Highpass Match for 868MHz
The next step is to change the lowpass pi network to a highpass network to further attenuate the 434MHz component. The 16nH PA bias inductor and the series capacitor (47nH) were not changed. The pi network, typically used as a lowpass filter for higher harmonic rejection, was changed to a simple highpass L network that transformed 50Ω at the antenna connector to 200Ω at the PA output pin. The simpler L network was chosen for this step, instead of a full pi network, to minimize the number of additional component changes and determine the effectiveness of this change. Because the load seen at the PA output using the L network is 200Ω (instead of 50Ω), the Tx-power current drain should be lower than it is for a 50Ω load.
Figure 6. Highpass L impedance transformation network.
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 7. Frequencies are rounded off to the nearest 1MHz.
Vdd = 2.7V, IDC = 18.1mA, IPLL = 2.06mA, IPA = IDC-IPLL = 16.04mA
P(434MHz) = +2.5dBm
P(868MHz) = +11.2dBm
P(1302MHz) = +4.0dBm
P(1736MHz) = -3.2dBm
Total radiated efficiency (all four frequencies) = 41.5%.
The 868MHz radiated efficiency = 30.4%.
Figure 7. Spectrum of the MAX7044EVKIT with 868MHz tank circuit and highpass L network.
The highpass L-network match attenuates the 434MHz component and significantly increases the efficiency of the desired 868MHz component to 30.5%. This demonstrates that an 868MHz signal with more than +10dBm of transmitted power into a 50Ω antenna can be produced with some simple changes to the existing matching network.
Summary of Simple Matching-Network Changes
The bias inductor to the MAX7044EVKIT was replaced with a smaller value that formed a resonant circuit with the combination IC and board capacitance at 868MHz. This made the 434MHz and 868MHz components equal in power. Replacing the harmonic filter with a simple highpass L-matching network improves the 868MHz-to-434MHz component by another 9dB, so that 868MHz is the dominant transmitted frequency. There is a small loss in power efficiency, but this circuit is still transmitting an 868MHz signal at more than +10dBm. There are more circuit changes that can be made to further enhance the 868MHz component with respect to the fundamental 434MHz frequency and its higher harmonics.
Suggestions for Future Work
These simple changes prove that external components can be modified to significantly enlarge the second harmonic power of a transmitter IC (compared to the fundamental frequency) while maintaining a high transmitted signal level. This is a good start, but more obstacles need to be overcome in order to transmit a signal that complies with operating regulations in the license-free bands of 868MHz in Europe and 915MHz in the U.S.
Additional Enhancement of the 868MHz Component
Enhancement of the 868MHz component is improved by increasing the Q of the resonant circuit, formed by the bias inductor and the capacitance to ground of the PA. This can be done by adding a capacitor to ground at the PA output pin and reducing the bias inductor. In this investigation, the bias inductor was reduced to 16nH to resonate with the stray capacitance on the board and in the IC. The inductor can be reduced to the 5nH to 10nH range and the total shunt capacitance increased to about 6pF, before the unloaded Q of the individual components significantly decreases the overall efficiency.
The highpass L-matching network’s 434MHz rejection can be improved by simply adding a shunt inductor in the C6 position of Figure 6 to form a highpass pi network and by adjusting the inductor values. Careful selection of the three pi-network components should increase the total rejection of the 434MHz component to 25dB or 30dB. This is still short of the 46dB rejection needed to satisfy the ETSI requirement that all spurious emissions be below -36dBm, if the 868MHz transmitted signal is +10dBm. There are more suggested methods for improving the rejection toward the end of this application note.
Maintenance of Transmitter Efficiency
The modifications made thus far concentrate on enhancing the 868MHz component and rejecting the 434MHz component. These changes reduce the PA efficiency from nearly 50% for a 434MHz transmission to about 30% for an 868MHz transmission. However, further attempts to reject the 434MHz signal may degrade efficiency even more. Early measurements in the development of matching networks for 434MHz transmissions showed that the DC-current drain increased when the 434MHz matching network was mistuned. Given that a typical filter rejects frequencies by presenting a poor match at those frequencies, it is surprising that the current drain in these tests did not increase more. How can the rejection of 434MHz be further enhanced without causing more increases in DC current and consequent decreases in efficiency?
The Diplexer Approach
A diplexer is normally used in dual-channel receiver systems to connect a common receive antenna to two receivers, each tuned to a different frequency. The diplexer forms a good match to the antenna at both of the two design frequencies. If the receive antenna is replaced by the PA, there is now a separate path for the 434MHz component and the 868MHz component. The 868MHz path connects to a transmitting antenna and the 434MHz path connects to a resistive load on the circuit board. The advantage of this configuration over a simple 868MHz filter is twofold: the 434MHz component is matched (thus keeping the current drain low), and it is also sent to a load that does not radiate. If the antenna on the 868MHz port is correctly matched and tuned, the rejection of the radiated 434MHz component will be very strong. To further reduce the supply current at 434MHz, the diplexer concept could be modified to present higher impedance to the 434MHz component than to the 868MHz component.
There is a potential flaw with this approach—it assumes a linear signal source with a 50Ω load. The PA, which has a switched amplifier output, does not fit any linear models.