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.

PA IF Mixer Mod Data in LO Noise LNA Mixer IF Amp IF Data out LO Dem Block Diagram of Simple One Way Microwave Communication System

Baseband:Data, Voice, Video,….etc BW? Modulation: Digital or Analog Transmitter Components: IF Filters Mixer (up conversion) FilterPARF FiltersAntenna Link Calculation: SatelliteTerrestrialRadarWireless Mobile, …..etc Receiver Components: AntennaRF Filters LNA Mixer (down conversion)IF FiltersIFAmplifier Demodulator Other Components: Control System& Power Supply Monitoring: Test and measuring components & Measuring tools

Receiver Calculations Noise (S o /N o ) min Receiver To demodulator Signal + Noise 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

Internal External NaturalManmade Thermal Noise Shot Noise Gen./Recomb. Flicker Noise (Mod. Noise) ……………. Atmospheric Cosmic Galactic Switching equipment Power generating equipment. Ignition Noise Interference Noise Sources

Thermal Noise The most basic type of noise being caused by thermal vibration of bound charges. Also known as Johnson or Nyquist noise. R T V n = 4KTBR R Available noise power P n = KTB Where, K = Boltzmann’s constant (1.38  J/ o K) T Absolute temperature in degrees Kelvin B IF Band width in Hz At room temperature 290 o K: For 1 Hz band width, P n = -174 dBm For 1 MHz Bandwidth P n = -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 KT a B. T a is the antenna temperature. The physical temperature of the antenna does not influence the value of T a. 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 F = (S/N) i /(S/N) o N i = Noise power from a matched load at T o =290 K; N i = KT o B. F is usually expressed in dB F(dB)=10 log F. Noise Figure (F) Two-port Network S i + N i S o + N o

T e = N o /KB, B is generally the bandwidth of the component or system T e = T o ( F – 1) T o is the actual temperature at the input port, usually 290 K R No No white noise source R R TeTe No No Equivalent Noise Temperature (T e ) 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 T e.

Examples: (1) the noise power of a bipolar transistor at 3 GHz is pW for a 1-MHz bandwidth. What is the noise temperature? Solution W N = KTB, T = W N /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 W N = KTB, T = W N /KB = 725  K F = 5.44 dB

Noise Figure of Cascaded Components T e = T o (F - 1) T s = T a + T e P n = KT s BG, where, G is the overall gain of the system F 2 – 1 G 1 F 3 – 1 G 1 G 2 F n – 1 G 1 G 2 ….. G n-1 F T = F …… + F1G1F1G1 F2G2F2G2 F N-1 G N-1 FNGNFNGN

Noise Figure of Passive and Active Circuits Passive Components: For Matching component: F = L (L Insertion Loss) T e = T o (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 -130 dBm dBm -130 dBm IL=7.5 dB F=7.5 dB LO RF IF KTB = -130 dBm 17 dBm -130 dBm Noise Figure = conversion loss

Noisy received signal: 17 dBm -130 dBm dBm dBm IL=7.5 dB F=7.5 dB -10 dBm -90 dBm -130 dBm LO RF IF

Noisy Local Oscillator: -10 dBm -130 dBm IL=7.5 dB F=7.5 dB dBm dBm -130 dBm 17 dBm -63 dBm LO RF IF Noise Figure = 40 dB

Example: F T ? T s ? N o ? Given IF bandwidth = 10 MH Noise LNA Mixer LO BPF G = 10 dB Ta = 15 K F = 2 dB L = 1 dB L = 3 dB F = 4 dB S o, N o S i, N i 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) F T = [ / /7.9] = 1.8 (2.55 dB) 3) T e = T o (F-1) = 290 (1.8 – 1) = 232 K 4) T s = T a + T e = 247 K 5)No = KTsBG, G is the overall Gain = G 1 ×G 2 ×G 3 ×….= 10 × 0.79 × 0.5 = 3.95 (~6dB) No = dBm

Dynamic Range, and 1-dB Compression Point Input power 1 dB compression point Output power 1 dB Dynamic range Noise floor

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 SystemSNR (dB) Analog telephone25-30 Analog television45-55 AMPS cellular18 QPSK (P e = )10

C i /T a Can be measured immediately following the receiver Detector: Removes the signal from the carrier S/N Can be determine Noise (C o /N o ) min Receiver Te F G To demodulator C i & T a

Example: FM modulated signal SNR = C/No - 10 log B + 20 log (f u /f max ) + q w (dB) Where C/No = carrier to noise density (dBHz) B = channel bandwidth (Hz) f u = test tone deviation at 0 dBm (Hz) f max = maximum frequency of baseband (Hz) qw = combined psophometric and preemphasis factors (dB) Sensitivity: (MDS) Receiver voltage sensitivity, usually shortened to simply the receiver sensitivity. V imin = (2ZoS imin ) 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 non-linearity of the RF stage and take advantage of low cost IF amplifiers G ~ dB.

Input and Output Receiver Dynamic Range Pr (dBm) DR r DR out Receiver Gain G Low gain High gain CiCi P b (V) PbPb ~ dB ~ 60 dB

IF AGC circuit IF Amp Demodulator LPF Variable gain amp/attenuator IF Input DC Amp DC Ref AGC detector

AGC Distributed Between RF and IF IF detector IF amplifier Filter LNA Mixer Filter LPF Comparator

Selection of IF frequency: f IF = |f RF - f LO | For lower side band selection f LO = f RF + f IF Frequency Conversion and Filtering Large IF eases the cutoff requirements of the image filter F IF > B RF /2 Image frequencies outside RF BW IF < 100 MHz Low cost f LO f RF Image IF

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 Tx Rx Baseband signal Baseband output R PtPt PrPr GtGt GrGr P t is the transmitted power G t is the transmit antenna gain G r is the receive antenna gain P r are the received power

The power density radiated by an isotropic antenna at a distance R is given by S avg = P t /4  R 2 W/m 2 The power density radiated by the given antenna is S avg = P t G t /4  R 2 W/m 2 The received power will be P r = S avg A e P t G t A e /4  R 2 W A e = G r 2 /4  m The received power can be expressed as P r = P t G t G r 2 /(4  R) 2 W

P r / Ni = (P t G t ) [ 2 /(4  R) 2 ] G r / KT A B = (P t G t ) [ 2 /(4  R) 2 ] (G r /T A )/KB = EIRP Path loss Figure of merit / KB where, EIRP is the equivalent isotropic radiated power T A 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 W t W t = P t G t ( q, f )/4 p R t 2 W/m 2 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

Microwave Relay System

Types of Microwave Devices Passive Devices No DC Power & No Electronic control Active Devices Uses DC Power or No Electronic control Duplexers Diplexers Filters Couplers Bridges Splitters Dividers Combiners Isolators Circulators Attenuators Cables Adapters Delay lines TL Waveguides Resonators R, L, C’s Dielectrics Antennas Opens, shorts, loads Switches Multiplexers Mixers Samplers Multipliers Diodes Transistors Oscillators Amplifiers RFICs MICs MMICs Modulators VCOs VTFs VCAtten’s VCAs Tuners Converters Synthesizer