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 50? 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.
ISM RF ICs are offered as 'use it as it is' parts, leveraging on their fulfillment of functional requirements. So, many users never even get to know the underlying RF basics. This article appreciably sheds a light on what is below the surface and may be a good track for some lab excercise.
From an engineering point of view, since there is plenty of ISM ICs designed for the 868/915 bands, the first question may be 'why should one try this for his application?'.
Good question about why one should try this for his application. You are correct in saying that there are a lot of ISMRF IC's around that work at 868 MHz, and some of them are already simple and inexpensive, as opposed to the more sophisticated ones that have shaped modulation, low phase noise, and high current drain. I think the motivation for this article was part lab experiment and part creative thinking for developing a very simple very inexpensive part in the future. - Larry Burgess
This is an interesting analysis and a new(sort of) application of a technique that radio amateurs have been using since at least 1950, which is the oldest dated publication that I have that references it. Hams call it "frequency doubling" or "frequency multiplication", and they have used a similar technique to obtain multiples of 2x, 3x, and 5x the input frequency. One thing to note is that because the operation is quite nonlinear, any amplitude modulation of the signal is quite distorted. Now, to address the comments from titux, we should understand this in order to assure that we avoid unintentionally transmitting signals on frequencies other than the intended ones. This is a problem associated with all nonlinear amplification, which is usually cheaper, more efficient, and simpler than linear amplification. And most designers aim for cheaper, simpler, and more efficient.
To respond and add to WKetel's comments: Part of this experiment was aimed at understanding the behavior of switched amplifiers driving circuits that they may not have been designed to drive. The model we use at 434 MHz is one where the output tank circuit is hit with a 25% duty cycle (at RF) pulse train and it is well documented, but what happens if the tank circuit is tuned to twice the intended frequency and it is still hit with a 25% duty cycle pulse train?