Yüksek sıklıkta çalışmaya uygun gilbert karıştırıcı tasarımı
Gilbert mixer design suitable for operating at high frequency
Yazar: EMRE ALTUNER
Danışman: PROF. DR. İSMAİL SERDAR ÖZOĞUZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı / Elektronik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Elektronik haberleşme = Electronic communication ; Karıştırıcılar = Mixers

Özet:
Karıştırıcılar radyo sıklığı devrelerinde kullanılan en temel bloklardan biridir. Karıştırıcılar için dikkat edilecek en önemli hususlar kazanç, doğrusallık ve gürültüdür. Kazanç ve doğrusallık karıştırıcıda ters orantılı olarak değişmektedir. Gürültü sayısı ise kazanç ile doğru orantılı olarak değişiklik gösterir. Doğrusallık bir karıştırıcı için en önemli parametredir. O yüzden tasarımda doğrusallık maksimum seviyeye çıkarılmaya çalışılmıştır. Bunu yaparken ise kazancın 1 in altına düşmemesi garantilenmiştir. Çünkü 1 in altındaki kazanç işaretin zayıflaması demektir. Bu ise istenmeyen bir durumdur.
Umc 90 nm teknolojisi kullanılarak 5 GHz sıklığında çalışan yüksek doğrusallıklı Gilbert karıştırıcısı tasarlanmıştır. Tasarıma başlanmadan önce dönüştürücü kazancı Av ≥ 1 ve IIP3 ≥ 10 dBm olacak şekilde alınmıştır. Daha sonra ise Gilbert karıştırıcısının dönüştürücü kazancı ve IIP3 değerinin teorik olarak formülü bulunmuştur. Bu formül ve bu değerler kullanılarak devreden geçen akım, transistör boyutları ve kutuplama gerilimlerinin alması gereken değerler bulunmuştur. Daha sonra ise giriş empedens uyumu için RS transistörlerinin giriş empedansı formulize edilip, giriş empedansının 50 ohm olması için hangi elemanların ve elemanların hangi değerlerde olması gerektiği bulunmuş ve devre tasarımı yapılmıştır. Bununla birlikte Gilbert karıştırıcısının gürültü gücü kuramsal olarak bulunmuştur.
Çıkışta oluşan gürültü gücünü en aza indirmek için ise devredeki dirençler 50 ohm gibi küçük bir değer olarak seçilmiştir.
Devre Cadence programı kullanılarak şematik ve serim çizilmiş daha sonra benzetim yapılmıştır. Devrenin besleme gerilimi 1.2 V olup çekilen toplam akım yaklaşık 14 mA olarak bulunmuştur. 12 mA Gilbert karıştıcısı tarafından kullanılırken, 2 mA akım öngerilim oluşturan transistörler tarafından kullanılmıştır. Yani toplam harcanan güç 16.8 mW olarak bulunmuştur. Benzetim sonucu şematik devrenin dönüştürücü kazancı 2.54 dB olarak görülürken, serim devresinin dönüştürücü kazancı 2.43 dB olarak gözlenmiştir. Giriş üçüncü derece kesişim noktası (IIP3) şematiğe göre 14 dBm iken serime göre 13 dBm, gürültü sayısı şematiğe göre 15.67 dB ve serime göre ise 15.77 dB olarak bulunmuştur. Bu sonuçlar ve Gilbert karıştırıcı tasarımı yapmış olan 5 adet çalışmanın sonuçları karşılaştırılmıştır. Diğer çalışmalara göre 1-dB bastırma noktası ve IIP3 değerleri gayet yüksek bulunmuştur.

Abstract:
A very high linear Gilbert mixer which is operating at 5 GHz is designed by using umc90 nm technology. The parameters which are most important for design are conversion gain and input third intercept point (IIP3). It is taken Av ≥ 1 ve IIP3 ≥ 10 dBm. After that, theoretical formulas of Av and IIP3 are obtained. With using these formulas and equations, the current which flows from the mixer, the sizes of transistors and the gate voltage are obtained. Next, the input impedance of RF transistors is formulized and the values of components which are necessary for impedance matching are found. Also, the noise power of the system is calculated.
The schematic circuit and the layout of the mixer are designed with using Cadence Virtuoso. After that, the schematic circuit and the layout are simulated. The supply voltage of the circuit is 1.2V and the current which flows from the circuit is 14 mA.   2 mA is used by Gilbert mixer and 12 mA is used by bias transistors. So the total power which is used from the circuit is 16.8 mW. The conversion gain is observed about 2.54 dB for the schematic and 2.43 dB for the layout in simulation results. Input third intercept point (IIP3) is found 17.91 dBm for the schematic circuit and 15.43 dBm for the layout. The noise figure of the system is observed about 15.67 dB for the schematic circuit and 15.77 dB for the layout. These results and the results which are from 5 articles are compared. 1 dB compression point and IIP3 results are much higher than the results of these articles.
Mixer is one of the main units in RF circuits. Mixers change the frequency of signals by multiplying. Downconversion mixers reduce the frequency of a high-frequency signal to a low-frequency. Upconversion mixers increase the frequency of a low- frequency signal to a high-frequency. A downconversion Gilbert Mixer is designed in this thesis. The Gilbert mixer is a dual balanced mixer circuit. Single balanced mixers have a high level of YO-AS continuous feed and high power consumption. For this reason, Gilbert mixer which is double balanced mixer is preferred over single balanced mixer.
When a mixer multiplies 2 waveforms which have different frequencies the result will be 2 waveforms. The frequency of one of these signals will be the sum of the 2 frequency of the multiplied signals and the frequency of the other signals will be the difference in the frequency of the multiplied signals. In downconversion mixer multiplied signals are radio frequency signal (RF), local oscillator signal (LO) and the output signal is intermediate frequency (IF). 
This is an ideal result of the output signal. When nonideality is taken into account the linearity of the system is broken.  When Vyo(t) = Acos(w1t)  and Vrf(t) = Bcos(w2t) are given to the mixer there will be signals which have w1,w2,w1+w2,w1-w2,2w1±w2,2w2±w1,3w1,3w2 frequencies. w1+w2,w1-w2  are the basic signals from the mixer. Other frequencies are called intermodulation products.
One of the most important things in a mixer is conversion gain. The downconversion mixers must have a sufficiently high gain to sufficiently suppress the noise to be transmitted to the next blocks. However, it is difficult to keep the gain high in low voltage mixers. Because, if the gain increases, the linearity goes away from the desired levels. Linearity is as important as gain for mixers. So the design of the mixer must be made taking conversion gain and linearity into account.
There are two basic criteria of linearity. These are the 1 dB suppression point and the third-order intercept point (IIP3). The output power must increase linearly with respect to the input power. However, since the circuit is not linear, this is not possible after a certain point. The ratio of output power to input power is expressed as gain. When the input power increases, the point where the gain goes out of linearity after a certain point and the output power drops 1 dB from what it should be is called a 1 dB suppression point.
Another criterion that determines linearity is the third-order intercept point (IP3). The power point at which the power of the first harmonic and third harmonic signal obtained at the output is equal to each other is expressed as the third-input intercept point. The point corresponding to the input power at this point is called the input third-order intercept point (IIP3) and the point corresponding to the output power is called the output third-order intercept point (OIP3).
The elements in the mixer structure have capacitances. Therefore, an undesired port to port feedthrough occurs between the ports in the mixer. Since the LO-IF feedthrough has a low-pass filter (LPF), a very large amount is suppressed. However, in the LO-RF feedthrough, it is transferred to the baseband with the RF signal. 
In RF-LO feed-through, RF causes interference to the LO signal and the distortions occur in the LO spectrum. Products resulting from a single degree of degradation of the RF signal result in a distorted IF signal in the RF-IF feedthrough.
In this thesis a downconversion Gilbert mixer is designed. There are 7 NMOS transistors and 2 resistors. RF transistors are M2 and M3, LO transistors are M4, M5, M6, M7 and the tail current transistor is M1, M2 and M3 transistors have high-frequency RF signals. These transistors are expressed as the gain stage because they perform voltage-current conversion. M4, M5, M6 and M7 transistors perform switching operation. The LO signal from the local oscillator is applied to these transistors. Resistors form the IF signal by making current-voltage conversion. A certain current flows from the circuit by applying bias voltage to the transistor M1 and thus the operation of the circuit at the desired level is provided. 
The circuit was designed using umc90 nm technology. The transistors used are 1.2LLLVTHRF NMOS transistors. The VDD value of these transistors should be 1.2 V. The first parameters determined in the mixer design will be the resistances and the dc voltage at which the IF signal will oscillate. Using these, the current flowing from the circuit is found. Then there are two important parameters to consider. These are conversion gain and linearity. The parameter we will look at in linearity will be the third input intercept point (IIP3). In the mixer, the gain and IIP3 values vary inversely. Therefore, it is necessary to select the two values in the most appropriate way. Although the main purpose of the mixers is not to make gain, it is important that the gain of the mixer is above a certain level, as it is mentioned at the beginning. In general, the gain is desired to be higher than 1 dB. Therefore, the circuit design is made for the gain greater than 1 and the maximum value of IIP3 that can be obtained. For this purpose, these parameters are formulated and the required drain currents, transistor dimensions and polarizing voltages are determined.
 The conversion gain of the Gilbert mixer is found as Av = (2/π)gmR and the most important criterion of linearity IIP3 (Third Input Intercept Point) is found. But gain expression changes due to the parasitic capacitances in the circuit and the non-ideal characteristics of the transistors. So when it is W2 = 24 um, the gain expression is less than 1. When the value is taken as W2 = 32 um in the circuit, the gain expression in the simulation results is greater than 1. So W2 = W3 = 32 um   and L1 = L2 = L3 = L4 = L5 = L6 = L7 = 32 um.
The W values of the transistors M4, M5, M6 and M7 which the LO signal is applied will be equal. Since these transistors will perform switching operation, the minimum value VGS – VTH must be selected. Thus W4 = W5 = W6 = W7 = 256 um.
Noise is one of the most important parameters in RF circuit design. In receiver circuits, the input noise of the mixer is divided by the LNA (low noise amplifier) gain, so the noise is relative to the RF signal input is found. In addition, the linearity of the mixer decreases as the gain of the LNA increases. Therefore, the noise figure and the linearity in downconversion mixers vary in relation to each other. In other words, since there is such an interaction between LNA and mixer stages in RF receiver circuits, these circuits should be designed as a single piece. 
Linearity is the most important parameter in mixers. Therefore, it is desirable to increase the linearity to the highest level in the mixers. However, when this is done by reducing the LNA gain, the noise figure will increase. This can be compensated by means of various additional circuits.
The total noise power generated at the output is determined with 2 frequency bands except the RF signal band at the input. These; noise from the LO signal and the noise produced by the transistors and resistors in the mixer in the IF signal. The noise from the LO signal changes continuously. Because these transistors operate in the form of switches, switching between saturation and cut-off modes. These transistors add little noise both in the cutoff mode and in the saturation mode. Because these transistors do not gain, they perform only switching operation.
The noise to be examined in CMOS transistors are thermal noise and flicker noise.
Resistances, transconductances of transistors and parasitic capacitances are the most important factors that increase thermal noise due to the thermal noise expression of the mixer. Therefore, the resistances are taken 50-ohm and the transistors are designed with high finger counts to reduce the effect of parasitic capacitances.
The flicker noise in Gilbert mixer can rise to considerable levels. Significant flicker noise occurs especially in the IF signal near zero frequency. Flicker noises from M1, M2, M3 transistors do not occur at the output. Because these noises interfere with the frequency of LO signal. So only flicker noises from M4, M5, M6, M7 occur at the output. Resistances and the current flows in the mixer are the most important factors that increase thermal noise due to the flicker noise expression of the mixer
Input impedance matching is very important in downconversion mixers. This enables the most efficient transport of power. The circuit is designed to match 50-ohm impedance. The RF input transistors in the circuit are M2 and M3 transistors. From the input, negative virtual impedance occurs due to the CGS parasitic capacitance of these transistors. The inductor with positive virtual impedance is therefore added. Thus, the virtual impedance is equal to zero. The actual impedance of 50-ohm is obtained by the resistors connected to the gate of the RF transistors.