A method for improving isolation of a millimeter wave sub-millimeter wave monolithic integrated modulator

Abstract

The application discloses a method for improving the isolation of a millimeter wave submillimeter wave single-chip integrated modulator, which comprises a digital controlled switch capacitor unit, each capacitor in the digital controlled switch capacitor unit is connected with an NMOS tube in series, wherein the opening and closing of each capacitor is controlled by the NMOS tube connected in series, the NMOS tube is a digital controlled switch, the switch voltage of the digital controlled switch is provided by a voltage source, and a series resistor functions as a protection MOS tube; a plurality of digital controlled switch capacitor units can be divided into two groups and are connected in parallel at a differential input end of a local oscillator of the modulator, the symmetry of the modulator circuit is compensated by adjusting an external power supply, so that the isolation of the modulator is improved.

Description

Technical Field

[0001] This invention provides a method for improving the isolation of millimeter-wave and submillimeter-wave monolithic integrated modulators, belonging to the field of millimeter-wave and submillimeter-wave monolithic integrated modulator technology. Background Technology

[0002] Modulators are crucial components in modern wireless communication systems. OOK modulation, also known as flash modulation, is a digital modulation technique that converts digital signals into analog signals with two different amplitudes. When the digital signal is 1, the transmitter outputs a high-amplitude carrier wave; when the digital signal is 0, the transmitter outputs a low-amplitude carrier wave. OOK modem circuits are widely used in communications, computers, and the Internet of Things (IoT). For example, in communications, OOK modem circuits can transmit voice and data signals from the transmitter to the receiver, enabling real-time voice and data transmission. In computers, OOK modem circuits can be used for wireless transmission, such as Bluetooth and Wi-Fi. In the IoT, OOK modem circuits can be used for data transmission in wireless sensor networks and smart homes. In OOK modulator circuits, the bit error rate (BER) is a critical indicator, reflecting the system's reliability and stability. A high BER can lead to excessively high data transmission error rates throughout the communication system. In differential OOK modulator circuits, circuit asymmetry can significantly affect the bit error rate. Traditional differential OOK modulators reduce circuit asymmetry by using symmetrical board layouts. However, in millimeter-wave and sub-millimeter-wave frequency bands, circuit parasitic effects worsen severely. These uncontrollable factors can greatly affect the asymmetry of the differential circuit, thereby affecting the isolation of the OOK modulator circuit. Summary of the Invention

[0003] Technical Problem: In order to overcome the shortcomings of the existing technology, the present invention provides a method for improving the isolation of millimeter-wave and submillimeter-wave monolithic integrated modulators. In the OOK modulator, the method controls the differential input port of the OOK modulator through multiple digitally controlled switched capacitor units to perform symmetry compensation on the overall OOK modulator circuit, thereby improving the isolation of the OOK circuit in the on and off states.

[0004] Initial technical solution: To achieve the above objectives, the technical solution adopted by the present invention for improving the isolation of millimeter-wave and submillimeter-wave monolithic integrated modulators is as follows: In the OOK modulator, multiple digitally controlled switched capacitor units are divided into two groups, and their output terminals are connected in parallel to the local oscillator differential input terminal of the OOK modulator. By adjusting the external power supply, the digitally controlled switched capacitor units compensate for the symmetry of the OOK modulator circuit, thereby improving the isolation of the modulator. The first and second digitally controlled switched capacitor units form the first group of digitally controlled switched capacitor units, and the third and fourth digitally controlled switched capacitor units form the second group of digitally controlled switched capacitor units. The first, second, third, and fourth CNC switched capacitor units have the same structure. Each capacitor is cascaded with a switching NMOS transistor. The switching on and off of each capacitor is controlled by the cascaded switching NMOS transistor, which acts as a CNC switch and is controlled by the switching voltage.

[0005] The first, second, third, and fourth CNC switched capacitor units have the same structure. Each CNC switched capacitor unit includes a fixed-value interdigitated capacitor, a switching NMOS transistor, and a protection resistor. The first protection resistor is connected in series between the DC tuned voltage source and the gate of the fifth NMOS transistor to protect the MOS transistor. The drain of the fifth NMOS transistor is cascaded with the first interdigitated capacitor, and the other end of the first interdigitated capacitor is connected in parallel to the local oscillator input of the OOK modulator, forming the first CNC switched capacitor unit. The CNC switched capacitor unit is controlled by the switching voltage and has two states: on and off. The first, second, third, and fourth CNC switched capacitor units are divided into two groups and connected in parallel to the local oscillator differential input terminal of the OOK modulator. Each single-ended input terminal is connected in parallel with two CNC switched capacitor units to form a 2-bit capacitor switch. The complete OOK modulator circuit forms a total of 4-bit capacitor switches. Tuning the 4-bit capacitor switches adjusts the local oscillator differential signal at the local oscillator differential input terminal of the OOK modulator, compensating for the symmetry of the entire circuit, thereby improving the isolation of the OOK modulator.

[0006] The OOK modulator has a local oscillator differential circuit composed of a first NMOS transistor, a second NMOS transistor, a third NMOS transistor, and a fourth NMOS transistor. The output terminals of the digitally controlled switched capacitor unit are connected in parallel to the local oscillator differential input terminal of the modulator.

[0007] Beneficial effects: The method for improving the isolation of millimeter-wave and submillimeter-wave monolithic integrated modulators provided by this invention has the following advantages compared with the prior art: 1. One existing method to improve the isolation of OOK circuits is to enhance the symmetry of the layout. However, this places extremely high demands on the board layout and causes severe parasitic effects in the millimeter-wave and sub-millimeter-wave frequency bands. These uncontrollable factors can seriously affect the symmetry of the circuit. This invention uses an externally controlled digitally switched capacitor unit to tune and compensate for the asymmetry of the circuit, thus avoiding the high requirements on the board layout and the uncontrollable parasitic factors in the millimeter-wave and sub-millimeter-wave frequency bands.

[0008] 2. Another existing method to improve the isolation of OOK circuits is to use two differential pairs to form a complete differential circuit, and connect a tunable current source to the common transmitter of each differential pair. By adjusting the DC bias point of the differential pair through the tunable current source, the symmetry of the differential circuit is improved. However, two differential pairs and an additional tunable current source pair will greatly increase the complexity of the circuit and increase the chip area cost. This invention only needs to use four digitally controlled switched capacitor units to compensate for the asymmetry of the OOK circuit. The digitally controlled switched capacitor unit has a simple structure, saves chip area, and the capacitance value can be fully simulated and optimized through electromagnetic simulation software, which greatly reduces the complexity of the circuit. Attached Figure Description

[0009] Figure 1 This is a circuit diagram for compensating for the symmetry of the OOK modulator to improve isolation in this invention; Figure 2 A schematic diagram of a digitally controlled switched capacitor unit for a monolithically integrated OOK modulator suitable for millimeter-wave and submillimeter-wave applications; Figure 3 This is a simulation diagram of the RF output signal waveform without the use of a digitally controlled switched capacitor unit OOK modulator; (local oscillator signal 130GHz); Figure 4 This is a simulation diagram of the RF output signal waveform using a digitally controlled switched capacitor unit OOK modulator (local oscillator signal 130GHz). Figure 5 This is a comparison of the simulated RF signal waveforms with and without a digitally controlled switched capacitor unit OOK modulator. (Local oscillator signal 130GHz).

[0010] The diagram shows: a first CNC switched capacitor unit 1, a second CNC switched capacitor unit 2, a third CNC switched capacitor unit 3, and a fourth CNC switched capacitor unit 4; a first NMOS transistor Q1, a second NMOS transistor Q2, a third NMOS transistor Q3, a fourth NMOS transistor Q4, a fifth NMOS transistor Q5, a sixth NMOS transistor Q6, a seventh NMOS transistor Q7, and an eighth NMOS transistor Q8; a first interdigitated capacitor C1, a second interdigitated capacitor C2, a third interdigitated capacitor C3, and a fourth interdigitated capacitor C4; and a first protection resistor R1, a second protection resistor R2, a third protection resistor R3, and a fourth protection resistor R4. Detailed Implementation

[0011] The invention will now be further described with reference to the accompanying drawings.

[0012] like Figure 1 The diagram illustrates a method for improving the isolation of a millimeter-wave and submillimeter-wave monolithically integrated OOK modulator. The numerically controlled switched capacitor unit includes interdigitated capacitors with fixed capacitance, a switching NMOS transistor, and a protection resistor. Taking the first numerically controlled switched capacitor unit 1 as an example, the first protection resistor R1 is connected in series between a DC-tuned voltage source and the gate of the fifth NMOS transistor Q5. The fifth NMOS transistor Q5 is cascaded with the first interdigitated capacitor C1. The other end of the first interdigitated capacitor C1 is connected in parallel to the local oscillator input of the OOK modulator. The switching voltage of the numerically controlled switched capacitor unit is provided by the DC-tuned voltage source and exists in two states: on and off. The first protection resistor R1, the fifth NMOS transistor Q5, and the first interdigitated capacitor... Capacitor C1 and the DC tuned voltage source together form the first digitally controlled switched capacitor unit 1. The four digitally controlled switched capacitor units, namely the first digitally controlled switched capacitor unit 1, the second digitally controlled switched capacitor unit 2, the third digitally controlled switched capacitor unit 3, and the fourth digitally controlled switched capacitor unit 4, are divided into two groups and connected in parallel to the local oscillator differential input terminal of the OOK modulator. Two digitally controlled switched capacitor units are connected in parallel to each single-ended input terminal to form a 2-bit capacitor switch. The complete OOK modulator circuit forms a total of 4-bit capacitor switches. Tuning the 4-bit capacitor switches adjusts the local oscillator differential signal at the local oscillator differential input terminal of the OOK modulator, compensates for the symmetry of the entire circuit, and thus improves the isolation of the OOK modulator.

[0013] like Figure 2 The diagram shows a digitally controlled switched capacitor unit. This invention controls the NMOS transistor's on / off state by controlling the gate voltage of the NMOS transistor. When the NMOS transistor is on, its drain is conducting, and the capacitor cascaded with it is turned on and connected to the circuit. Simultaneously, due to the DC blocking characteristic of the capacitor, the digitally controlled switched capacitor unit does not affect the DC bias of the OOK modulator circuit. The digitally controlled switched capacitor unit only changes the amplitude and phase of the differential input signal to compensate for the circuit's imbalance.

[0014] The differential OOK modulator circuit suffers from local oscillator leakage due to circuit asymmetry. Even when the modulation signal is zero, it still outputs a radio frequency signal with a certain amplitude, significantly increasing the modulator's bit error rate. This invention addresses this by connecting a digitally controlled capacitor switching unit in parallel to the local oscillator differential input of the OOK modulator. By switching the capacitors on and off, the amplitude and phase of the input differential signal are adjusted, thus compensating for the inherent asymmetry of the OOK modulator's differential circuit by altering the balance of the differential input signal's amplitude and phase. The OOK modulator circuit contains four digitally controlled switched capacitor units, divided into two groups connected in parallel to the differential input of the OOK modulator. Each input can tune two switched capacitors, allowing for sufficient tuning to compensate for the circuit's inherent asymmetry.

[0015] The following are the chip simulation results: To verify the feasibility of the method of the present invention, a D-band tuned isolation OOK modulator was designed using 28nm CMOS process. Figure 3 This is the RF signal output waveform of the OOK modulator. It can be clearly seen that when the modulation signal is 0, the OOK modulator's RF output signal still has amplitude. Ideally, when the modulation signal is 0, the OOK modulator circuit has no DC bias. However, due to the asymmetry of the OOK modulator's differential circuit itself, local oscillator leakage occurs. The local oscillator signal of the OOK modulator leaks directly to the output terminal without being modulated by the OOK modulator itself. The OOK modulator using digitally controlled switched capacitor unit tuning technology has an RF output signal amplitude of 25mV when the modulation signal is 1, a 16.7% reduction compared to the 30mV output signal without the digitally controlled switched capacitor unit. However, when the modulation signal is 0, the OOK modulator with the digitally controlled switched capacitor unit has an output signal amplitude of 1mV, a 50% reduction compared to the 2mV output signal amplitude without the digitally controlled switched capacitor unit. Therefore, the digitally controlled switched capacitor unit does not simply reduce the gain of the OOK modulator, but rather compensates for the asymmetry of the OOK modulator's differential circuit and improves the isolation of the OOK modulator circuit.

[0016] The OOK modulator described above is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and the invention can be extended to other types of modulator structures. These improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for improving the isolation of a millimeter-wave and submillimeter-wave monolithic integrated modulator, characterized in that: In the OOK modulator, multiple digitally controlled switched capacitor units are divided into two groups, and their output terminals are connected in parallel to the local oscillator differential input terminal of the OOK modulator. By adjusting the external power supply, the digitally controlled switched capacitor units compensate for the symmetry of the OOK modulator circuit, thereby improving the isolation of the modulator. Among them, the first digitally controlled switched capacitor unit (1) and the second digitally controlled switched capacitor unit (2) form the first group of digitally controlled switched capacitor units, and the third digitally controlled switched capacitor unit (3) and the fourth digitally controlled switched capacitor unit (4) form the second group of digitally controlled switched capacitor units. The first CNC switched capacitor unit (1), the second CNC switched capacitor unit (2), the third CNC switched capacitor unit (3), and the fourth CNC switched capacitor unit (4) have the same structure. Each capacitor is cascaded with a switching NMOS transistor. The opening and closing of each capacitor is controlled by the cascaded switching NMOS transistor. The switching NMOS transistor acts as a CNC switch and is controlled by the switching voltage. Each digitally controlled switched capacitor unit includes a fixed-value interdigitated capacitor, a switching NMOS transistor, and a protection resistor; the first protection resistor (R1) is connected in series between the DC voltage source and the gate of the fifth NMOS transistor (Q5) to protect the MOS transistor; the drain of the fifth NMOS transistor (Q5) is cascaded with the first interdigitated capacitor (C1), and the other end of the first interdigitated capacitor (C1) is connected in parallel to the local oscillator input of the OOK modulator, forming the first digitally controlled switched capacitor unit (1). The digitally controlled switched capacitor unit is controlled by the switching voltage and has two states: on and off; the amplitude and phase of the input differential signal are adjusted by turning the capacitor on and off, and the asymmetry of the differential circuit of the OOK modulator is compensated by changing the balance of the amplitude and phase of the differential input signal. The digitally controlled switched capacitor unit controls the NMOS transistor's turn-on and turn-off by controlling the gate voltage of the NMOS transistor. When the NMOS transistor is turned on, the drain is turned on, and the capacitor cascaded with it is turned on and connected to the circuit. At the same time, due to the DC blocking characteristic of the capacitor, the digitally controlled switched capacitor unit will not affect the DC bias of the OOK modulator circuit. The digitally controlled switched capacitor unit only changes the amplitude and phase of the differential input signal to compensate for the imbalance of the circuit.

2. The method for improving the isolation of millimeter-wave and submillimeter-wave monolithic integrated modulators according to claim 1, characterized in that: The two sets of digitally controlled switched capacitor units are connected in parallel to the local oscillator differential input terminal of the OOK modulator. Each single-ended input terminal is connected in parallel with two digitally controlled switched capacitor units to form a 2-bit capacitor switch. The complete OOK modulator circuit forms a total of 4-bit capacitor switches. Tuning the 4-bit capacitor switches adjusts the local oscillator differential signal at the local oscillator differential input terminal of the OOK modulator, compensates for the symmetry of the entire circuit, and thus improves the isolation of the OOK modulator.

3. The method for improving the isolation of millimeter-wave and submillimeter-wave monolithic integrated modulators according to claim 2, characterized in that: The OOK modulator has a local oscillator differential circuit composed of a first NMOS transistor (Q1), a second NMOS transistor (Q2), a third NMOS transistor (Q3), and a fourth NMOS transistor (Q4). The output terminals of the digitally controlled switched capacitor unit are connected in parallel to the local oscillator differential input terminal of the modulator.

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