1. Introduction
As time passes, switching circuits are increasingly used in millimeter-wave systems such as radar, communication, and measurement to turn on and off certain signals [
1]. The microwave switch is a common microwave control circuit and is an important component in radar and measurement systems [
2,
3]. To avoid a transmission distortion between the antenna end and the receiving and transmitting ends, as well as a distortion induced by reflection at the antenna end by its own terminal, the switch must have a consistent insertion loss and reflection loss during its operation. Radio frequency (RF) switches can be broadly classified into two main categories: transistor switches and diode switches [
4,
5,
6]. While the diode switch requires a DC bias voltage for it to open and close, unlike the transistor switch, it can handle more power in signal transmission [
7]. The diode switch outperforms the transistor switch in terms of both switching characteristics due to the impact of the beginning resistance on switch isolation and the effect of the off-state capacitance on insertion loss [
8].
Within the transceiver architecture, the antenna will transfer the received signal to the input terminal of the low noise amplifier. It is necessary to ensure that the input signal does not pass through to the power amplifier. Thus, in relation to the criteria for switching characteristics, it is necessary for the switch to possess a minimal insertion loss in order to prevent an excessive attenuation of the input signal, and the low noise amplifier being unable to precisely identify the signal. Simultaneously, the switch must possess a high level of linearity in order to prevent the signal power of the power amplifier from diminishing as a result of saturation during output [
9]. This study presents the design of two types of single-pole double-throw switches specifically for K-band applications. The first type of switch employs diodes as the means of switching. Applying a forward bias voltage to the diode will activate it. Applying a reverse bias voltage causes the diode to deactivate. The length of the selected secondary tube can only be altered by modifying its gate width and the quantity of parallel diodes, due to limitations imposed by the process. Increasing the total area of the diode decreases the on-resistance, which helps to reduce the insertion loss. However, it also increases the on-capacitance, resulting in a leakage of current while the switch is off and diminishing the isolation of the switch [
10]. The second type of switch employs transistors as a means of switching and utilizes the impedance conversion of the quarter-wavelength transmission line as a method of matching to achieve the circuit signal path switching. The frequency band that is matched is directly determined by the length of the transmission line. The switch is capable of operating in both the K band and the Ka band [
11].
4. Measured Results and Discussion
The implementation of the K-band diode and FET SPDT switch chips is achieved using a 0.15 GaAs pHEMT technology. The chip photographs are depicted in
Figure 7 and
Figure 8, and the chips have a final area of 1 mm × 2 mm.
Figure 9 depicts the utilization of the S-parameter test platform for this particular single-pole double-throw switch. In the diagram, DUT refers to the circuit chip that has been specifically created for testing purposes in this research study. On either side of the circuit, G-S-G and G-S-G-S-G are directly mounted on the wafer. The Agilent N5247A PNA-X (Santa Clara, CA, USA) is a device used for analyzing microwave networks. Simultaneous measurements can be taken for the return loss, gain, and reverse isolation of the three ports. Prior to the test, the measuring line is calibrated using the test piece, and the frequency width is fine-tuned to 100 Hz in order to enhance the precision of the measurement. When conducting tests on the diode single-pole double-throw switch, it is important to follow a specific sequence. First, the power supply must be turned on. For testing from P1 to P2, the voltage should be turned on in the order of VA and VB, followed by the testing of the S-parameter. Conversely, when testing from P1 to P3, the voltage should be turned on in the order of VB and VA, and then the S-parameter can be tested.
Figure 10 displays the test platform used for assessing linearity. The linearity test involves two parameters, namely the 1 dB compression point and the input third-order intercept point. Due to the double-ended nature of the linearity test, a 50Ω terminal resistor is employed as a virtual resistor at the closed port. The Agilent E8257D (Santa Clara, CA, USA) is a device that generates signals. Initially, the functionality of the diode single-pole double-throw switch is assessed. During the testing of the 1 dB compression point, the output of the Agilent E8257D is connected directly to the P1 port of the circuit described in this paper. The open end of this circuit, either P1 or P2, is then connected directly to the Agilent E4448A (Santa Clara, CA, USA) spectrum analyzer. The voltage opening sequence of the power supply is dictated by the opening end. If the P2 end is opened, the sequence is VA followed by VB. If the P3 terminal is opened, the sequence is VB followed by VA. After that, the signal generator is configured to generate output signals for testing. Next, we evaluated the functionality of the FET single-pole double-throw switch. During the testing of the 1 dB compression point, the output of the Agilent E8257D is connected directly to the P1 port of the circuit described in this paper. The open end of this circuit (P2 or P3) is then connected directly to the Agilent E4448A spectrum analyzer. The power supply’s voltage switching sequence is determined by the switching end. When P2 is turned on, the voltage switching sequence consists of VG1, VM1, VG2, and VM2. When P3 is enabled, it follows the sequence of VG2, VM2, VG1, and VM1. After that, the signal generator is configured to generate output signals for testing. For the third-order cut-off point test, two Agilent E8257D instruments are used as the output sources. To combine the power from these sources, a power divider is required between the two instruments. The combined signal is then connected to the P1 terminal of the circuit, and the received signal is tested at the output terminal of the circuit (P2 or P3).
The test results for a diode single-pole double-throw switch are outlined as follows. Upon opening the P1 to P2 channels, the VA voltage measures 1.5 V while the VB voltage measures −2 V. Upon opening the P1 to P3 channel, the VA and VB voltages measure −2 V and 1.5 V, respectively. During the test, a current of 65 mA is observed to flow through the pad with a positive voltage, however no current flows through the pad with a negative voltage. The combined power consumption of the circuits in both operational modes is 97.5 milliwatts. Upon opening the P1 to P2 path, the switching circuit parameters outlined in this paper are examined and the findings are presented in
Figure 11. Subsequently,
Figure 11a displays the chart of the return loss results,
Figure 11b illustrates the diagram of the test results for the insertion loss and isolation, and
Figure 11c showcases the test result of the 1 dB compression point at the operating frequency of 20 GHz. In the obtained measurement result, the value has reached the maximum frequency that the measuring instrument can output, but it has not yet reached the point where the power is compressed by 1 dB. Therefore, we can only conclude that the value is greater than 15 dBm.
Figure 11d represents the diagram illustrating the measurement result of the third-order intercept point. The circuit’s working frequency throughout the test is 20 GHz, with a channel spacing of 5 MHz. The third-order cut-off point, computed using the heterodyne method, is 28 dBm.
When the P1 to P3 path is activated, the switching circuit described in this paper undergoes testing for various parameters. The test results are presented in
Figure 12, with
Figure 12a representing the chart for the return loss results,
Figure 12b depicting the diagram for the test results of the insertion loss and isolation, and
Figure 12c illustrating the comparison between the test results of the 1 dB compression point and the simulation results at the operating frequency of 23 GHz. In the obtained results, the value of the measured quantity reaches the maximum frequency that the measuring instrument can output, but it does not yet reach the point where the power is compressed by 1 dB. Therefore, it can only be concluded that the value is greater than 15 dBm.
Figure 12d shows the diagram representing the measured results of the third-order intercept point. The circuit’s working frequency throughout the test is 23 GHz, with a channel interval of 5 MHz. The third-order cut-off point, computed using the heterodyne method, is 39.5 dBm.
The test results for a transistor single-pole double-throw switch are outlined as follows. When the P1 to P2 channels are activated, the voltages of VG1, VM1, VG2, and VM2 are measured to be −2.1 V, 0 V, −2.1 V, and −0.9 V accordingly. Meanwhile, the chip’s current is only a few nA, and its power consumption is almost negligible. When the P1 to P3 channel is activated, the voltages of VG1, VM1, VG2, and VM2 are 0 V, −3 V, −0.9 V, and −2.1 V, respectively. Simultaneously, the chip’s overall current is merely a few nA, and the entire power consumption is nearly negligible. Upon opening the P1 to P2 path, the parameters of the switching circuit outlined in this study are examined and the findings are displayed in
Figure 13.
Figure 13a depicts the diagram of the test results for the return loss;
Figure 13b illustrates the diagram of the test results for the insertion loss and isolation;
Figure 13c displays the test result diagram for the 1 dB compression point at the operating frequency of 20 GHz, with a measured value of 6 dBm; and
Figure 13d shows the measured result diagram for the third-order intercept point. The circuit operates at a frequency of 20 GHz during the test. The channel interval is 5 MHz, and the third-order cut-off point is obtained as 19 dBm using the heterodyne method.
Upon opening the P1 to P3 path, the parameters of the switching circuit outlined in this work are examined and the findings are displayed in
Figure 14.
Figure 14a displays the diagram of the test results for the return loss;
Figure 14b shows the diagram of the test results for the insertion loss and isolation;
Figure 14c represents the test result diagram of the 1 dB compression point at the operating frequency of 20 GHz, with a measured value of 7 dBm; and
Figure 14d illustrates the measured result diagram of the third-order intercept point. The circuit’s working frequency during the test is 20 GHz, with a channel interval of 5 MHz. The third-order cut-off point of 22 dBm was determined using the heterodyne method.
To conduct a thorough performance comparison between the two K-band single-pole double-throw switches and the other literature sources, the relevant characteristics have been compiled in
Table 1. The optimal coefficient figure-of-merit (
FOM) is expressed as follows:
Return Loss refers to the loss of power in a signal that is reflected back from a device or system. Insertion Loss, on the other hand, refers to the loss of power that occurs when a signal is inserted into a device or system. Lastly, Area refers to the size or dimensions of a chip.
When compared to a diode switching circuit, this circuit has a smaller chip area and a greater optimization coefficient. Transistor circuits exhibit reduced insertion losses and increased signal isolation. The input return loss of the two is comparable, and the figure of merit (FOM) value is higher than that reported in other research.