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RF2905

433/868/915MHZ FM/FSK/ASK/OOK TRANSCEIVER

RF Micro Devices 

RF2905

RF2905 Theory of Operation and Application Information

The RF2905 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 433/

868MHz ISM bands or 915MHz US ISM band.This IC

has been implemented in a 15GHz silicon bipolar pro-

cess technology that allows low-power transceiver

operation in a variety of commercial wireless products.

In its basic form, the RF2905 can implement a two-way

half duplex FSK transceiver with the addition of some

crystals, filters, and passive components. There are

two reference crystals that allow for the transmit carrier

and the receiver LO to be independently generated

with a common PLL and VCO. The receiver IF section

is optimized to interface with low cost 10.7MHz

ceramic filters but has a -3 dB bandwidth of 25MHz

and can still be used (with lower gain) at higher fre-

quency with the other type of filters. The PA output and

LNA input are available on separate pins and are

designed to be connected together through a DC

blocking capacitor. In the Transmit mode, the PA will

have a 50Ω impedance and the LNA will be a high

impedance. In Receive mode, the LNA will have a 50Ω

interface and the PA will have a high impedance. This

eliminates the need for a TX/RX switch and allows a

single RF filter to be used in transmit and receive

modes. Separate access to the PA and LNA allow the

RF2905 to interface with external components such as

higher power PA’s, lower NF LNA’s, upconverters, and

downconverters for a variety of implementations.

FM/FSK SYSTEMS

The MOD IN pin drives an internal varactor for modu-

lating the VCO. This pin can be driven with a voltage

level needed to generate the desired deviation. This

voltage can be carried on a DC bias to select the

desired slope (deviation/volt) for FM systems. Or, a

resistor divider network referenced to Vcc or ground

can divide down logic level signals to the appropriate

level for a desired deviation in FSK systems.

On the receiver demod, two outputs are available, an

analog FM output and a digital FSK output. The FM

output is a buffered signal coming off of the quadrature

demodulator. The digital output is generated by a data

slicer that is DC coupled differentially to the demodula-

tor. An on-chip 1.6MHz RC filter is provided at the

demodulator output to filter the undesired 2xIF product.

This balanced data slicer has a speed advantage over

a conventional adaptive data slicer where a large

capacitor is used to provide DC reference for bit deci-

sion. Since the balanced data slicer does not have to

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charge a large capacitor, the RF2905 exhibits a very

fast response time. For best operation of the on-chip

data slicer, FM deviation needs to exceed the carrier

frequency error anticipated between the receiver and

transmitter with margin.

The data slicer itself is a transconductance amp and

the DATA OUT pin is capable of driving rail to rail out-

put only into a very high impedance and small capaci-

tance. The amount of capacitance will determine the

bandwidth of the DATA OUT. At a 3pF load, the band-

width is in excess of 500kHz. The rail to rail output of

the data slicer is also limited by the frequency deviation

and bandwidth of the IF filters. With the 180kHz band-

width filters on the eval boards, the rail to rail output is

limited to less than 140kHz. Choosing the right IF

bandwidth and deviation vs. data rate (mod index) is

important in evaluating the applicability of the RF2905

for a given data rate.

While this type of data slicer is best for wideband devi-

ation, it can also work for narrowband if care is taken to

minimize frequency differences. By loading down the

DATA OUT pin, the output will be limited to a small data

signal on a DC carrier. With this signal, an external

data slicer can be used to achieve higher data rates or

improve performance in narrow deviations. Alterna-

tively, an AFC loop can be added to correct for fre-

quency errors with a few external components.

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For FM or FSK modulation, an internal varactor is used

to directly modulate the VCO with the baseband data.

The primary consideration when directly modulating

the VCO is the data rate verses PLL loop bandwidth.

The PLL will track out the modulation to the extent of

its loop bandwidth which distorts the modulating data.

Therefore, the lower frequency components of the

modulating data should be 5 to 10 times the loop band-

width to minimize the distortion. The lower frequency

components are generated by long strings of 1’s or 0’s

in the data stream. By limiting the number of consecu-

tive, same bits, the lower frequency component can be

set. In addition, the data stream should be balanced to

minimize distortion. Using a coding pattern such as

Manchester is highly recommended to optimize system

performance.

The PLL loop bandwidth is important in several other

system parameters. For example, switching from trans-

mit to receive requires the VCO to retune to another

frequency. The switching speed is proportional to the

loop bandwidth, the higher the loop bandwidth, the

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