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Maxim > Design Support > Technical Documents > Application Notes > Wireless and RF > APP 4929
Keywords: ISM, RF, transmitter, short-range, RFIC, switching power amplifier, ETSI
APPLICATION NOTE 4929
Adapting Low-Band ISM Transmitters for High-Band
Operation
Aug 23, 2011
Abstract:
ISM 300MHz
to 450MHz band RF transmitters already serve the European 434MHz market, as well
as the most important frequencies in the U.S. 260MHz to 470MHz band. This application note explores
whether an 868MHz transmitter can be created from existing 300MHz to 450MHz RF IC products. An 868MHz
transmitter would serve Europe's license-free band from 868MHz to 870MHz. Specifically, the article discusses
a series of tests and analyzes how much power can be transmitted at 868MHz from one or more of the ISM-
band RF transmitters designed for the 300MHz to 450MHz ISM band.
A similar version of this article appeared in the June 27, 2011 issue of
EE Times magazine.
Introduction
Low-band (300MHz to 450MHz) ISM RF transmitters already serve the
European 434MHz market, as well as the most important frequencies in
the U.S. 260MHz to 470MHz band. This application note explains how
an 868MHz transmitter can be created from existing low-band RF IC
devices to serve Europe's license-free 868MHz to 870MHz band.
The article specifically discusses a series of tests and analyzes how much power can be transmitted at
868MHz from one or more of the ISM-band RF transmitters designed for the 300MHz to 450MHz range.
The Theoretical Challenge
The switching power amplifier (PA) in most low-band ISM transmitters produces a second harmonic that is
only 3dB below the fundamental frequency. If some efficiency and power can be sacrificed, would it be
possible to create a serviceable 868MHz ASK transmitter from an IC designed for 434MHz operation? Since
the phase-noise density is just low enough to meet the European Telecommunications Standards Institute
(ETSI) out-of-band emission standards at Europe's 434MHz license-free band, the phase-noise density would
not meet the more stringent requirements for the 868MHz band. However, that does not mean that there is no
value in devising an 868MHz ASK transmitter. Some customers will have applications for very low transmitting
power, or perhaps some modifications can be made to the oscillator on the low-band ICs without the need a
completely new design?
RF Spectrum of a Switching Power Amplifier
The switching PA found in most ISM low-band RF transmitters produces a periodic series of 0.25 duty-cycle
pulses, where the pulse period is the period of the carrier frequency. The theoretical frequency spectrum of
this pulse train is a set of evenly spaced lines at multiples of the carrier frequency. The amplitude of each line
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