Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Spectral density wikipedia , lookup
Spectrum analyzer wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Opto-isolator wikipedia , lookup
Multidimensional empirical mode decomposition wikipedia , lookup
Wireless power transfer wikipedia , lookup
Regenerative circuit wikipedia , lookup
MICROWAVE SYSTEM DESIGN CONSIDERATIONS Misunderstanding of complete system System will surely fail Without a solid understanding of complete communications system from the transmitter’s modulator input to the receiver’s modulator output, including everything in between, and how the selection of various components, circuits, and specifications can make or break an entire system, any wireless design will surely fail. Block Diagram of Simple One Way Microwave Communication System IF Data in Mixer PA Mod LO IF Amp Data out IF Noise Mixer Dem LO LNA Baseband: Data, Voice, Video,….etc BW? Modulation: Digital or Analog Transmitter Components: IF Filters Filter PA RF Filters Mixer (up conversion) Antenna Link Calculation: Satellite Terrestrial Radar Wireless Mobile, …..etc Receiver Components: Antenna RF Filters LNA Mixer (down conversion) IF Filters IFAmplifier Demodulator Other Components: Control System & Power Supply Monitoring: Test and measuring components & Measuring tools Receiver Calculations Receiver To demodulator Signal + Noise Noise (So/No)min Internal Noise Noise is added into an RF or IF passband and degrades system sensitivity The receiving system does not register the difference between signal power and noise power. The external source, an antenna, will deliver both signal power and noise power to receiver. The system will add noise of its own to the input signal, then amplify the total package by the power gain Noise behaves just like any other signal a system processes Filters: will filter noise Attenuators: will attenuate noise Noise Sources External Manmade • Switching equipment • Power generating equipment. • Ignition Noise • Interference Internal Natural • Atmospheric • Cosmic • Galactic • Thermal Noise • Shot Noise •Gen./Recomb. •Flicker Noise (Mod. Noise) ……………. Thermal Noise The most basic type of noise being caused by thermal vibration of bound charges. Also known as Johnson or Nyquist noise. R R Vn = 4KTBR T Available noise power Pn = KTB Where, K = Boltzmann’s constant (1.3810-23J/oK) T Absolute temperature in degrees Kelvin B IF Band width in Hz At room temperature 290o K: For 1 Hz band width, Pn = -174 dBm For 1 MHz Bandwidth Pn = -114 dBm Shot Noise: Source: random motion of charge carriers in electron tubes or solid state devices. Noise in this case will be properly analyzed on based on noise figure or equivalent noise temperature Generation-recombination noise: Recombination noise is the random generation and recombination of holes and electrons inside the active devices due to thermal effects. When a hole and electron combine, they create a small current spike. Antenna Noise In a receiving system, antenna positioned to collect electromagnetic waves. Some of these waves will be the signals we are interested and some will be noise at the same frequency of the received signal. So filters could not be used to remove such noise. Antenna noise comes from the environment into which the antenna is looking. The noise power at the output of the antenna is equal to KTaB. Ta is the antenna temperature. The physical temperature of the antenna does not influence the value of Ta. The noise temperature of the antenna can be reduced by repositioning it with respect to sources of external noise Assumptions ■ Antenna has no earth-looking sidelobes or a backtobe (zero ground noise) ■ Antenna is lossless ■ h is antenna elevation angle (degrees) ■ Sun not considered ■ Cool. temperate-zone troposphere Equivalent Noise Temperature and Noise Figure Noise Figure (F) Si + N i Two-port Network F = (S/N)i/(S/N)o Ni = Noise power from a matched load at To =290 K; Ni = KTo B. F is usually expressed in dB F(dB)=10 log F. So + N o Equivalent Noise Temperature (Te) If an arbitrary noise source is white, so that its power spectral density is not a function of frequency, it can be modeled as equivalent thermal noise source and characterized by Te. white noise source No R Te R No R Te = No/KB, B is generally the bandwidth of the component or system Te = To( F – 1) To is the actual temperature at the input port, usually 290 K Examples: (1) the noise power of a bipolar transistor at 3 GHz is 0.001 pW for a 1-MHz bandwidth. What is the noise temperature? Solution WN = KTB, T = WN/KB = 72.5 K F of the transistor is 0.97 dB (2) the noise power of a mixer at 20 GHz is 0.01 pW for a I MHz bandwidth. what is the noise temperature ? Solution WN = KTB, T = WN/KB = 725K F = 5.44 dB Noise Figure of Cascaded Components F1 G1 FT = F1 + F2 G2 F2 – 1 G1 + FN-1 GN-1 F3 – 1 FN GN + …… + G 1 G2 Te = To (F - 1) Ts = Ta + Te Pn = KTsBG, where, G is the overall gain of the system Fn – 1 G1 G2 ….. Gn-1 Noise Figure of Passive and Active Circuits Passive Components: For Matching component: F = L (L Insertion Loss) Te = To (L-1) F Increases if the component is mismatched. Active Devices: It is generally easier and more accurate to find the noise characteristics by direct measurement Conversion Noise Noise Free signal and Local Oscillator: -10 dBm IL=7.5 dB F=7.5 dB RF -17.5 dBm IF -130 dBm -130 dBm 17 dBm LO KTB = -130 dBm Noise Figure = conversion loss -130 dBm Noisy received signal: -10 dBm IL=7.5 dB F=7.5 dB RF IF -17.5 dBm -97.5 dBm -90 dBm 17 dBm LO -130 dBm -130 dBm Noisy Local Oscillator: IL=7.5 dB F=7.5 dB -10 dBm RF -17.5 dBm IF -97.5 dBm -130 dBm -130 dBm LO 17 dBm -63 dBm Noise Figure = 40 dB Example: FT? Ts ? No ? L = 3 dB F = 4 dB Mixer Given IF bandwidth = 10 MH Noise BPF LNA L = 1 dB G = 10 dB F = 2 dB Si , Ni So , No LO Ta = 15 K 1) dB to numerical values LNA G = 10 dB (10) BPF: G = -1 dB (0.79) Mixer: G = -3 dB (0.5) F = 2 dB (1.58) F = 1 dB (1.26) F = 4 dB (2.51) 2) FT = [ 1.58 + 0.26/10 + 1.51 /7.9] = 1.8 (2.55 dB) 3) Te = To(F-1) = 290 (1.8 – 1) = 232 K 4) Ts = Ta + Te = 247 K 5) No = KTsBG, G is the overall Gain = G1×G2×G3×….=10 × 0.79 × 0.5 = 3.95 (~6dB) No = -98.7 dBm Dynamic Range, and 1-dB Compression Point Output 1 dB power 1 dB compression point Input power Noise floor Dynamic range Minimum Detectable Signal (MDS) MDS is dependent of the type of modulation used in receiving systems as well as the noise characteristics of the antenna and receiver. For a given system noise power, the MDS determines the minimum signal to noise ratio (SNR) at the demodulator of the receiver. The usable SNR depends on the application, with some typical values below System SNR (dB) Analog telephone 25-30 Analog television 45-55 AMPS cellular 18 QPSK (Pe = 10-5) 10 To demodulator Receiver Ci & Ta (Co/No)min Noise Te F G Ci/Ta Can be measured immediately following the receiver Detector: Removes the signal from the carrier S/N Can be determine Example: FM modulated signal SNR = C/No - 10 log B + 20 log (fu/fmax) + q w (dB) Where C/No = carrier to noise density (dBHz) B = channel bandwidth (Hz) fu = test tone deviation at 0 dBm (Hz) fmax = maximum frequency of baseband (Hz) qw = combined psophometric and preemphasis factors (dB) Sensitivity: (MDS) Receiver voltage sensitivity, usually shortened to simply the receiver sensitivity. Vimin = (2ZoSimin)0.5 Receiver Dynamic range: DRr = (maximum allowable signal power) / MDS Defined by the third-order intercept point Automatic Gain Control (AGC) Why? DR(at the output of the receiver) < DR(at the input) Avoid receiver non-linearity Receiver Gain: should be distributed throughout the RF, IF, & Baseband to avoid nonlinearity of the RF stage and take advantage of low cost IF amplifiers G ~ 80-100 dB. Input and Output Receiver Dynamic Range Pb (V) Pr (dBm) Low gain 1 0 -20 -40 DRr -60 Ci ~ 80-100 dB -80 -100 -120 Receiver Gain G DRout ~ 60 dB 0.1 0.01 0.001 High gain Pb IF AGC circuit IF Amp IF Input Variable gain amp/attenuator Demodulator AGC detector LPF DC Ref DC Amp AGC Distributed Between RF and IF Filter LNA Filter Mixer IF amplifier IF detector LPF Comparator Frequency Conversion and Filtering Selection of IF frequency: fIF = |fRF - fLO| fLO fRF For lower side band selection fLO = fRF + fIF Image IF IF Large IF eases the cutoff requirements of the image filter FIF > BRF/2 IF < 100 MHz Image frequencies outside RF BW Low cost Transmitter Radiate electromagnetic signal Output: Desired signal power Harmonic Spurious outputs Wideband noise and phase noise, Critical parameters: Frequency and amplitude stability Signal’s peak and average powers Transmitted noise will raise the noise floor of the receiver Link Budgets Baseband signal Baseband output Gr Gt Pr Pt Rx Tx R Pt is the transmitted power Gt is the transmit antenna gain Gr is the receive antenna gain Pr are the received power The power density radiated by an isotropic antenna at a distance R is given by Savg = Pt/4pR2 W/m2 The power density radiated by the given antenna is Savg = Pt Gt /4pR2 W/m2 The received power will be Pr = Savg Ae Pt Gt Ae/4pR2 W Ae = Grl2/4p m The received power can be expressed as Pr = Pt Gt Grl2 /(4pR)2 W Pr / Ni = (Pt Gt) [l2 /(4pR)2] Gr / KTAB = (Pt Gt) [l2 /(4pR)2] (Gr /TA)/KB = EIRP Path loss Figure of merit / KB where, EIRP is the equivalent isotropic radiated power TA is the antenna noise temperature G/T is a useful figure of merit for a receive antenna because it characterizes the total noise power delivered by the antenna to the input of a receiver. The power density of the transmitted wave at the target location is Wt Wt = Pt Gt(q,f)/4pRt2 W/m2 RCA Radar cross section area (echo area). It depends on the angle of incidence, on the angle of observation, on the shape of the scatterer, on the EM properties of the matter that it is built of, and on the wavelength. Some Other Microwave Systems Baseband Microwave Radio RF Multiplexing technique Baseband Repeater Types of Microwave Devices Passive Devices Active Devices No DC Power & No Electronic control Uses DC Power or No Electronic control Duplexers Diplexers Couplers Bridges Dividers Combiners Circulators Attenuators Adapters Delay lines Waveguides Resonators Dielectrics Antennas Opens, shorts, loads Switches Samplers Transistors Amplifiers MMICs VTFs Tuners Synthesizer Filters Splitters Isolators Cables TL R, L, C’s Multiplexers Multipliers Oscillators RFICs Modulators VCAtten’s Converters Mixers Diodes MICs VCOs VCAs