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RF2917

433/868/915MHZ FM/FSK RECEIVER

RF Micro Devices 

RF2917

RF2917 Theory of Operation and Application Information

The RF2917 is part of a family of low-power RF trans-

ceiver IC’s developed for wireless data communication

devices operating in the European 433/868MHz ISM

bands or the U.S. 915MHz ISM band. This IC has

been implemented in a 15GHz silicon bipolar process

technology that allows low-power transceiver operation

in a variety of commercial wireless products. The

RF2917 realizes a highly integrated, single-conversion

FM/FSK receiver with the addition of a reference crys-

tal, intermediate frequency (IF) filtering, and a few pas-

sive components. The LNA (low noise amplifier) input

of the RF2917 is easily matched to a front-end filter or

antenna by means of a DC blocking capacitor and

reactive components. The receiver local oscillator (LO)

is generated by an internalized VCO, PLL and phase

discriminator in conjunction with the external reference

crystal, loop filter and VCO resonator components.

The receiver IF section is optimized to interface with

low cost 10.7MHz ceramic filters, and its -3dB band-

width of 25MHz also allows it to be used (with lower

gain) at higher frequencies with other types of filters.

FM/FSK SYSTEMS

The receiver output functionality is determined by the

tri-state MUTE input. The three output configurations

are linear FM, FSK and mute. An on-chip 1.6MHz RC

filter, which follows the demodulator output, filters the

harmonics of the IF signal from the output data.

When in the FM mode, the FM OUT signal is the buff-

ered output from the quadrature demodulator. The out-

put signal has a fixed DC offset of VCC-1.0V, while the

AC level is dependent on the FM deviation, with a max-

imum level of 240mVP-P. For optimum operation in

either FM or FSK mode, FM deviation needs to exceed

(with margin) the carrier frequency error anticipated

between the receiver and transmitter.

When in the FSK mode, the FM OUT signal is clipped,

having a rail-to-rail output level. The FM OUT pin is

only capable of driving rail-to-rail output into a very

high impedance and small capacitance, with the

amount of capacitance determining the FM OUT band-

width. For a 3pF load, the bandwidth is in excess of

500kHz. The rail-to-rail output is also limited by the fre-

quency deviation and bandwidth of the IF filters. With

the 180kHz bandwidth filters on the evaluation boards,

the rail-to-rail output is limited to less than 140kHz.

Choosing the right IF bandwidth and deviation versus

data rate (modulation index) is important in evaluating

the applicability of the RF2917 for a given data rate.

Rev B2 010118

AM SYSTEMS

The RF2919 is recommended for use in ASK/OOK

applications, however, the RF2917 may be utilized in

an AM system by using the RSSI (received signal

strength indicator) output to recover the modulation.

The FM output mode should be selected for AM opera-

tion because of the higher RSSI resolution in FM

mode.

RSSI

The RSSI output signal is supplied from a current

source and therefore requires a resistor to convert it to

a voltage. The RSSI is linear over the same range of

input power for both FM and FSK modes, but the FM

mode has higher RSSI resolution. For a 51kΩ resistive

load, the RSSI will range from 1.0V to 2.6V in FM

mode and from 0.8V to 1.5V in FSK mode (3.6V sup-

ply). A small parallel capacitor is suggested to limit the

bandwidth and filter noise.

APPLICATION AND LAYOUT CONSIDERATIONS

The RX IN pin is DC-biased, requiring a DC blocking

capacitor. If the RF filter has DC blocking characteris-

tics, such as a ceramic dielectric filter, then a DC block-

ing capacitor is not necessary. When in power down

mode, the RX IN impedance increases. Therefore, in a

half-duplex application, the RF2917 RX IN may share

the RF filter with a transmitter output having a similar

11 high impedance power down characteristic. Care must

be taken in this case to account for loading effects of

the transmitter on the receiver, and vice versa, in

matching the filter to both the transmitter and receiver.

The VCO is a very sensitive block in this system. RF

signals feeding back into the VCO by either radiation or

coupling of traces may cause the PLL to become

unlocked. The trace(s) for the anode of the tuning var-

actor should also be kept short. The layout of the reso-

nators and varactor are very important. The capacitor

and varactor should be closest to the RF2917 pins and

the trace length should be as short as possible. The

inductors can be placed further away and any trace

inductance can be compensated by reducing the value

of the inductors. Printed inductors may also be used

with careful design. For best results, the physical layout

should be as symmetrical as possible.

When using loop bandwidths lower than the 5kHz

shown on the evaluation board, better supply filtering

at the resonators (and lower VCC noise as well) will

help reduce the phase noise of the VCO; a series

resistor of 100Ω to 200Ω and a 1µF or larger capacitor

11-135

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