the overall RF/IF system performance. This is because the

high gain and the low NF of the RF front end have effectively

reduced the noise contribution of the MC2800 to the overall

system.

FDB Interface

This pager board uses an 11–pin surface mount socket to

interface with the pager baseband board or the FLEX

Development Board (FDB). This interface provides 3 digital

output pins (BDOut, D1 & D2), one audio output pin, 4 control

pins (EN, CL, R1 & R2) and 3 supply pins (VCC, VDD–E &

Gnd). The pin descriptions are summarized in the following

table and the footprint of this socket is depicted in Figure 3. In

order to provide a default state for these input pins, pull up

resistors (R17 & R18) are used at both R1 & R2 input pins

whereas a pull down resistor (R19) is connected at the CL

input pin.

Note: For general information regarding FLEX products,

please contact the local Motorola SPS Sales Office or the

web at http://www.mot–sps.com

Pin

Symbol

1

Audio Output

2

R2

3

R1

4

CL

5

VDD–E

6

VCC

7

EN

8

D2

9

D1

10

BD Out

11

Gnd

PIN FUNCTION DESCRIPTION

Description

Symbol rate filter output.

Control bits of the MC2800 bit–rate filter. Default value = ’1’.

Pre–charge and reset of the MC2800. Default value = ’0’.

Operating voltage of FLEX decoder used.

Battery supplies of MC2800.

MC2800 enable pin.

2–Bit digital outputs of the MC2800.

Low battery detector output.

Ground pin.

Figure 3. The Footprint of the 11 Pin Interface Socket

MOTOROLA RF/IF DEVICE DATA For More Information On This Product,

5

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