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Introduction to RF-Microwave Systems

Introduction to RF-Microwave Systems - ECE 166 Microwave...

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1 ECE 166 – Microwave Circuits Introduction to RF/Microwave Systems Gabriel M. Rebeiz – © 2006 RF/Microwave engineers spend a lot of time designing and specifying systems with the purpose of receiving and/or transmitting a specific signal, filtering out all the interferers from the receive path, and making sure that the transmitted power is spectrally clean and does not contain any out-of-band signals. One has to specify the noise, gain, linearity, etc. of the system, and build components to achieve these functions (amplifiers, mixers, oscillators, etc.). Typical received powers can be as low as -104 dBm in GSM signals and typical transmit powers are around 1-3 W (30-35 dBm). The difference between transmit and receive signal levels can be as much as 10 14 in the same phone and one must make sure that the transmit signal does not saturate the receiver (this is why we believe that digital is “easy” –hard work is in analog RF). Also, while receiving a -100 dBm signal, someone may be standing 1 m from you and transmitting as much as 2 W at a slightly different frequency. How do we get rid of their signal and not let it interfere with the (wanted) received signal? Welcome to system-level design at RF/Microwave frequencies. There are few basics which need to be covered first before we go to a typical system. They will be done in a question and answer mode (pioneered by Galileo in the 15 th century). 1. Why do we use RF to Microwave Frequencies (100 MHz to 6 GHz) for Wireless Communications? The basic information which we are transmitting, being voice, video or data has a bandwidth of 4-20 kHz (voice) and 0.1-2 MHz (video and data). Sometimes the bandwidth is given in bits-per-second (Kilo or Mega-bits per second) since the information is basically in digital bits. In general, it is possible to put 2 bit per hertz of bandwidth (in modern systems only), so a 1 Mbps video data will take around 500 kHz or frequency bandwidth. However, these low frequencies cannot be transmitted wirelessly since antennas operate efficiently when their dimensions are a fraction of a wavelength (typically 0.25-0.5 λ ). Therefore, an efficient 1 MHz antenna will be 75-150 meter is length! There is another reason why we do not transmit low frequencies: A 2 Mbps signal contains all frequencies from nearly DC to 1 MHz, and this is virtually a 200:1 bandwidth (5 KHz to 1 MHz), and it is very hard, if not impossible, to make an antenna at any frequency with such a wide bandwidth. Therefore, in order to solve the antenna problem, we modulate the information that we wish to send on a carrier signal. The carrier signal is heavily regulated by the FCC (Federal Communications Commission) and some are reserved for cell phones, some for police bands, hospital bands, radio amateur bands, etc.
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