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 Physical constraints for broadband operation
If you are a microwave hardware engineer you most likely have had a few sobering experiences when you test your new design the first time, particularly wideband high frequency modules –they almost never look like the model. Many of the bigger issues come down to physics. Broadband modules e.g. 2 to 18 GHz virtually always have gain roll-off at the high end of the band, from a combination of transistor and package parasitic L’s & C’s give rise to gain roll-off and VSWR mismatch losses, while interconnect transmission lines have increasing loss with frequency.
To achieve good EW module signal fidelity over such frequency ranges requires thin dielectric interconnect substrates for:
1) Low ground via inductance for SMT amplifiers and RF components
2) 50 ohm line widths commensurate with SMT component signal pads
3) High isolation at high frequencies.
Further, any components in the RF signal path should have a signal contact width that closely matches the 50 ohm line width to prevent reflections (VSWR) which degrade signal fidelity. Typical construction has transmission line widths in the 0.010 to 0.02 inch line widths to achieve 50 ohm characteristic impedance. A consequence of module measured performance deviating from model prediction is an in ability to pre-plan effectively to compensate for gain slope issues.
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