Tuning the MAX2248 for 2.4GHz Applications
Abstract
The MAX2248 is a single-supply, low-voltage power amplifier (PA) that is specifically designed for applications in the 1.9GHz RF band. However, the MAX2248 can be tuned to 2.4GHz applications. This application note describes what changes are required to tune the MAX2248 evaluation kit to provide optimal gain and harmonic distortion in the 2.4GHz frequency band.
Introduction
The MAX2248 is a single-supply, low-voltage power amplifier (PA) IC with integrated digital power-control circuitry and logic-level shutdown. It is designed specifically for applications in the 1880MHz to 1930MHz frequency band. The PA provides a nominal +20dBm output power in the highest power mode.
With some modification of the input and output matching networks, the MAX2248 can be optimized for applications in the 2.4GHz to 2.5GHz frequency band. With the proper matching network, the PA can deliver +19dBm output power from a 3.2V supply, at the highest gain setting.
See Table 1 for the performance of the PA over supply voltage and frequency variations. See Table 2 for the performance of PA including output power, current consumption, and second-order harmonic distortion at different gain settings.
Tuning Methodology
The MAX2248 is a high-frequency power amplifier that requires a relatively small number of external components for matching. The components installed on the evaluation kit (EV kit) are guaranteed to give optimal performance over the 1.9GHz band, but these components can be tuned to get optimal performance over the 2.4GHz band. This section details how the tuning for 2.4GHz can be done on the MAX2248 EV kit (MAX2248EVKIT#).
Input Match
For the input side, a two-element LC match optimizes the input return loss for 2.4GHz. Remove the existing L and C on the EV kit and short the transmission line. Using a calibrated network analyzer, measure the input return loss (s11) over the 2.4GHz band and do port extension up to the pins of the IC. Doing so provides the de-embedded s-parameters, which can be imported into any tuning software to find the optimum L and C network values that minimize s11. In the software model, make sure to account for the transmission line segment from the pin of the IC to the inductor. The optimal L and C values obtained through simulation can be further fine-tuned experimentally to find the best values that give lowest input return loss.
See Figure 1 for exact values of L2 and C100 that make up the input match for 2.4GHz band.
Output Match
The primary power-matching structure at the output is a lowpass network formed by the series transmission line section and the open-stub transmission line section. This transmission line network acts like a series inductance and shunt capacitance.
To minimize the second-order harmonic distortion, the length of the open-ended transmission line needs to be adjusted. Start by reducing the length of this open-ended transmission line until the desired harmonic distortion is achieved. There is a trade-off between getting better harmonic performance versus higher output power that needs to be determined. On the EV kit the optimum value of harmonic distortion and output power is achieved with the length of open stub (TL2) being 46 degrees at 2.4GHz.
To achieve better gain at 2.4GHz, the length of series transmission line can be reduced. On the EV kit this is achieved by cutting the longer transmission line and connecting to the shortest path (TL1) that is 46 degrees at 2.4GHz. The capacitor C14 is also removed from the output.
See Figure 1 to see how the output match is achieved for 2.4GHz band on the MAX2248 EV kit.
2.4GHz Tuning Solution
The MAX2248 EV kit was tuned to operate optimally at 2.4GHz band. At the input matching network, L2 was changed to 5.1nH, C13 was removed, and a shunt cap C100 of 0.7pF was added after the inductor. The output stage match was achieved by using the shortest available series transmission line (TL1), which is 46 degrees at 2.4GHz and cutting the open-ended stub (TL2) so that its length is 46 degrees at 2.4GHz, as shown in the layout in Figure 2. Figure 3 shows the EV kit tuned for 2.4GHz.
Figure 1. MAX2248 EV kit schematic changes for 2.4GHz tuning.
PCB Layout Change
Figure 2. MAX2248 EV kit PCB changes for 2.4GHz tuning.
Figure 3. MAX2248 EV kit tuned for 2.4GHz.
| Reference | Qty | Value | Tolerance | Description | Part Number | Manufacturer |
|---|---|---|---|---|---|---|
| C1 | 1 | 1µF | ±10% | 0805 ceramic capacitor, SMT | GRM21BR71C105KA01 | Murata |
| C2 | 1 | 0.01µF | ±10% | 0402 ceramic capacitor, SMT | C0402C103K3RAC; GRM155R71E103KA01D; C1005X7R1E103K | KEMET; Murata; TDK |
| C3, C6, C7 | 3 | 470pF | ±5% | 0402 ceramic capacitors, SMT | GCM1555C1H471JA16; GRM1555C1H471JA01 | Murata |
| C4, C9, C12 | 3 | 1000pF | ±5% | 0402 ceramic capacitors, SMT | GRM1555C1H102JA01; C1005C0G1H102J050 | Murata; TDK |
| C5, C11 | 2 | 220pF | ±5% | 0402 ceramic capacitors, SMT | GRM1555C1H221JA01 | Murata |
| C8 | 1 | 18pF | ±5% | 0402 ceramic capacitors, SMT | C0402C180J5GAC; GRM1555C1H180JA01J; C1005C0G1H180J050 | KEMET/Murata/TDK |
| C10 | 1 | 8pF | ±0.25pF | 0402 ceramic capacitor, SMT | GRM1555C1H8R0CZ01D | Murata |
| C13 | DNI | 1pF | ±0.05pF | 0402 ceramic capacitor, SMT | GJM1555C1H1R0WB01 | Murata |
| C14 | DNI | 1pF | ±0.05pF | 0201 ceramic capacitor, SMT | GRM0334C1H1R0WA01 | Murata |
| C1000 | 1 | 0.7pF | ±0.1pF | 0402 ceramic capacitor, SMT, 50V | GRM1555C1HR70BA01D | Murata |
| L1 | 1 | 22nH | ±10% | 0603 ceramic inductor, SMT | LQG18HN22NJ00 | Murata |
| L2 | 1 | 5.1nH | ±0.2nH | 0402 Wirewound inductor, SMT | LQW15AN5N1C00 | Murata |
| R1, R2, R3 | 3 | 8.2W | ±5% | 0402 thick film resistors | RMC1/16S-8R2J | Kamaya |
| J1, J5 | 2 | 142-0701-851 | 2-pin connector, end launch jack receptacle | 142-0701-851 | Johnson Components | |
| J2, J3, J4 | 3 | HEADER_3P | 3-pin connector, male, through hole | 800-10-003-10-001000 | Mill-Max | |
| SU2, SU3, SU4 | 3 | STC02SYAN | 2 (1 x 2) position shunt connector black | STC02SYAN | Sullins Connector Solutions | |
| TP1 | 1 | N/A | Test point, red | 5000 | Keystone | |
| TP2 | 1 | N/A | Test point, black | 5001 | Keystone | |
| U1 | 1 | MAX2248 | MAX2248, 16 TQFN-EP; PACKAGE CODE T1633+5 | MAX2248 | Maxim | |
| TL1 | 1 | Zo = 60Ω, 46 degrees at 2.4GHz | Series transmission line segment | |||
| TL2 | 1 | Zo = 65Ω, 46 degrees at 2.4GHz | Open transmission line segment |
Performance of Tuned MAX2248 at 2.4GHz
This section shows the performance achieved on a tuned MAX2248 EV kit in the 2400MHz to 2500MHz RF band. An output power of +19dBm can be achieved at the highest gain setting, with the typical 3.2V supply while consuming only 88mA of current. The harmonic distortion is also a decent -10.6dBm. The output power can be further increased to +22dBm with a 4.5V supply voltage.
| f = 2400MHz | f = 2450MHz | f = 2500MHz | ||||
|---|---|---|---|---|---|---|
| VCC (V) | POUT (dBm) | ICC (mA) | POUT (dBm) | ICC (mA) | POUT (dBm) | ICC (mA) |
| 3.2 | 19.0 | 88 | 18.6 | 86 | 18.2 | 84 |
| 3.6 | 20.2 | 96 | 19.8 | 93 | 19.3 | 90 |
| 4.5 | 22.2 | 110 | 21.4 | 103 | 20.5 | 90 |
| Digital Control Inputs | Supply Current, Output Power and 2nd Harmonic Distortion | |||
|---|---|---|---|---|
| D1 | D0 | ICC (mA) | POUT at 2.4GHz (dBm) | 2nd Harmonic Distortion at 4.8GHz (dBm) |
| 0 | 0 | 69 | -0.9 | -29.5 |
| 0 | 1 | 71 | 7.5 | -21.5 |
| 1 | 0 | 76 | 13.9 | -17.5 |
| 1 | 1 | 88 | 19.0 | -10.6 |
Conclusion
With some modifications to the input and output match networks, the MAX2248 EV kit can be tuned to give optimal performance in the 2.4GHz to 2.5GHz frequency band. It can deliver up to +19dBm of output power at the maximum gain setting while consuming only 88mA of supply current.
The MAX2248 EV kit (MAX2248EVKIT#) can be tuned following the guidelines provided in this application note. The de-embedded s11 s-parameter file for MAX2248 can also be downloaded from the MAX2248 QuickView webpage under Design Resources.