2.4-2.7 GHz radyo-röle alt band çevirici tasarımı ve gerçekleştirilmesi

Abstract

2.4-2.7 GHz Radyo-Röle alt band çevirici olarak adlandırılan bu sistem, 300 MHzilik mikrodalga işaret bandının UHF bölgesinde 560-860 MHz bandına indirmek için tasarlanmıştır. Bu çeşit bir alt band çevirici uydu alıcısı olarak veya mikrodalga TV yayınlarının alınmasında rahatlıkla kullanılabilir. Arabsat uydusunda alıcı olarak kullanılabileceği gibi, aynı frekans bandında TV yayınlarına alıcı olarak da kullanılabilmektedir. Bu tür yayınlarda 16 TV kanalı bulunmaktadır ve yaklaşık olarak 186 MHz band genişliği kaplamaktadır. Bu tez çalışmasında, gerçekleştirilen düzende bulunan blokların, tasarımları, uygulamada bir araya getirilişleri, sonuçta yapılan ölçüm ve deneylerle istenen şartları ne ölçüde sağlandığı gösterilmiştir. Sistem genel olarak beş ana bloktan oluşmuştur. Düşük gürültülü RF kuvvetlendirici, süzgeç katı, yerel osilatör (YO) katı, karıştırıcı ve ara frekans (AF) kuvvetlendiricisi. Düşük gürültülü RF kuvvetlendiricisi tasarımında NEC in NE72084 GaAs MESFET'i kullanılmıştır. Bu eleman la 8 GHz 'e kadar, yeterli düzeyde yüksek kazanç ve düşük gürültü sayısı (NF) elde edilebilmektedir. Ayrıca düşük maliyetli olması da önem taşır. Süzgeç katında ara bölmeli, üç ve iki elemanlı süzgeçler kullanılmıştır. RF ve YO çıkışında band geçiren türden süzgeçler kullanılmıştır. Yerel osilatör olarak bir evre kilitli çevrim (PLL) yardımıyla frekans sentezleme yoluna gidilmiş tir. Bu şekilde 1839 MHz'de kristal sabitliğinde bir yerel osilatör işareti elde edilmiştir. Böylece giriş te düşük gürültülü RF kuvvetlendiriciden ve daha sonra band geçiren süzgeçten geçen işaret yerel osilatör ile beraber tek diyodlu karıştırıcı katında karıştırılarak fark frekanslı terim elde edilmiş olur. Son olarak ara frekans kuvvetlendirici (AF) katında 560-860 MHz bandına getirilen işaret yaklaşık olarak 15dB kadar kuvvetlendirilerek çıkışa ulaştırılır. Çıkıştan DC besleme alma düzeneği de ayrıca eklenmiş ve bu şekilde band çevirici sistem tasarımı tamamlanmış olur. Tasarlanan sisteme ait tanımlayıcı ölçümler de yapılmış ve ilgili bölümde sunulmuştur.

The main idea of designing this system is to convert approximately 300MHz frequency band from 2.4- 2.7GHz to 560-860MHz. This kind of a down-converter can be used as a satellite receiver or a custom microwave TV distribution system. Arabsat is one of the satellites for which the device can be used as a receiver. On the other hand it is possible to receive the TV channels which are transmitted in the frequency band mentioned above. It is known that 32 TV channels are transmitted (half reverse polarized) in the 2.5-2.68GHZ frequency band. The device, which is the result of this thesis is capable of receiving these TV channels as well. Five sub-circuits are used in order to achieve this result: low-noise RF amplifier, filter block, local oscillator (LO), mixer and intermediate amplifier (IF) amplifier. The design approach of these circuits are summarized below. RF Amplifier Design : The input signal level is extremely low. There fore it should be designed as a low-noise amplifier. NEC 720 series GaAs FETs are suitable for this applica tion. The NE72084 is NEC's low cost 1.0 /4m recessed ga te GaAs FET, offering a low noise figure and high gain through 8GHz. It is designed for consumer applications. For low-noise design, the transistor data must include S-parameters at the low noise bias and four noise parameters. One common noise parameter set is:[l] F. Minimum noise figure ( <3dB ) rain R = N/G" Noise resistance n 6 Y_ = G_ + jB_ Optimum noise admittance On On On r The noise figure of the two-port is determined by the generator admittance (or impedance) presented to the [1] (v) input terminals and is calculated from : The output port is tuned for the maximum available gain if the amplifier is single-stage. In a two-stage, low- noise design the interstage circuit will probably be tu ned for minimum second-stage noise figure. The N-para- meter is invariant to lossless transformations. The noise performance of a transistor is often visualized by plotting noise circles on the Tg plane for bipolar transistors and for FETs. Some additional ef fects on low-noise performance are 3dB Lange coupler losses, microstrip-line losses, non-optimum dc biasing, and resistive losses. Feedback can also be used in low- noise designs to vary Fmin and Yon. When feedback is used, the noise figure may be reduced, but the available gain is also reduced. The minimum noise measure is in variant to lossless feedback elements. It is not inva riant to the common lead, and is usually lowest for the common-emitter or common-source transistors. In the design of a single stage amplifier, the specifications are usually clearly focused on either gain, noise figure, or output power. When the additio nal specification of input and/or output match is added by the system designer, a design trade-off will be ne cessary. For example, if we consider the design of a low-noise amplifier using the parameters at low noise bias there are three basic designs to consider :[2-6] 1. Design M for low-noise, Tq = Ton Design M_ for r - <$ '" - i «s * 12 21 0n ri - S22 - L S*2 1 " S^ J22 = [ This gives Fmin, high input VSWR, and low-output VSWR, which is the most common design technique. 2. Design M1 for low-noise, Tg = Ton (vi) Design M2 for j * _ S12S21FL which gives S,, = r n = S", + 11 On 11 S + r * r 11 On L " d - s r * 22 On i - s r 22 On This case gives Fmin, low-input VSWR, and high- output VSWR with a possibility of oscillation (if kl ). If Mi is used to provide a particular value of Tq, only one degree of freedom remains available and an option has to be made for input match only, for output match only or a suit able compromise between the two. Filter Block : The filter block includes two different filters. One of them filters the RF input signal between 2.4-2.7 GHz and the other one filters the LO signal at 1839MHz. Both of these filters are short-end interdigital fil ters. The output of the RF amplifier is combined with three element filter and the output of the LO is com bined with two element filter connected at the input of the mixer diode [7-11]. The RF filter has got 5dB loss at the passband of 2.4 - 2.7GHz. The LO filter loss is in neglectable range. (vii) Mixer : A single diode mixer is used in order to get the difference frequency band between RF input and LO. Today single-diode mixers exhibit 8dB noise figure performance in access of 100GHz. The burden of establis hing receiver sensitivity is still largely dependant on the mixer throughout the microwave frequency range. Re cently, low-noise GaAs FET amplifiers are being used be low 50GHz to improve the system noise figure, but above this frequency the diode is the only device that can be used for frequency-conversion applications [1]. The mixer, which can consist of any device ca pable of exhibiting nonlinear performance, is essen- tialy a multiplier. Unfortunately, no physical nonli near device is a perfect multiplier. Thus, they contri bute noise and produce last number of spurious frequency components. For example, the voltage-current relation ship for a diode can be described as an infinite power series, I = a + a V 01 a^v2 + ^v3 where V is the sum of both input singals and I is the total signal current. If the RF signal is substantially smaller than the LO signal and modulation is ignored, the frequency components of the current I are «J = nw ± d) d p 8 The desired output signal is the difference fre quency. Therefore, the mixer output contains the nece ssary microstrip elements in order to vanish the sum frequency and other mixer products. Local Oscillator : A Phase Locked Loop (PLL)is used in order to get a constant oscillation at 1839 MHz. The voltage con trolled oscillator (VCO) functions between 1500-2000 MHz. A common emitter amplifier is added to the output of the oscillator. Then a two step division is perfor med so as to use the PLL integrated circuit. The crystal which is used to compare with VCO output signal, opera tes at 7.183 MHz [7]. (viii) The synthesised LO signal is performed to the interdigital filter so as to suppress the second harmo nic products and other parasitic signals. IF Amplifier Design : IF amplifier has been designed in order to get maximum flat gain (G « 15dB) between 560-860MHz (UHF band). The input and output ports of the active device have been matched in order to provide the above condi tion. The active device used in IF amplifier is BFR92A (SMD type). The given S-parameters in the catolog of this device at the desired band were not enough, so mo deling of the device was required and then missing S- parameters have been calculated. The results of the measurements show that all of the required specifica tions are obtained [11-16].