Broadband quadrature (i.e., 90 degree) combiners have been the primary means to sum the outputs of multiple RF/microwave power amplifiers (PA), preferred because of their inherent interstage matching, open-loop gain flatness, load tolerant efficiency, spectral stability and third-order harmonic suppression performance. The main drawback with “quads” is they only work with pairs of amplifier modules, i.e., they combine in binary: 2, 4, 8, etc. In many applications with power density and weight restrictions—such as IED jammers, airborne EW and communications jamming—two amplifier modules do not provide enough power and four have too much power, unacceptable efficiency, cost or weight.
It is not that broadband, three-port combiners do not exist. They do. However, they have only been available in 0 degree format, and in-phase power combining lacks the benefits of the quadrature combiner. To have the advantages that quads offer, three-port combiners need to be phased with 60 degree differentials between ports. CPC has developed a new broadband technology that meets this requirement. Figure 1 shows the signal flow of the CPC model CM-10-130-1E3-3P in a divider configuration. The unit covers 10 to 130 MHz and, when used as a combiner, handles a combined output power of 500 W CW, 1 kW pulsed. Insertion loss is specified at 0.5 dB typical, 0.8 dB maximum and worst-case isolation between any of the three ports is 20 dB. With all input ports terminated in 50 Ω, the typical return loss at the combined output is 20 dB, 15 dB worst-case. With the input ports open or shorted, the return loss at the output port degrades to 15 dB typical and 10 dB worst-case. The phase tolerance around the nominal 60 degree phase difference among the three ports is ±15 degrees.
Figure 1 Signal flow of the 60 degree, 3-way divider/combiner used as a divider.
The RoHS-compliant combiner has female SMA connectors on the inputs and a type N connector on the combined output. The operating temperature range is 0°C to +40°C, and the unit will withstand 40 Grms, six-axis random vibration.
The true test of this new 60 degree combiner/divider is how it performs in an amplifier application compared to a 0 degree combiner/divider. A thorough evaluation confirms it provides the same desirable traits as a quadrature combiner.
Figure 2a shows the input |S11| of three combined amplifiers using 0 degree divider/combiners. Best case, the |S11| is on the order of ‐10 dB. Using the 60 degree divider/combiners (see Figure 2b), the |S11| is dramatically improved to ‐20 dB average. This improvement in |S11| translates to an enhancement in gain flatness, shown in Figure 3. The 0 degree combined amplifiers have ±1.4 dB flatness, and the 60 degree combined amplifiers show a modest improvement to ±1.2 dB. A more dramatic performance enhancement is observed with third-order harmonics, shown in Figure 4. The 60 degree configuration achieves, on average, 13.4 dB lower third-order harmonics than with the 0 degree configuration.
Figure 2 Amplifier |S11| using 3-way, 0 degree divider/combiner (a) vs. 3-way, 60 degree divider/combiner (b).
Using the 60 degree divider and combiner helps maintain amplifier load tolerance or constant efficiency. Comparing amplifiers driven into 2:1, 3:1 and 5:1 load VSWRs with discrete 45 degree phase rotations at 100 MHz, the 60 degree power combiner maintains the efficiency within ±2.85 percent
(see Figure 5). Using the 0 degree combiners resulted in a ±13 percent variation in efficiency. The data indicates there are certain phase rotations where the 0 degree combiner yields higher efficiency; however, as the magnitude of the load VSWR increases, the efficiency drops below 30 percent at certain load phase angles. This is highly problematic for an RF PA designer, since the length of cable connected to the amplifier in the field cannot be controlled.
Figure 3 Amplifier gain flatness using 3-way, 0 degree divider/combiner (a) vs. 3-way, 60 degree divider/combiner (b).
Until now, broadband RF PA engineers had to sacrifice several performance parameters to combine amplifiers with three-port networks. However, CPC’s novel 60 degree divider/combiner architecture provides the means to maintain RF amplifier performance while achieving the optimum power level—not attainable with binary (2, 4, 8, etc.) combiner networks.
Figure 4 Third-order harmonic performance comparison of an amplifier using the 0 degree divider/combiner and the 60 degree divider/combiner.
Figure 5 Efficiency vs. load for a power amplifier using the 60 degree divider/combiner. The load was varied across 45 degree phase steps around the Smith chart at VSWRs of 2:1, 3:1 and 5:1.
Communication Power Corp. (CPC)
Hauppauge, N.Y.
www.cpcamps.com
info@cpcamps.com