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Why Is It So Difficult To Package Devices, Modules And Subsystems At Microwave And Millimeter-wave Frequencies?

As the operating frequency of a circuit increases, the challenges also increase. Fundamentally, this is due to the fact that as frequency increases, the wavelength is shorter. This causes the circuit elements to begin to perform differently since their physical size is an applicable fraction of a wavelength. For instance, lumped element devices no longer perform properly. Interconnects such as wire bonds behave like inductors or worse, an antenna. Simple interconnects such as conductor lines can behave electrically as inductors, capacitors and can resonate. Two simple examples are given below.

Consider A Simple Circuit: An Inductor

This can be demonstrated by considering a simple inductor. The figure below shows the insertion loss of an ideal inductor and measured performance of a real inductor. From the curve, it can be seen that the ideal inductor curve is monotonic with increasing insertion loss as frequency increases. This is exactly what one would expect for an ideal inductor since its reactance increases as frequency increases. However, the measured data for the real inductor shows that the frequency range over which it performs like an inductor is only to about 1.25GHz. Above that frequency, the insertion loss deviates significantly from the ideal, expected performance.

Moreover, the measured data shows that the inductor actually resonates at about 3.5GHz. At this resonant frequency, the inductor has completely deviated from performing as an inductor. If the inductor is used in the frequency range of that it is resonating, it can cause undesired circuit effects such as amplifier oscillation or compromised filter performance if it is used in these types of circuits.

These non-ideal characteristics are due to the fact that as frequency increases, the inductor's size becomes an appreciable fraction of a wavelength and effects that can be ignored at lower frequencies become very important at high frequencies. Successful product development at higher frequencies requires a solid understanding of these effects.

Consider An Interconnect: Conductive Trace (Transmission Line)

Most electric circuits use electrical conductors to connect between components. For instance, consider a conductive trace on a printed circuit board. At high frequencies, these traces can be 52 ohm transmission linemodeled as transmission lines. The electrical performance of a simple trace is shown in the figure below. The trace is a 53 ohm transmission line connected between a 50 ohm source and a 50 ohm load. To make this a bit more concrete, consider this a metal trace between two amplifiers. The first amplifier output is an ideal 50 ohm source and the second amplifier's input is an ideal 50 load.

The transmission line performance in the plot shows that the insertion loss (S21) is very near zero which means most of the energy is being transferred from the first amplifier to the second. Also the return loss (S11) is less than -25dB which means very little of the energy from the first amplifier is being reflected back to it. In short, the interconnect is performing ideally.

However, consider the effect of just changing the width of the conducting trace. The figure below shows the Effect of a mismatched transmssion lineresult. The conductor was narrowed so that its impedance is now 75 ohms. Note how the insertion loss (S21) is no longer near zero due to mismatch loss. Energy that should be transferred to the second amplifier is now being reflected back to the first. The return loss is approximately 6dB and is causing the mismatch loss.

This simple transmission line analysis is an example of how effects that can be neglected at lower frequencies must be considered carefully when operating at high frequencies.

Conclusions

These two simple examples show one reason for the difficulties of designing at microwave and millimeter-wave frequencies. There are other reasons such as increased power density for high power amplifiers, hermeticity requirements, high signal gain, interconnect alignment issues, operation over temperature range and hydrogen poisoning. It is critically important to choose your partner for your product development and contract manufacturing if your products operate at high frequencies.