5G kablosuz iletişim sistemleri için aşağı dönüşüm Gilbert hücre mikser tasarımı
Down-conversion Gilbert cell mixer design for 5G wireless communications systems
Yazar: FATMANUR TALAY
Danışman: DOÇ. DR. MUSTAFA BERKE YELTEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı / Elektronik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Karıştırma = Mixing ; Milimetre dalga frekansı = Milimeter wave frequency ; Radyofrekans = Radiofrequency

Özet:
Mikserler kablosuz iletişim sistemlerindeki RF alıcı ve verici yapılarının her ikisinde de bulunan kablosuz haberleşmenin en önemli bloklarından biridir.  Alıcı ve verici sistemlerinde farklı işlevleri yerine getirmesi için kullanılmasına rağmen genel olarak belli bir frekansa sahip giriş sinyalini bir taşıyıcı sinyali ile karıştırarak farklı bir frekansa sahip çıkış sinyali oluşturma işlemini gerçekleştirirler. Alıcı sistemlerinde kullanılan mikserler yüksek frekanslı bir sinyali düşük frekanslı bir sinyale dönüştürdüğü için aşağı dönüşüm mikser olarak adlandırılırken, verici sistemlerinde kullanılan mikserler düşük frekanslı bir sinyali yüksek frekanslı bir sinyale dönüştürdüğü için yukarı dönüşüm mikser olarak adlandırılır.  Bir mikser devresi içinde bulunduğu alıcı ve verici sistemin çalışmasını doğrudan etkileyeceği için belirli performans gereksinimlerini sağlaması gerekmektedir. Mikserin dönüşüm kazancı, gürültü katsayısı, doğrusallık ve güç tüketimi gibi performans parametrelerine bakılarak yüksek ya da düşük performansta çalıştığı yorumlanabilmektedir. Bir mikserin performansını belirleyen en önemli kriter tasarım mimarisidir. Uygulamaya özel yapılan mikser tasarımlarında yüksek kazanç gerektiren uygulamalar için aktif mikser mimarisi, düşük güç tüketimi ve doğrusallık gerektiren uygulamalar için ise pasif mikser mimarisi sıklıkla tercih edilmektedir.  Bu tez çalışmasında yüksek kazanç ve yüksek doğrusallık hedef alınarak en yaygın aktif mikser mimarisi olan aşağı dönüşüm Gilbert hücre mikser tasarımı yapılmıştır. Tasarlanan mikser devresinden yüksek kazanç elde edebilmek için devreye akım basma tekniği eklenmiştir. Ayrıca mikserin performansını arttırmak ve empedans uyumlamasını kolaylaştırmak amacıyla tasarıma çıkış tampon kuvvetlendiricisi eklenmiştir. Devrenin tüm bağlantı noktaları 50 Ω porta bağlanmış, olabilecek herhangi bir sinyal sızıntısını önlemek amacıyla tüm bağlantı noktaları için 50 Ω'a göre empedans uyumu yapılmıştır.   Önerilen mikser devresi için TSMC firmasının 40 nm CMOS teknolojisi kullanılmıştır ve devre 28 GHz milimetre-dalga frekans bandında çalıştırılmıştır. Cadence Virtuoso programında şematik ve serim tasarımı gerçekleştirilmiş ve mikserin performansını ölçmek için analizler yapılmıştır.  Aşağı dönüşüm mikser 28 GHz frekansında ve −10 dBm gücüne sahip RF sinyalini, 26.4 GHz frekansında ve 4 dBm gücüne sahip LO sinyali ile karıştırarak 1.6 GHz frekansındaki IF sinyalini oluşturmaktadır. Dönüştürme işlemini yaparken 3 GHz (26.5 GHz – 29.5 GHz) bant genişliği sağlamaktadır. Serim tasarımı yapılan mikser devresi ile −2.32 dB dönüşüm kazancı elde edilmiş, gürültü katsayısı 9.604 dB oluşmuştur. Mikserin doğrusallık performansını belirleyen giriş P1dB değeri  4.818 dB ve IIP3 değeri 12.667 dBm bulunmuştur. Mikser devresi 50-60 dB arasında LORF izolasyonu, 30-40 dB arasında LO-IF izolasyonu ve 25-35 dB arasında RF-IF izolasyonu sağlamaktadır. Tasarlanan devre 1.1 V gerilim kaynağı ile beslenmiş ve devreden çekilen akım 43.61 mA olmuştur. Devre 27.148 mW mikser çekirdek bölgesinden ve 20.823 mW çıkış tampon kuvvetlendiricilerinden olmak üzere toplam 47.941 mW güç tüketmektedir. Mikser performans değerleri ve literatürde bulunan bazı mikser çalışmalarının sonuçları ile karşılaştırılmıştır ve tasarımın performansı değerlendirilmiştir

Abstract:
One of the most popular application areas of wireless communication is cellular communication systems. Cellular communication systems are a technique developed to increase spectral efficiency and user capacity. The massing of cellular communication only started with the development of first generation (1G) cellular technologies that support the transmission of analog voice data. With the transmission of digital voice data and the development of the short message system, 2G technology was adopted. Developed cellular communication systems could not meet the increasing user demands and third generation (3G) cellular communication systems were developed in line with the need for more advanced data transmission mechanism. 4G technology includes not only cellular systems but also broadband wireless access systems. Recently, with the development of technology, increasing user demands such as high data rates, high capacity and wide frequency band in communication systems have made it necessary to develop 5G cellular systems with more advanced data transmission mechanism communication system is expected to emerge. With 5G technology, a new wireless technology that can reach gigabit and higher speeds, delays in the network will be less than 1 millisecond, data capacity will have a unit area that will reach 1000 times the current capacity, energy efficiency will improve up to 1000 times, will provide coverage and uninterrupted communication in all areas. 5G wireless communication systems attract attention as multi-functional systems in which more than one feature works at the same time. In this context, the antennas to be used in these systems should have a very wide band feature covering the relevant operating frequencies. It is thought that these requirements in 5G communication systems can be met with the use of millimeter wave frequency band. Millimeter wave is frequently preferred in this technology because it offers advantages such as high bandwidth, high data transfer rate and the ability to reach high frequencies to 5G communication systems. In order to achieve high data rates, frequency ranges with different bandwidths in different countries have been allocated to the 5G communication system. 27.5-28.5 GHz (USA), 24.25-27.5 GHz (Europe), 25-29 GHz (Latin America) and 26.5-29.5 GHz (Asia) are some of the different frequency bands allocated for 5G communication systems. The basis of 5G wireless communication systems is based on RF transceiver structures that include LNA, mixer, power amplifier and VCO blocks. Mixers are one of the most important blocks of wireless communication in both RF receiver and transmitter structures in wireless communication systems. In the design of RF transceiver systems, they are generally placed after an LNA block or before a PA block. Mixers have three ports, two inputs and one output. These are indicated as the RF port where the radio frequency signal is applied, the LO port where the local oscillator signal is applied, and the IF port where the mixer output is received, that is, the intermediate frequency signal is received. Although they are used to perform different functions in receiver and transmitter systems, they generally perform the process of creating an output signal with a different frequency by mixing the input signal with a certain frequency with a carrier signal. Mixers used in receiver systems are called down-conversion mixers because they convert a high-frequency signal to a low-frequency signal. In contrast, mixers used in transmitting systems are called up-conversion mixers because they convert a low-frequency signal to a high-frequency signal. There are some performance parameters that need to be examined in order to interpret how a mixer works and to determine the advantages and disadvantages it will provide in its circuit. Since a mixer circuit will directly affect the operation of the receiver and transmitter system, it must provide certain performance requirements. By evaluating the performance parameters such as conversion gain, noise figure, linearity, and power consumption of the mixer, it can be interpreted that it operates at high or low performance. The most important criterion determining the performance of a mixer is the design architecture. Active mixer architecture is generally preferred for applications that require high gain in mixer designs, whereas passive mixer architecture is often preferred for applications that require low power consumption and high linearity. However, active mixer structures provide high isolation between connection points and create high noise figures. On the other hand, passive mixers provide low isolation and low noise figure. CMOS has gained importance in the last 20-30 years due to analog IC design, mixedsignal applications and the digital world. With the developing advanced CMOS technologies, the increase in unit gain frequencies of transistors (𝑓𝑇 ) several times has increased the use of CMOS technologies in radio frequency circuit designs. When we consider a whole chip design, advanced CMOS technology comes to the fore with its advantages such as low space, low power consumption, average cost and high speed performance. Gain and linearity are inversely variable performance parameters in mixer designs. A change made to increase the performance of one of these parameters may cause a decrease in the performance of the other. For this reason, circuit design should be done to obtain maximum efficiency by trade-off both gain and linearity performance parameters in certain amounts. In this thesis, a down-conversion Gilbert Cell mixer design, which is the most common active mixer architecture, was designed by targeting high gain and high linearity. In order to obtain high gain from the designed mixer circuit, the current bleeding structure was introduced to the circuit. With this structure, the current flowing from the LO MOSFETs in the switching stage and the voltage falling on the output load are increased without changing the current flowing from the RF MOSFETs in the transconductance stage, thus increasing the conversion gain performance parameter of the circuit. By using the drain current structure, the advantage is provided that the RF MOSFETs in the transconductance stage can operate at a lower gate source voltage, thus offering a compact size. Also, an output buffer amplifier has been added to the design to increase the mixer's performance and facilitate impedance matching. With the buffer circuit, 180° phase difference occurs in the IF output signals. Input impedance matching is very important in downconversion mixers. This enables the most efficient transport of power. Impedance matching has been performed at all ports to 50 Ω to prevent any signal reflection. Ltype matching structure is used for all input output ports. The layout design of the mixer circuit, whose schematic design was made, was made. Substrate coupling problem in Analog and RF IC designs was taken into consideration while designing the layout. To achieve this, NMOS transistors were surrounded by Nwell protection ring in the p-base and this N-well protection ring was connected to VDD, while PMOS transistors were surrounded by p-diffusion protection ring in Nwell and p-diffusion protection ring was connected to VSS connected. TSMC 40 nm LP CMOS technology was used for the proposed mixer circuit, and the circuit was operated in the 28 GHz millimeter-wave frequency band. Schematic and layout design was carried out in the Cadence Virtuoso program, and analyses were made to measure the performance of the mixer.  The downconversion mixer creates an IF signal with a frequency of 1.6 GHz by mixing the RF signal with a frequency of 28 GHz and a power of −10 dBm with an LO signal at 26.4 GHz with a power of 4 dBm. Thus, the mixer provides 3 GHz (26.5 GHz – 29.5 GHz) bandwidth while performing the conversion process. A conversion gain of −2.32 dB was obtained for the mixer circuit with the layout design, and the noise figure was 9.604 dB. The P1dB and IIP3 values, which determine the linearity performance of the mixer, were found to be 4.818 dBm and 12,667 dBm, respectively. The mixer circuit provides LO-RF isolation between 50-60 dB, LO-IF isolation between 30-40 dB, and RF-IF isolation between 25-35 dB. The designed circuit was fed with a 1.1 V voltage source. The circuit consumes a total of 47.9 mW of power, 27.15 mW from the mixer core region and 20.75 mW from the output buffer amplifiers. The mixer performance values and the results of similar mixer studies in the literature were compared, whereby the performance of the design was evaluated. According to the comparison, it has been seen that the mixer circuit designed has good performance to meet the needs of 5G wireless communication systems.