Comtech EF Data PCB-4000 - Manual (Page 18)

PCB-4000 1+1 Phase Combiner Revision 2
Introduction MN/PCB4000.IOM
1–4
• The RF signals from the PCCB are fed to the RF input of the respective SSPA via a set of
phase‐matched cables. Again, the phase length (and matching) of these cables is
essential to system operation and no substitutes should be used. The routing should not
be such to cause extreme bends.
• The output waveguide combining system consists of balanced waveguide lengths, two
waveguide transfer switches, a “Magic‐Tee” combiner with termination, and couplers
for test and alignment.
1.3 Theory of Operation
As mentioned previously, phase combining is a common technique to increase the available
output power of an amplifier system. Referring back to the Figure 1‐2 system block diagrams,
when two signals of equal phase and amplitude are fed into the “Magic‐Tee” combiner, the
individual power of each SSPA is summed at the output port and cancelled in the termination
port.
In real systems, the phase and amplitude of the two signals are never exactly equal, so there is a
small amount of power that is absorbed by the load termination. This terminated port is
sometimes referred to as the “wasted” power port. In practice, however, it is possible to keep
phase and amplitude imbalances at low enough levels such that overall combining losses are
only in the 0.2 to 0.5 dB range.
Figure 1‐3 shows the effects of phase imbalance on the power combining efficiency.
Figure 1-3. Combining Loss vs. Phase Imbalance Summary of Specifications
1.3.1 Phase and Gain Equalization
Naturally, two separate amplifiers are likely to have different phase and gain characteristics. The
PCB‐4000 1+1 system is designed such that the phase difference between the two amplifier
paths is compensated by adjusting the phase shifter in the Phase Combiner Control Box. This is
done at the factory for the full amplifier bandwidth, and should not normally require further
adjustment in the field unless an amplifier has been replaced. (See alignment procedure).