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.
Vdd = 2.7V, I = 16.83mA, IPLL = 2.06mA, IPA = I - IPLL = 14.77mA
P(434MHz) = +9.0dBm
P(868MHz) = +8.65dBm
P(1302MHz) = +4.5dBm
P(1736MHz) = -3.0dBm
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.