Introduction to RF and Microwave Systems

RF and Microwave Frequency Bands RF (“radio frequency”) is used to indicate the frequency band from hundreds of MHz to about 3 GHz Microwave frequencies start from 300 MHz and goes up to 30 GHz, ( wavelength of 1m to 0.01m) The frequency bands above 30 GHz is called Millimeter waves, and extend up to 300 GHz. Its technology is very similar to microwaves.

Electromagnetic Spectrum

Electromagnetic Spectrum (comparable)

What is different about the RF/Microwave band? Circuit theory / transmission lines / electromagnetics all needed, because: – The size of the circuit (let’s call it d) for RF and microwave circuits – Question: what relationship between d and exists for (a) low frequency circuits (kHz range), and (b) optical circuits (where the wavelength is on the order of µm), keeping in mind that the circuits themselves are all on the order of cm? Microwave networks are harder to analyze than their low-frequency counterparts. The reason is that a microwave circuit is about the same size as the wavelength, so phase variation along a part of a circuit cannot be ignored like we do at lower frequencies. In other words, KirchhoÆ's laws do not apply, since they assume that the circuit is much smaller than a wavelength. On the other hand, in optics, everything is many thousands of wavelengths large, so we can use another approximation, and deal with rays and geometrical optics. The microwave region is the trickiest one to deal with mathematically. Rigorous analysis uses electromagnetic Øeld theory, starting from Maxwell's equations, and is very complicated in most practical cases. Fortunately, we do not need all the information that such a Øeld theory analysis gives us, and in many cases transmission-line theory is applicable.

Advantages of the use of higher frequencies Larger instantaneous BW for much information, Higher resolution for radar, imaging and sensing, bigger doppler shift, Reduced dimensions for components, Less interference from nearby applications Higher speed for digital systems, signal processing, data transmission Less crowded spectrum Difficulty in jamming (military)

Disadvantages of the use of higher frequencies More expensive components, Higher atmospheric losses, Reliance on GaAs instead of Si technology Higher components losses, lower output powers from active devices, Less accurate design tools, less mature technologies. The electron mobility in GaAs is higher than that in silicon. Therefore, GaAs devices can operate at higher frequencies and speeds.

RF and Microwave Applications Wireless Communications (space, cellular phones, cordless phones, WLANs, Bluetooth, satellites etc.) Radar and Navigation (Airborne,vehicle, weather radars, GPS, MLS, imaging radar etc.) Remote sensing (Meteorology, mining, land surface, aviation and marine traffic etc.) RF Identification (Security, product tracking, animal tracking, toll collection etc.) Broadcasting (AM,FM radio, TV etc.)

RF and Microwave Applications Automobiles and Highways (Collision avoidance, GPS, adaptive cruise control, traffic control etc.) Sensors (Temperature, moisture sensors, robotics, buried object detection etc.) Surveillance and EW (Spy satellites, jamming, police radars, signal/radiation monitoring etc.) Medical (MRI, Microwave Imaging, patient monitoring etc.) Radio Astronomy and Space Exploration (radio telescopes, deep space probes, space monitoring etc.) Wireless Power Transmission (Space to space, space to ground etc. power transmission)

Radiated Power and Safety Organic tissue absorbs RF and microwave energy and converts it to heat (e.g. Microwave oven) This is not a good thing when the tissue is you! Heating is dangerous to areas such as brain, eyes, and stomach organs Radiation may cause cataracts, cancer, and sterility ANSI/IEEE standard sets safety standard for exposure limits (e.g. limited to 10 mW/cm2 above 15 GHz where radiation is absorbed by the skin) Handheld cell phones limited to maximum radiated power of 0.76 W, while base stations are limited to 500 W.

The main purpose of the course is to provide the following questions: At what upper frequency does conventional circuit analysis become inappropriate? What characteristics make the high-frequency behavior of electric components so different from low-frequency behavior? What “new” circuit theory has to be employed? How is this theory applied to practical design of high-frequency analog circuits?

Sample Tranceiver The entire block diagram can be called a transceiver, since it incorparates both transmitter and receiver circuits and uses a single antenna for communication. In this configuration the input signal is first digitally processed. If the inut signal is a voice signal, as is the cas in cellular phones, it is first converted into digital form;then compressed to reduce the time of transmission; and finally appropriately coded to suppress noise and communication errors. After the input signal has been digitally preprocessed, it is converted to back to analog form via a DAC. This low frequency signal is mixed with a high frequency carrier signal provided by local oscillator. The combined signal is subsequently amplified through a PA and then routed to the antenna, whose task is to radiate the encoded information as EW into free space.

Power Amplifier: Circuit Power amplifier of the cell phone is used to amplify the signal coming from the antenna (or going to antenna) to feed the mixer. An example, seen in Fig.5, is a dual- stage amplifier. Input signal is fed through a DC blocking capacitor into a matching circuit that helpt matching the imput impedance of the transistor to the mixer output (or antenna). Matching is necessary for maximum power transfer at higher RF and microwave frequencies, and we will learn later how matching circuits are implemented using discrete(lumped) and distributed components. The interstate matching network is to match the output of the transistor to the second stage of the power amplifier that is not shown in Fig. 5. The key elements in matching networks are micorstrip lines shown in Fig. 6. We should also need some other circuits indicated in Fig. 5. They are required to bias the the transistors(biasing networks/circuits). The separation from the RF signals from the DC supply is achieved via the RF blocking networks at where RFCs (radio frequency coils) are employed as shown in Fig. 5.

Power Amplifier: PCB layout Actual dual stage implementation of the power amplifier (PCB layout) is shown in Fig. 6. They are realized via printed circuit boards. Microstrip lines and other components are mounted on the surface of the PCB. The microstrip lines as copper traces are of specific lengths and widths. Chip capacitors and inductors are also attached on the microstrip lines for matching and biasing purposes as shown in Fig. 6.

RF Behavior of Passive Components

Lumped(discrete) or distributed elements: Inductor