GaAs Switches Are a High-Performance Alternative to SOI for Test & Measurement Instrumentation

The RF switch is an important element in test & measurement instrumentation for routing signals and for filtering them, utilizing switch manifolds and filters. One unique challenge for switches in the test & measurement market is the requirement to pass relatively high-level signals of greater than +20 dBm at very low frequencies, often down to 9 kHz. It seems that 9 kHz is an unusual frequency until you consider that the frequency limits for IEC/CISPR 11, EN 55011 for conducted and radiated emissions of ISM RF equipment are 9 kHz to 400 GHz, and that prevailing test equipment must be capable of measuring to these limits. Speed, including settling time is also of paramount importance in test and measurement, especially for Automatic Test Equipment (ATE). In ATE systems, SOI switch delays that are 100 times greater than the few tens of ns exhibited by GaAs switches must often be avoided, as these delays can become cumulative throughout an instrument’s signal chain and over the course of long, complex measurement routines. The final challenge for the designer is cost containment, particularly when equipment is designed with a significant number of switches.
Switches using both GaAs and SOI technologies are possibilities when designing modern test & measurement instrumentation such as spectrum analyzers, vector network analyzers, signal generators, or for complex interface testing using channel emulators for semiconductor test applications. In this application note we discuss why GaAs switches are re-emerging as an attractive option in test & measurement due to technology advances, and the key metrics that make choosing a GaAs RF switch an easy decision.
The GaAs Switch – A Natural Fit
Even though a mistaken belief has arisen recently that SOI has somehow taken over as the preferred technology for RF switching in test and measurement, the fact remains that GaAs RF switches are superior to SOI in the frequency range beginning at 9 kHz, a range that has long been essential to the test and measurement community. In this lower frequency range, GaAs switches simply outperform SOI by orders of magnitude in speed and input power handling, while also exhibiting significantly greater operating temperature.
GaAs is roughly 100 times faster and handles up to 30 times more thru path power than SOI at frequencies as low as 9 kHz. The reason that greater speed and power is important is that test equipment such as signal generators, vector network analyzers, and spectrum analyzers are often specified to operate at 9 kHz and make measurements per the IEC/CISPR 11, EN 55011 limits that begin there. An SOI switch would not only operate more slowly than a GaAs switch, effectively slowing the instrument’s response time, but the SOI switch would also cause problems when trying to source or measure signals near the instruments’ maximum RF power levels at low frequencies. Mini-Circuits’ recent introduction of the M3SWA2-63DRC+ and the M3SWA2-34DR+ ultrafast, absorptive RF switches has given designers an unbeatable solution when it comes to choosing an RF switch for test and measurement applications. Designers have now discovered that GaAs is a natural fit, and that advances in GaAs technology provide the speed and power handling at frequencies as low as 9 kHz that signal generators, spectrum analyzers, and vector network analyzers all require.
GaAs vs. SOI RF Switches in Test & Measurement – A Comparison of Key Parameters
A comprehensive parametric comparison of the Mini-Circuits’ wideband GaAs MMIC M3SWA2-34DR+ switch to two SOI competitive offerings was performed and the results are shown in Table 1. A comparison of more than a dozen specification parameters for the Mini-Circuits M3SWA2-34DR+ wideband GaAs MMIC switch and two wideband SOI MMIC switches is included. One of the first parameters to examine is frequency coverage. One benefit of GaAs RF switches is their ability to operate at very low frequencies, essentially down to DC. For decades, GaAs switches were the device of choice for high-frequency operation as well, and still are in many applications. SOI devices are constructed with very tiny geometry MOS transistors to reduce parasitic capacitance and to achieve very high-frequency operation (tens of GHz).

These tiny transistors have low breakdown voltages that are easily exceeded at low frequencies, especially at 9 kHz, which compromises the low-frequency maximum thru path power of SOI. Case in point, the Brand X SOI switch in Table 1 can only support a maximum thru path power level of +10 dBm at 10 kHz, which is 15 dB less than the power level that the Mini-Circuits M3SWA2-34DR+ GaAs MMIC switch can sustain! Additionally, the Brand X SOI switch exhibits ON, OFF, and 0.05 dB settling times that are over 100 times slower than those of the M3SWA2-34DR+ GaAs switch! Likewise, the Brand Y SOI switch turns ON and OFF nearly 40 times slower than the M3SWA2-34DR+ GaAs switch, has a 0.05 dB settling time that is over 300 times slower, and handles a maximum of just +15 dBm of thru path power at 10 kHz.
The M3SWA2-34DR+ GaAs switch turns ON and OFF in roughly 20 ns and settles to 0.05 dB of the final RF output signal in approximately 30 ns. Now that’s fast! The maximum CW thru path power that the M3SWA2-34DR+ GaAs switch can handle at 10 kHz and +105⁰C (not +85⁰C, as with SOI Brand X and Brand Y) is +25 dBm, plenty to support most signal generator and vector network analyzer outputs.
Maximum Thru Path Power Handling Levels and Derating
As discussed in the previous section, Mini-Circuits’ M3SWA2-34DR+ GaAs RF switch excels in maximum thru path power handling, particularly at very low frequencies such as 9 or 10 kHz. Table 1 lists the maximum thru path power at the low end of the frequency band (10 kHz, 10 MHz, and 100 MHz). A graphical representation of thru path power handling for the three RF switches is shown in Figure 1.

Below 900 kHz, the Mini-Circuits’ M3SWA2-34DR+ GaAs switch handles greater thru path power than its Brand X and Brand Y SOI counterparts, as it derates less than 1.5 dB while the SOI switches derate 12 to 14 dB. The continuing reduction in SOI switch power handling with declining frequency is due to the tradeoff made in constructing the SOI switches with very tiny transistor geometries. High frequency operation is achieved, but limitations on low-frequency performance are introduced, especially where power handling is concerned.
GaAs and Go
In this application note, we drew a detailed comparison between the Mini-Circuits’ M3SWA2-34DR+ GaAs MMIC RF switch and two SOI counterparts. Over a dozen parameters were examined to show how the M3SWA2-34DR+ GaAs switch performance is very similar to the SOI competition, although at frequencies of 1 MHz to as low as 9 kHz, all similarity ends. For the test and measurement market, the M3SWA2-34DR+ dominates by handling high levels of thru path power, even at low frequencies, while exhibiting lightning-quick switching speeds. Before you fall victim to any rumors of SOI taking over all switching functions, take a look at Mini-Circuits’ cost-effective GaAs alternatives. Their speed endurance at lower frequencies will surprise you.