<figure><img src="image_1.png" /></figure>

# High rejection self-oscillating up-conversion mixer for fifth-generation communications

Abdelhafid Es-saqy<fnref n="1" />, Maryam Abata<fnref n="1" />, Mohammed Fattah<fnref n="2" />, Said Mazer<fnref n="1" />, Mahmoud Mehdi<fnref n="3" />,
Moulhime El Bekkali<fnref n="1" />, Catherine Algani<fnref n="4" />

**Article Info**

**Article history:**

Received Oct 3, 2022

Revised Apr 7, 2023

Accepted Apr 7, 2023

**Keywords:**

Fifth-generation circuits

Mixer

Mm-Wave

Self-oscillating mixer

Up-conversion

**ABSTRACT**

This paper presents the design of a pseudomorphic high electron mobility transistor (pHEMT) self-oscillating mixer (SOM) for millimeter wave wireless communication systems. The 180° out-of-phase technique is chosen to both improve the desired lower sideband (LSB) signal and to achieve a satisfactory rejection of the unwanted signals (LO, USB and IF). This SOM is designed on the PH15 process of UMS foundry which is based on 0.15 µm GaAs pHEMT. The signal is up-converted from 2 GHz-IF frequency to 26 GHz-LSB frequency, using an autogenerated 28 GHz-LO signal. Simulations were performed using the advanced design system (ADS) workflow. They show 6.4 dB conversion gain and a signal rejection rate of 29.7 dB for the unwanted USB signal. The chip size is 3.6 mm<fnref n="2" />.

This is an open access article under the **CC BY-SA** license.

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**Corresponding Author:**

Abdelhafid Es-Saqy

AIDSES Laboratory, Sidi Mohamed Ben Abdellah University

Fez, Morocco

Email: abdelhafid.essaqy@usmba.ac.ma

## 1. INTRODUCTION

In these times of pandemic caused by coronavirus disease 2019 (COVID-19), communities' way of life has been changed [1]. During social distancing, connectivity allows us to keep informed, continue working and maintain our mental and physical health balance [2]. As current communication systems cannot meet our needs, developing new communication systems for fifth-generation (5G) has become a great necessity [3]–[5]. And to achieve the performance promised by this new generation, new design approaches and semiconductor technologies are being adopted.

Radio frequency (RF) transceivers must be as compact as possible to effectively reduce power consumption in 5G millimeter-wave systems [6]. High integration also leads to a tangible reduction in the costs of semiconductor technologies [7], [8]. In this context, the research presented in Winkler et al. [8] and Lee et al. [9] is carried out. They propose the design of self-oscillating mixer (SOM) circuit, which combines the harmonic generation, and the frequency conversion functions in a single RF chip.

Sawada and Imai [10] propose the design of an optoelectronic self-oscillating using a heterojunction bipolar transistor (HBT). The designed circuit performs the up-conversion of the 1 GHz-IF signal to a 10.6 GHz-RF signal, using a 9.6 GHz LO. On the other hand, low-power injection-locked zero-IF SOM for 40 GHz-RF frequency is developed in Burasa et al. [11]. The experimental results exhibit a conversion gain of -30 dB under -38 dBm input power. An up-conversion SOM for 77 GHz applications is presented in the paper [9]. This circuit has a high conversion gain of 3.87 dB. However, both lower sideband (LSB) and universal

<footnote n="1">¹AIDSES Laboratory, ENSA, Sidi Mohamed Ben Abdellah University, Fez, Morocco</footnote>

<footnote n="2">²IMAGE Laboratory, EST, Moulay Ismail University, Meknes, Morocco</footnote>

<footnote n="3">³Microwaves Laboratory, Lebanese University, Beirut, Lebanon</footnote>

<footnote n="4">⁴ESYCOM Laboratory, Université Gustave Eiffel, CNRS, Paris, France</footnote>
<figure><img src="image_3.png" /></figure>

serial bus (USB) signals have the same output power, meaning no image frequency rejection exists. In [8], an integrated receiver using a balanced SOM for 5.8 GHz-RF frequency is presented.

The purpose of this paper is to analyze and design a SOM circuit. It consists of a low-phase noise voltage controlled oscillator (VCO) and a double-balanced mixer with high LO and USB signal rejections. The mixer circuit, introducing couplers and phase-shifting lines, allows a conversion gain of 6.385 dB. Thus, among published SOMs, this work exhibits the best GC around 26 GHz, with a high rejection of unwanted harmonics, making it more attractive for 5G wireless systems.

This paper is organized as follows. The pseudomorphic high electron mobility transistor (pHEMT) is introduced in section 2. In section 3, the double-balanced mixer architecture is studied and designed. While in section 4, the designed SOM circuit and its simulation results are discussed. Then a conclusion is reported in section 5.

## 2. PHEMT TECHNOLOGY

There is explosive growth in mobile traffic demand, broadband transmissions and improved system performances are required [12]–[14]. Circuit designers are then focused on the device technology [15]: transistors with high transducer gain and low noise are privileged in our study. GaAs pHEMTs are nowadays the basis for monolithic microwave integrated circuits (MMIC) circuits [16]. Thanks to the development of these SC devices, researchers have succeeded in designing very efficient communication systems for the next generation 5G [17]–[24].

The circuit presented in this paper is designed with a 0.15 µm GaAs pHEMT transistor from the UMS foundry. A cutoff frequency characterizes this device $f_T$ of 110 GHz, a maximum transconductance $g_m$ of 640 (mS)/mm, a maximum Ids current of 220 mA/mm, a pinch-off voltage $V_{pinch}$ of -0.7 V, and a gate-drain breakdown voltage of more than 4.5 V [25]. The transistor gate length is 0.15 µm, associated with a gate width of 30 µm. The pHEMT structure is shown in Figure 1. The typical electrical parameters of the used pHEMT are summarized in Table 1.

<figure><img src="image_4.png" /><figcaption>Figure 1. Structure of the pHEMT Technology [26]</figcaption></figure>

<table><caption>Table 1. Typical electrical parameters of the pHEMT technology</caption><thead><tr><td>Parameters</td><td>Values</td></tr></thead><tbody><tr><td>Power density</td><td>300 mW/mm</td></tr><tr><td>Gate length</td><td>0.15 µm</td></tr><tr><td>Idssat</td><td>220 mA/mm</td></tr><tr><td>V<sub>BDS</sub></td><td>&gt; 4.5 V</td></tr><tr><td>Cutoff frequency</td><td>110 GHz</td></tr><tr><td>pinch-off voltage</td><td>- 0.7 V</td></tr><tr><td>Gm max</td><td>640 ms/mm</td></tr><tr><td>Noise/Gain</td><td>1.9/6 dB @ 60 GHz</td></tr></tbody></table>

## 3. DOUBLE BALANCED MIXER DESIGN

### 3.1. Proposed architecture

LO, IF, and USB rejections are performed by filtering, in the case of a carrier frequency in a few hundred MHz, or by the method of phase opposition, especially for a carrier frequency in the millimeter-wave band, where filtering becomes difficult because of the impedance matching difference at the different ports of
a balanced structure and in a large bandwidth. Two input signals that are phase opposite and with the same amplitude cancel each other at the output when added.

As shown in Figure 2, the signals LO1, LO2, LO3, and LO4, successively shifted in phase by 90°, drive the four transistor gates of four cascode cells acting as mixer cells. The other four transistor gates are excited by the signals IF1, IF2, IF3, and IF4, successively phase shifted by 90°, to. Table 2 presents the phase balance of the main signals present at each output of the four cascode cells, corresponding to transistor drains. Vector addition of the signals RF1, RF2, RF3 and RF4, leads to eliminating the signals in the opposite phase, i.e., IF, LO, and USB, while it amplifies the LSB signal in the millimeter-waves for 5G.

<figure><img src="image_5.png" /><figcaption>Figure 2. Synoptic diagram of the proposed double balanced mixer</figcaption></figure>

<table><caption>Table 2. Phase balance of the signals present on the drains of the four cascode cells</caption><thead><tr><th>Signal Output</th><th>FI</th><th>OL</th><th>LSB</th><th>USB</th></tr></thead><tbody><tr><td>RF1</td><td>0°</td><td>0°</td><td>0°</td><td>0°</td></tr><tr><td>RF2</td><td>90°</td><td>90°</td><td>0°</td><td>180°</td></tr><tr><td>RF3</td><td>180°</td><td>180°</td><td>0°</td><td>0°</td></tr><tr><td>RF4</td><td>270°</td><td>270°</td><td>0°</td><td>180°</td></tr></tbody></table>

## 3.2. Post-layout simulations

The mixer presented in this paper can generate RF signal in the first millimeter-wave band assigned to 5G by the International Telecommunication Union (from 24.25 to 27.5 GHz). Figure 3 shows the conversion gain CG as a function of LO frequency, varying from 26.25 to 29.5 GHz. CG is positive and higher than 2 dB. In addition, to verify the harmonic rejection at IF, LO, and USB frequencies, its output power spectrum is presented in Figure 4. In the LO frequency bandwidth, the mixer has a good level of rejection. The rejection of the USB signal is maximum and about 25 dB for a LO frequency of 28.25 GHz. And the rejection of the IF signal exceeds 30 dB in the LO bandwidth. At 27.4 GHz-LO frequency, the LO rejection reaches its maximum value of 15 dB at the mixer output, and it decreases for the other frequencies, higher than 4 dB for the worst case.

<figure><img src="image_6.png" /><figcaption>Figure 3. Variation of the conversion gain versus LO frequency (for IF_power=-11.5 dBm and LO_power=7.5 dBm and IF_freq=2 GHz)</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Figure 4. Variation of IF, LO, USB and LSB power versus LO frequency (for IF_power=-11.5 dBm and LO_power=7.5 dBm and IF_freq=2 GHz)</figcaption></figure>

The same frequency analysis was performed, varying the IF frequency. Figure 5 shows the CG curve versus IF frequency. It shows a conversion gain ripple of 3 dB for an IF frequency variation of 100 MHz (between 1.95 and 2.05 GHz). The output power spectrum is presented in Figure 6, showing a good rejection all over the IF frequency band: IF signal rejection is higher than 30 dB, the maximum USB rejection is 22 dB, whereas its minimum value is 5 dB, and finally, the LO rejection varies from 8 to 32 dB.

<figure><img src="image_9.png" /><figcaption>Figure 5. Variation of the conversion gain versus IF frequency (for IF_power=-11.5 dBm and LO_power=7.5 dBm and LO_freq=27 GHz)</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 6. Variation of IF, LO, USB and LSB power versus IF frequency (for IF_power=-11.5 dBm and LO_power=7.5 dBm and LO_freq=27 GHz)</figcaption></figure>

# 4. SELF-OSCILLATING MIXER DESIGN

## 4.1. Layout design

To ensure the LO signal generation and the IF signal conversion inside the same circuit, we integrate the mixer circuit into a VCO. Figure 7 illustrates the process of this integration. Figure 7(a) shows the LO and Mixer blocks before the integration and Figure 7(b) shows the resulting SOM block. The VCO circuit has been presented and studied in detail in the paper [25]. It is a perfectly symmetrical circuit based on “pHEMT varactor” to vary the capacitance value of the resonator and thus control the oscillation frequency.

The layout of the SOM circuit is presented in Figure 8. Its area dimension is 3.6 mm². It consists of the doubly balanced mixer (Part I), the VCO whose oscillation frequency is controlled by the control voltage Vtune (Part II), and the impedance matching circuit (Part III). The matching circuits are provided to achieve impedance matching between the output of the VCO and the LO port of the mixer.
<figure><img src="image_11.png" /><figcaption>Figure 7. LO and Mixer blocks (a) before integration and (b) after integration</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Figure 8. 5G mm-wave SOM layout</figcaption></figure>

## 4.2. Post-layout simulations

The conversion gain of the SOM circuit is presented in Figure 9. For an IF power of -15.4 dBm, the CG equals 6.385 dB. On the other hand, the variation of the LSB output power is presented in Figure 10. For an IF power of -12.5 dBm, it is equal to -7.394 dBm.

To verify the rejection of IF, LO and USB harmonics, the Figure 11 present the power spectrum at the SOM output. In the LO frequency bandwidth, the SOM has a good level of rejection. The rejection of LO signal is better than 8 dB in the entire bandwidth. In addition, the rejection of IF signal exceeds 22 dB. And the USB rejection reaches its maximum value of 29.7 dB for a voltage Vtune lower than 1 V. Whereas it is about 20 dB for the other values of Vtune. Finally, the effect of technological dispersion is evaluated. Figure 12 shows the Monte Carlo analysis of IF, LO, USB and LSB signals. This analysis is performed at 100 iterations.

<figure><img src="image_13.png" /><figcaption>Figure 9. Variation of the conversion gain versus IF power (for IF_frequ=2 GHz, and LO_power=7.5 dBm and Vtune=0.8 V)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Figure 10. LSB power versus IF power (for IF_frequ=2 GHz, and LO_power=7.5 dBm and vtune=0.8 V)</figcaption></figure>
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>Figure 11. Variation of IF, LO, USB and LSB power versus vtune (for IF_power=-11.5 dBm and IFfreq=2 GHz)</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Figure 12. Monte Carlo analysis of IF, LO, USB and LSB power (for IF_power=-11.5 dBm and IFfreq=2 GHz)</figcaption></figure>

Table 3 summarizes the main characteristic parameters of SOMs published in the literature. The conversion gain of our SOM is 6.385 dB which is greater than the one of the structures studied in [9], [11], [27], [28]. In addition, our circuit allows high rejection of unwanted harmonics at its output. This allows to eliminate IF, LO and USB harmonics without the use of a bandpass filter. Therefore, this circuit meets the requirements of the new communication systems.

<table><caption>Table 3. Performance summary and comparison of SOMs</caption><thead><tr><th></th><th>Technology</th><th>RF freq (GHz)</th><th>IF freq (GHz)</th><th>CG (dB)</th><th>Chip size (mm²)</th></tr></thead><tbody><tr><td>This work</td><td>0.15 µm GaAs pHEMT</td><td>26</td><td>2</td><td>6.385</td><td>3.6</td></tr><tr><td>[11]</td><td>65 nm CMOS</td><td>40</td><td>NA</td><td>-30</td><td>0.24</td></tr><tr><td>[27]</td><td>0.13 µm CMOS</td><td>30</td><td>0.5</td><td>-30</td><td>NA</td></tr><tr><td>[9]</td><td>0.10 µm GaAs pHEMT</td><td>38.5/77.38</td><td>0.3</td><td>3.87</td><td>5.98</td></tr><tr><td>[28]</td><td>0.20 µm GaAs pHEMT</td><td>10</td><td>1.57</td><td>-19</td><td>NA</td></tr></tbody></table>

## 5. CONCLUSION

New design approaches are needed to achieve the performance promised by 5G, which relies on millimeter wave frequencies. This paper shows the efficiency and feasibility of an up-conversion SOM with IF, LO, and USB rejection for 5G millimeter-wave systems. The circuit enables up-conversion of a 2 GHz IF signal to a 26 GHz LSB signal. The circuit has a conversion gain of 6.385 dB. The rejection of the LO signal is better than 8 dB in the entire bandwidth. In addition, the rejection of IF signal exceeds 22 dB. And the USB rejection reaches a maximum value of 29.7 dB. The circuit layout occupies an area of 3.6 mm²; it is designed based on 0.15 µm GaAs pHEMT of PH15 process from UMS foundry. This circuit will constitute the core of a fully integrated communication system that we intend to design based on pHEMT Technology.

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## BIOGRAPHIES OF AUTHORS

<figure><img src="image_19.png" /></figure>

Abdelhafid Es-saqy was born in TISSA, Morocco, in January 1992. Received his master's degree in Microelectronics from Faculty of Sciences Dhar EL Mahraz Fez Morocco, in 2018. He is now Ph.D. student in Artificial Intelligence, Data Sciences and Emerging Systems Laboratory at the University of Sidi Mohamed Ben Abdellah Fez. He can be contacted at email: abdelhafid.essaqy@usmba.ac.ma.
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

**Maryam Abata** ID SC born in 1989 in Nador, Morocco, obtained master's degree in 2012 at the National School of Applied Sciences. She received the Ph.D. in Telecommunications and Micro-waves Devices from Sidi Mohamed Ben Abdellah University of Fez. She can be contacted at email: maryam.abata@usmba.ac.ma.

<figure><img src="image_22.png" /></figure>

**Mohammed Fattah** ID SC received his Ph.D. in Telecommunications and CEM at the University of Sidi Mohamed Ben Abdellah (USMBA) Fez, Morocco, 2011. He is a professor in the Electrical Engineering Department of the High School of Technology at the Moulay Ismail University (UMI), Meknes, Morocco and he is a responsible of the research team ‘Intelligent Systems, Networks and Telecommunications’, IMAGE laboratory, UMI. He can be contacted at email: m.fattah@umi.ac.ma.

<figure><img src="image_23.png" /></figure>

**Said Mazer** ID SC born in 1978. He received the Ph.D. degree in electronics and signal processing from the University of Marne-La-Vallée, Champs-surMarne, France. He is currently a full Professor with the National School of Applied Sciences of Fez, Morocco. He is membre of IASSE Laboratory, University of Sidi Mohamed Ben Abdellah Fez. His research interests include the development of microwave-photonics devices for radio-over fibre and wireless applications, and he is also involved in network security. He can be contacted at email: said.mazer@usmba.ac.ma.

<figure><img src="image_24.png" /></figure>

**Mahmoud Mehdi** ID SC was born in Beirut, Lebanon, in 1974. He received his Ph.D. in high frequency communication systems from the university of Paris Marne la Vallée, France 2005. He is an associate professor in the Physics Department of the Faculty of sciences at the Lebanese University, Beirut, Lebanon. His research interests include monolithic microwave integrated circuits, micro-electro-mechanical systems (MEMS), radiofrequency, double balanced and distributed mixers, local oscillator, TW amplifiers, optoelectronic mixer for LIDAR system, optical systems and design, and photo-detectors. He is course leader in microwave devices for the Masters program in Electronics. He can be contacted at email: mahmoud.mehdi@ul.edu.lb.f.

<figure><img src="image_25.png" /></figure>

**Moulhime El Bekkali** ID SC holder of a doctorate in 1991 from the USTL University-Lille 1-France, he worked on antennas printed in X-band and their applications to microwave radar. Since 1992, he was a professor at the Graduate School of Technology, Fez (ESTF) and he was a member of the Transmission and Data Processing Laboratory (LTTI). In 1999, he received a second doctorate in electromagnetic compatibility from Sidi Mohamed Ben Abdellah University (USMBA). Since 2009, Pr. El Bekkali has been Vice-President of Research and Cooperation at the Sidi Mohamed Ben Abdellah University (USMBA) in Fez-Morocco until 2018. Currently, he is a professor at the National School of Applied Sciences (ENSAF) and member of the LIASSE laboratory at Sidi Mohamed Ben Abdellah University. He can be contacted at email: moulhime.elbekkali@usmba.ac.ma.

<figure><img src="image_26.png" /></figure>

**Catherine Algani** ID SC born in 1963. Ph.D. in electronics at the University Pierre and Marie Curie (Paris VI). University professor at the National Conservatory of Arts and Crafts, CNAM-Paris. She is the responsible of the research team “communication systems” at ESYCOM laboratory. She can be contacted at email: catherine.algani@lecnam.net.