Terahertz wave phase modulation circuit

Abstract

To provide means for performing phase modulation on terahertz waves by a simple manner and further realizing quadrature modulation in which both phase modulation and amplitude modulation are performed.SOLUTION: In a terahertz wave modulation circuit, a phase shift circuit is inserted before a multiplier that generates the local oscillation frequency of a mixer, thereby generating an amplitude-modulated or amplitude-phase-modulated terahertz wave.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for configuring a modulation circuit for terahertz wave band wireless communication, mainly at 100 GHz or higher. [Background technology]

[0002] The terahertz wave band is a frequency band that is expected to be used for high-speed, high-capacity wireless communications. A component called a mixer is used to modulate high-frequency signals. Signal modulation methods include amplitude modulation, phase modulation, and amplitude-phase modulation, which is a combination of the two. If a phase modulation circuit for the terahertz wave band were to be created as an extension of microwave and millimeter-wave technology, one possible method would be to prepare two mixers, one for I phase and one for Q phase, and mix the outputs. However, this method requires high mechanical processing precision as well as an accurate high-frequency source and an accurate 90-degree phase shifter, and in reality, this is difficult to achieve due to the variations and instability of individual parts. Other approaches include generating terahertz waves from differences in the frequency of laser light, and research into creating metamaterials, but these have not yet been put to practical use. With conventional technology, only amplitude modulation was possible in the terahertz wave band, and phase modulation was difficult. Summary of the Invention [Problem to be solved by the invention]

[0003] An object of the present invention is to provide a means for performing phase modulation on terahertz waves in a simple manner and further realizing quadrature modulation in which both phase modulation and amplitude modulation are performed. [Means for solving the problem]

[0004] The configuration of the present invention will be explained using Figure 1. A high-frequency oscillator, a phase shift circuit, a frequency multiplier (AMC), and a high-frequency mixer (SHM) that operates in the terahertz wave band are prepared. A phase shift circuit is a circuit that can continuously change the phase of an input signal, and in this case, it can be continuously changed according to the applied analog voltage. The amount of shift that can be achieved is about 5 to 6 degrees. A frequency multiplier is a high-frequency component called an AMC (Amplifier Multiplexer Chain), which combines a multiplier and an amplifier in a single housing. An AMC can generate N times the frequency of an input frequency. The SHM (Subharmonic Mixer) is a high-frequency mixer that performs modulation at twice the input frequency. High-frequency oscillators require a signal source with stable frequency and low phase noise (jitter), because the output frequency of the high-frequency oscillator is multiplied by 2×N by AMC and SHM to generate the LO, and the jitter contained in the output of the high-frequency oscillator is also multiplied by 2×N. The feature of this invention is that it utilizes the phenomenon that the jitter of a high frequency oscillator is magnified by multiplication, and inserts a phase shift circuit into the output of the high frequency oscillator, which is an extremely stable frequency source, to significantly change the phase of the LO. Normally, the area between the high frequency oscillator and the multiplier is very sensitive, so no circuit is inserted in this location. In the configuration shown in Figure 1, when a constant voltage is applied to the IF input of the mixer (SHM), an RF signal with constant amplitude and twice the LO frequency is output. When the phase of the output of the high-frequency oscillator is shifted, the amount of shift is amplified by N times by the multiplier, and then further amplified by two times by the SHM. In this way, terahertz waves with constant amplitude and phase modulation can be obtained (Figure 2). By inputting a signal that varies in timing with the phase shift rather than a constant voltage into the IF input, it is possible to modulate both amplitude and phase simultaneously (Figure 3). The phase shift amount does not necessarily have to be continuous, but can also be discrete. For example, if the shift amount is set to two levels and the IF input is also changed in two levels simultaneously, QPSK modulation can be performed. If the shift amount is set to four levels and the IF input is also changed in four levels simultaneously, QAM16 modulation can be performed. Digital modulation is also possible if the IF and phase shift amount are set appropriately (Figure 4). [Effects of the Invention]

[0005] According to the present invention, a terahertz wave phase modulation circuit can be created using only one mixer and a simple phase shift circuit. Because there are no branching or joining waveguides, high precision is not required for machining or assembling the waveguide components. This allows for significant cost reductions and downsizing of the device. Generally, phase modulation is more power efficient than amplitude modulation, enabling noise-resistant communications with less power, making it an advantageous modulation method in the terahertz band, where only weak power is available. Digital modulation can also be performed by simultaneously varying the phase and amplitude in a discrete manner, which allows the communication bandwidth to be doubled compared to amplitude modulation for the same power. [Brief explanation of the drawings]

[0006] [Figure 1] Configuration diagram of the present invention [Figure 2] Phase Shift Principle [Figure 3] Principle of Amplitude and Phase Modulation [Figure 4] Digital modulation resulting from amplitude and phase modulation [Figure 5] SHM structure and operating principle [Figure 6] Phase shift circuit configuration [Figure 7] Phase modulation experimental setup [Figure 8] SHM phase detection output [Figure 9] Detector output when phase modulated with a sine wave [Figure 10] Detector output when phase modulated with a square wave [Figure 11] Phase modulation circuit configured using conventional technology DETAILED DESCRIPTION OF THE INVENTION

[0007] The operating principle of the present invention will be explained with reference to FIG. The modulation circuit of the present invention is composed of a master oscillator, a high frequency oscillator, a phase shift circuit, an AMC, and an SHM. The master oscillator is a precise 10MHz oscillator that is used as the reference clock for the high-frequency oscillator, which uses a PLL to multiply the 10MHz reference input by 1600 to generate a 16GHz high-frequency signal. The high-frequency signal is input to a phase-shift circuit, where it is shifted by a maximum of approximately 6 degrees. The output of the phase-shift circuit is connected to the input of the AMC, where it is multiplied by 8 to generate a 128GHz local oscillator (LO) frequency. There are two types of high frequency mixers: harmonic mixers and sub-harmonic mixers (SHM). A harmonic mixer multiplies the signal input from the LO port by the IF signal and outputs it from the RF port, while a sub-harmonic mixer multiplies the IF signal by twice the frequency of the signal input from the LO port. Either type can be used, but in the terahertz wave band, sub-harmonic mixers are easier to use and there are more types available. The 128 GHz signal from the AMC is connected to the LO input of the SHM (Sub-Harmonic Mixer) via a waveguide. The SHM is a high-frequency component with three ports: IF input, LO input, and RF output.

[0008] The structure and operating principle of the SHM are shown in Figure 5. When the voltage from the LO input is high, the anti-parallel diode pair inside the SHM turns ON, and the voltage input from the IF is dropped to GND through the stub. Because the diodes turn ON twice per LO cycle, a waveform obtained from RF is the IF input switched at twice the frequency of the LO input. For example, if the LO is set to 128 GHz and a baseband signal with a bandwidth of about 1 GHz is input to the IF, an intermittent waveform at 256 GHz will be output. In normal use of SHM, the frequency given to the mixer's LO is constant, and it is considered better to have less phase fluctuation, as this will result in a more stable output. This is because if the LO phase fluctuates, the RF phase will also fluctuate. Therefore, the high frequency generator that generates the LO must have extremely low phase noise. For example, if a high frequency generator is outputting 16 GHz (period 62.5 picoseconds) and a 1 picosecond fluctuation occurs in the output clock, the deviation for one period is 1.6%. However, since there is also a 1 picosecond fluctuation in the 256 GHz carrier wave (period 3.9 picoseconds) generated by multiplying 16 GHz, there is a 25% fluctuation in the 256 GHz, or a phase fluctuation of approximately 90 degrees. As the phase noise increases due to the multiplication by AMC and SHM, a slight phase shift can be amplified by inserting a phase shift circuit into the high frequency generator and AMC, resulting in a large phase change.

[0009] The configuration of the phase shift circuit is shown in Figure 6. A hybrid circuit generates 0-degree and 90-degree signals from a 16 GHz signal input. The 90-degree signal is pulled down to GND by a HEMT inserted in the capacitor-coupled section. This HEMT is not used for amplification; it is used as a resistor controlled by the gate voltage. The closer the gate voltage is to zero, the lower its resistance becomes, so the 90-degree signal is weakened and then mixed using a Wilkinson divider. In other words, the 0-degree signal and the moderately weakened 90-degree signal are mixed to form a continuously controllable phase shift circuit. The phase shift that can be achieved by this circuit is approximately 5 to 6 degrees, but this is amplified by 16 times by the multiplier circuit, allowing the LO phase to be shifted from 0 to approximately 90 degrees.

[0010] Figure 7 shows the experimental setup used to confirm the phase shift. The transmitter and receiver AMCs are fed with frequencies distributed from the same high-frequency oscillator. The transmitted signal is demodulated by the receiver mixer, but the mixer used in this experiment has minimum output when the phase difference between RF (RX) and LO (RX) is 0 degrees, and maximum output when the phase difference is 90 degrees. Note that some mixers have maximum output at 0 degrees and minimum output at 90 degrees, but the essence is the same regardless of the type.

[0011] Figure 8 shows a simulation of the relationship between the phase difference between RF (RX) and LO (RX) input to the receiving mixer and the mixer output voltage. When the received power is constant, it works as a phase detector that outputs a sine function of the phase difference between RF and LO. Experiments were conducted using the equipment configured as above. When a 0.4V DC current was applied to the IF (TX) of the transmitting mixer, a continuous wave (CW) of approximately 100μW was obtained at 256GHz. It is known that the phase shift circuit used in this experiment has the largest phase shift when the gate voltage is in the range of -0.4V to -0.5V.

[0012] Therefore, when a sine wave of -0.425V to -0.475V was applied to the gate, the waveform shown in Figure 9 was obtained from the output of the LNA (low noise amplifier). This is because the IF voltage on the transmitting side is constant, so the transmission power is constant, but the detection output changes because the phase is modulated.

[0013] Figure 10 shows the gate voltage transitioned digitally. It can be seen that the detection output from the receiving mixer also changes in a similar rectangular wave. This demonstrates that the present invention can be used to phase-modulate 256 GHz terahertz waves. While the above explanation uses 256 GHz as an example, the high-frequency components used in this experiment support frequencies from 220 GHz to 320 GHz and are not dependent on the 256 GHz frequency. Furthermore, while the IF was fixed in this experiment, changing the voltage applied to the IF simultaneously with the LO shift enables amplitude quadrature modulation, which modulates both amplitude and phase simultaneously. Phase modulation also has advantages in terms of power. In amplitude modulation, for example, if 50% and 100% amplitudes are assigned to the two states of "0" and "1," then only 25% of the power is used while transmitting 50% amplitude. With current technology, terahertz wave mixers can only obtain around 100 microwatts of power, and power amplifiers are almost nonexistent. As a result, communication in the terahertz wave band essentially has to use weak radio waves. While amplitude modulation transmits information by further weakening weak power, phase modulation only shifts the phase at 100% amplitude, making effective use of less power.

[0014] If we were to create a phase modulation circuit as an extension of conventional microwave and millimeter-wave technology, we could consider a method of preparing two mixers, one for I phase and one for Q phase, and mixing their outputs. As shown in Figure 11, if we provide the two mixers with local oscillator frequencies (LO) that are 90 degrees out of phase, and set the IF inputs of the mixers to x and (1-x), then theoretically we can obtain terahertz waves that are phase-modulated up to 90 degrees. However, this method, which uses two mixers, presents various problems. At this frequency, the wiring from the two mixers to the mixer and the wiring to the 90-degree shift circuit connected to the LO of the two mixers are waveguides. Because the wavelength is about 1 mm, the processing precision of the waveguides must be on the order of micrometers, and the length must be precisely matched during assembly. Any fluctuation in this precision will cause an imbalance in the I / Q. Unlike the wiring in electronic circuits, the length cannot be changed. Furthermore, it requires a precision high-frequency source with ultra-low jitter and a 90-degree phase shifter, and simply assembling commercially available parts and devices does not provide satisfactory performance due to the individual differences and operational fluctuations of each part. As such, it has been difficult to achieve phase modulation for terahertz waves by simply extending millimeter-wave and microwave technologies.

Claims

1. a mixer having an IF input, an LO input and an RF output; a multiplier that multiplies an original oscillation by N times to generate an LO to be supplied to the mixer; a phase shift circuit disposed between the multiplier and a master oscillator; A modulation circuit that performs phase modulation by utilizing the fact that the shift amount of the phase shift circuit is magnified by N times in the RF output.

2. 2. The modulation circuit according to claim 1, wherein a sub-harmonic mixer is used instead of said mixer, and the amount of phase shift is amplified by 2*N times.

3. 3. The modulation circuit according to claim 1, wherein the phase change caused by said phase shift circuit is discrete.

4. 3. The modulation circuit according to claim 1, wherein the phase shift amount and the IF input are changed simultaneously to simultaneously perform amplitude and phase modulation.

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