Matching-Network Topology for 434MHz Operation
The MAX7044EVKIT was modified for 868MHz operation, using the component topology already in place for 434MHz operation. The matching networks of all the ISM RF transmitter EV kits in the 300MHz to 450MHz band have the same topology, shown in Figure 3. The reference designators are identical to those in the MAX7044EVKIT.
Figure 3. Matching-network and reference designators for MAX7044EVKIT.
There are several ways to realize a matching network to a 50? load with this topology. The most straightforward method is to populate the C2-L3-C6 pi network as a 50? lowpass filter for harmonic rejection. Next, use the C1-L1 combination as an “L” narrowband impedance transformation network that converts 50? to a higher impedance. With the exception of the MAX7044 and MAX7060 280MHz to 450MHz programmable transmitter, all Maxim ISM RF low-band transmitters are the most power efficient when they drive an impedance between 125? and 250?. The MAX7044 achieves its highest power in the low band (+13dBm with 2.7V supply) when it drives a 50? to 60? load. Lower power levels and lower supply currents can be achieved by increasing the impedance presented to the transmitter PA output. For normal operation in the low band, the inductors and capacitors are chosen to present the desired impedance to the PA at the design frequency. For the MAX7044EVKIT, the values chosen present a good match to 50? at 433.92MHz.
The purpose of the experiments that follow is to change the matching components in a 433.92MHz EV kit (to present a good match at 868MHz) and to reduce the transmitted power at 434MHz.
PA-Output Tank Circuit Tuned to 868MHz
The first step in developing a matching network for 868MHz is to try the simplest possible match, which is an 868MHz tank circuit at the PA output, connected to a 50? resistor. This approach is used to produce the baseline spectrum in Figure 1. However, in this case the bias inductor is chosen to resonate the stray capacitance of the PA pin at 868MHz (instead of 434MHz). To produce the schematic presented in Figure 4, the PA bias inductor was changed in the MAX7044EVKIT from 62nH (for a resonant circuit at 434MHz) to 16nH (for a resonant circuit at 868MHz). Additionally, the shunt capacitors were removed from the pi network and the series inductor was replaced with a 0? shunt. Finally, the series capacitor C1 was changed between the pi network and bias inductor to 47pF, effectively a DC block at 868MHz.
Figure 4. Simple tank-circuit matching network for the MAX7044EVKIT at 868MHz.
Power measurements of the 434MHz fundamental frequency and the first four harmonics are listed below. The spectrum of the 434MHz and 868MHz components is shown in Figure 5. Frequencies are rounded off to the nearest 1MHz.
Total PA efficiency (power in all four frequencies/(Vdd × IPA)) = 46.6%.
The 868MHz PA efficiency = 18.4%.
Figure 5. Spectrum of MAX7044EVKIT with tank circuit tuned to 868MHz.
Because the bandwidth of the 868MHz tank circuit is narrower than the bandwidth of the 434MHz tank circuit (the stray capacitance remains the same, so the inductor needs to be reduced by a factor of 4), there is enough rejection of the 434MHz fundamental frequency to make the power in the fundamental and second harmonic almost equal. This simple change in the tank circuit improves the power ratio of the 868MHz component to the 434MHz component by approximately 3dB.
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?
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.
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?'.