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RF2919

915MHz ASK/OOK receiver

RF Micro Devices 

RF2919

RF2919 Theory of Operation and Application Information

The RF2919 is a part of a family of low-power RF

transceiver IC's that was developed for wireless data

communication devices operating in the European

433MHz/868MHz ISM bands or U.S. 915MHz ISM

band. This IC has been implemented in a 15GHz sili-

con bipolar process technology that allows low-power

transceiver operation in a variety of commercial wire-

less products. The RF2919 realizes a highly inte-

grated, single-conversion ASK/OOK receiver with the

addition of a reference crystal, intermediate frequency

(IF) filtering, and a few passive components. The LNA

(low-noise amplifier) input of the RF2919 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 inter-

nalized VCO, PLL and phase discriminator in conjunc-

tion 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 bandwidth of 25MHz also allows it

to be used (with lower gain) at higher frequencies with

other types of filters.

OPERATION

The ASK/OOK demodulation is accomplished by an

on-chip data comparator. The RSSI output is internally

routed through 50kΩ resistors to provide the inputs

(DATA IN+ and DATA IN-) to the data comparator.

Either input may be used as the data input with the

other input used as the reference. A shunt capacitor

can be added to the data input to provide filtering of

noise and the second IF harmonic. The value of the

data filtering capacitor is calculated by

=

----------------1-----------------

2πF ⋅ 16.7kΩ

where F is the desired 3dB bandwidth. The factor of

16.7kΩ is the net internal impedance (50kΩ in parallel

with a 25kΩ comparator input impedance).

A large filtering capacitor may be used on the refer-

ence input to remove the modulation signal, leaving a

DC reference for the comparator. Because this refer-

ence filter may have a long time constant, a longer pre-

amble may be required to allow the DC reference to

stabilize. The data pattern also affects the stability of

the DC reference and the reliability of the received

data. Since a string of consecutive data 'ones' (or

'zeroes') will result in a change to the DC reference, a

coding scheme such as Manchester should be used to

Rev A12 001113

improve data integrity. Hysteresis can be added by

placing a resistor between the input and the output.

The DATA OUT pin is only capable of driving rail-to-rail

output into a very high impedance and small capaci-

tance, with the amount of capacitance affecting the

DATA OUT bandwidth. For a 3pF load, the bandwidth

is in excess of 500kHz. The rise and fall times of the

RSSI are limited by the bandwidth of the IF filters,

thereby limiting the effective data rate.

The RSSI output signal is supplied from a current

source and therefore requires a resistor to convert it to

a voltage. For a 24kΩ resistive load, the RSSI will typi-

cally range from 0.4V to 1.5V (3.6V supply). 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 RF2919 RX IN may share

the RF filter with a transmitter output having a similar

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 match-

ing the filter to both the transmitter and receiver.

11

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 RF2919 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 phase noise of the VCO; a series resistor

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

used. Phase noise is generally more critical in narrow-

band applications where adjacent channel selectivity is

11-149

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