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U9397A/C FET Solid State Switches (SPDT)

Technical Overviews

The Benefits of GaAs FET

GaAs FET switches have inherently low video leakage which makes them more suitable for measuring devices that require low maximum input power ratings. Sensitive components such as receivers, traveling wave tube (TWT) and handset power amplifiers typically have maximum input power ratings of < 13 dBm and can be easily damaged or over-driven by the high video leakage of PIN switches. Keysight Technologies, Inc. U9397A/C switches have < 10 mVpp video leakage compared to PIN switches which typically have 3 1 V video leakage.

GaAs FET switches have RF response extending down to DC, whereas in PIN switches there is a practical lower limit to the frequency range in which the diodes behave as linear resistors. Generally, PIN diode switches perform poorly below 10 MHz; the ON and OFF switching uses the same path as the RF, so they can not operate well at low frequencies.

Historically, the main drawback of GaAs FET switches has been a long settling time. The settling time of a switch is defined as 50% of TTL drive to 0.01 dB (99.88% Vfinal) of the final RF value as shown in Figure 1. Settling time includes: the time delay of the switch, switching speed and the time it takes to settle within 0.01 dB of its final value.

Typical GaAs FET switches have settling time in the order of tens of ms. This is mainly caused by the slow transients or the “gate lag” effect. Gate lag occurs when electrons become trapped at the surface of the GaAs device. The conventional method of reducing gate lag in GaAs devices is usually achieved by controlling the gate trough geometry so that the gates fit “tightly” in the bottom of trough. However, this approach reduces the breakdown voltage and power handling of the device. As shown in Figure 1, the typical transient behavior of FET switches from OFF state to ON state has a slow tail effect that increases the settling time.

Keysight U9397A/C FET switches patented design eliminates the gate lag effect (i.e. slow tail), resulting in a settling time of approximately 500 μs.

Key Features

  • Prevent damage to sensitive components with low video leakage < 10 mVpp
  • Minimize crosstalk with exceptionally high isolation 100 dB @ 8 GHz
  • Maintain fast throughput with settling time for FET switches of approximately 500 μs
  • Integrated TTL/CMOS driver eliminates the need for external drivers

Description

Keysight U9397A and U9397C FET

solid state switches, SPDT provide superior performance in terms of video leakage, isolation, settling time and insertion loss across a broad operating frequency range. The U9397A/C are particularly suitable for measuring

sensitive devices and components, such as mixers and amplifiers, where video leakage may cause damage or reliability issues. High isolation minimizes crosstalk between measurements, ensuring accurate testing and improving yields. A settling time of 500 μs makes these ideal for high-speed RF and microwave SPDT switching applications in instrumentation, communications, radar, and many other test systems.

The U9397A/C incorporate a patented design which reduces the settling time to approximately 500 μs (measured to 0.01 dB of the final value). Other FET switches available today have a typical settling time of > 50 ms.

The U9397A/C switches have a GaAs FET MMIC at each RF port, and the integrated TTL/CMOS driver is configured in such a way that when either the RF1 or RF2 port is not

selected to RFCOM, the port is terminated to 50 Ohm.

Applications

Mixer measurements

Figure 4 shows a mixer test setup which is used to test two devices simultaneously. The LO signal is omitted in the diagram since it is a fixed LO. When the first device is being tested for s-parameters, the second device is being measured for harmonics or spurious signals. The high isolation of the switches plays an important role in achieving accurate measurements when measuring spurious signals as low as –120 dBm. In this test setup, the test signal of the first device-under-test (DUT) goes through switches “B” and “D” and appearing as spurious signals for the second DUT. The spurious signal can be as low as –120 dBm for second DUT, so the total isolation between each DUT must be greater than 140 dB. Hence, each switch needs to have at least 70 dB isolation to get accurate measurements.

Dual-band mobile handset power amplifier testing

Figure 5 shows a simplified test setup of a dual-band mobile handset power amplifier. A signal generator with digital modulation capability supplies the test signal to the power amplifier and a vector signal analyzer (VSA) is used to measure the output signal from the power amplifier. Two switches are used to switch between the DCS and GSM bands and two attenuators are placed at the output of the power amplifier to protect the switches. The triggering signal (frame trigger) from the signal generator is used to synchronize the VSA and trigger the switches to test the correct band of the power amplifier at the right time.

Switch selection is very important in this application for two reasons: First, the switch must have a settling time that is fast enough to allow the VSA to capture any timeframe of the of the Signal. Figure 6 shows a timing diagram for a GSM/EDGE signal, as you can see one slot equals 577 μs. Thus, when the signal generator sends a frame trigger signal out, the switches must switch and settle within 577 μs so the VSA can start to capture data within the time frame of the slot 1 signal to ensure accurate measurements. The second reason careful switch selection is needed is video leakage. Typical PIN switches have video leakage of 3 1V due to the nature of PIN switch design. This can potentially cause permanent damage to power amplifiers because their maximum input power is typically < 13 dBm. The other alternative, electro-mechanical switches, have low or no video leakage but the switching speed (typically in ms) is too slow for application.

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