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 article 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 is weighted by a sinc (sinx/x) curve that contains zeroes at multiples of 4 times the carrier frequency. Figure 1 illustrates the first six lines of the spectrum for a 434MHz carrier frequency. The amplitude of the 868MHz component (the second harmonic) is only 3dB down from the fundamental 434MHz component. In practice, the switched amplifier drives a tuned circuit, whose characteristics depend on the desired rejection of the harmonics of the fundamental frequency. If the tuned circuit has a relatively wideband characteristic, it should radiate the 868MHz component at a power level that is not much more than 3dB below the fundamental frequency.
Figure 1. Theoretical power contribution from fundamental and harmonics of a 25% duty-cycle RF pulse train at 434MHz.
The 3dB difference was verified by removing the harmonic filter from the evaluation (EV) kit for a 300MHz to 450MHz high-efficiency, crystal-based +13dBm ISM transmitter and changing the bias inductor to 62nH, a value that resonates with the approximately 2pF to 2.5pF of stray capacitance. The resonant circuit formed by this L-C combination has a wide enough bandwidth. Thus, it does not attenuate the 868MHz harmonic significantly when the PA output is connected directly to a 50Ω load. Figure 2 shows the spectrum analyzer trace of the 434MHz and 868MHz components. The 868MHz component is 3.5dB lower than the 434MHz component, which represents only 0.5 dB reduction by the wider resonant circuit.
Figure 2. Spectrum of MAX7044EVKIT ISM transmitter with tank circuit tuned to 434MHz.
The next step is to modify the matching-network components to enhance the 868MHz second harmonic and attenuate the 434MHz fundamental frequency.