RFRXD0420 View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

RFRXD0420

UHF ASK/FSK/FM Receiver

Microchip Technology 

rfRXD0420/0920

3.0 APPLICATION CIRCUITS

This section provides general information on applica-

tion circuits for the rfRXD0420/0920 receiver. The

following connections and external components

provide starting points for designs and list the minimum

circuitry recommended for general purpose

applications.

Performance of the radio system (transmitter and

receiver) is affected by component selection and the

environment in which it operates. Each system design

has its own unique requirements. Specifications for a

particular design requires careful analysis of the appli-

cation and compromises for a practical implementation.

3.1 General

This subsection lists connections and components that

are common between applications. The following

subsections give specific circuit connections and

components for ASK, FSK and FM applications.

3.1.1 BYPASS CAPACITORS

Bypass capacitors should be placed as physically close

as possible to VDD pins 8, 14, 17, 27 and 32

respectively. Additional bypassing and board level low-

pass filtering of the power supply may be required

depending on the application.

3.1.2 FREQUENCY PLANNING

The rfRXD0420/0920 receivers are single-conversion

superheterodyne architecture with a single IF

frequency. The receive frequency is set by the crystal

frequency (fXTAL) and intermediate frequency (fif). For

a majority of applications an external crystal is

connected to XTAL (Pin 26). Figure 3-1 illustrates an

example circuit with an optional trim capacitor.

FIGURE 3-1: XTAL EXAMPLE CIRCUIT

WITH OPTIONAL TRIM

CAPACITOR

26

C TRIM

(OPTIONAL)

X1

The crystal load capacitance should be specified to

include the internal load capacitance of the XTAL pin of

15 pF plus PCB stray capacitance (approximately 2 to

3 pF). A trim capacitor can be used to trim the crystal

on frequency within the limitations of the crystal’s trim

sensitivity and pullability. Figure 3-2 illustrates the

effect the trim capacitor has on the receive frequency

for the rfRXD0420 at 433.92 MHz. Keep in mind that

this graph represents one example circuit and the

actual results depends on the crystal and PCB layout.

FIGURE 3-2:

434.10

434.05

434.00

433.95

433.90

433.85

433.80

433.75

RECEIVE FREQUENCY VS.

TRIM CAPACITANCE

Trim Capacitor (pF)

Note that a 0 Ω resistor, in the lower left of the graph,

represents an infinite capacitance. This will be the

lowest frequency obtainable for the crystal and PCB

combination.

Calculation of the crystal frequency requires knowl-

edge of the receive frequency (frf) and intermediate

frequency (fif). Figure 3-3 is a worksheet to assist the

designer in calculating the crystal frequency. Table 3-1

lists crystal frequencies for popular receive frequen-

cies. Table 3-2 lists crystal parameters required for

ordering crystals. For background information on

crystal selection see Application Note AN826, Crystal

Oscillator Basics and Crystal Selection for rfPICTM and

PICmicro® Devices.

TABLE 3-1: CRYSTAL FREQUENCIES FOR

POPULAR RECEIVE

FREQUENCIES

Receive

Frequency

Crystal

Frequency

rfRXD0420

315 MHz

20.35625 MHz (2)

433.92 MHz

26.45125 MHz (1)

868.3 MHz

915 MHz

rfRXD0920

26.8 MHz (1)

28.259375 MHz (1)

(1) Low-side injection (2) High-side injection

TABLE 3-2: CRYSTAL PARAMETERS

Parameter

Value

Frequency:

(see Figure 3-1)

Mode:

Fundamental

Load Capacitance:

15-20 pF

ESR:

60 Ω Maximum

These values are for design guidance only.

 2003 Microchip Technology Inc.

Preliminary

DS70090A-page 9

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