Transcript reflection
Trasmission lines Lightwave Analogy to RF Energy Incident Reflected Transmitted Lightwave DUT RF Why Do We Need to Test Components? • Verify specifications of “building blocks” for more complex RF systems • Ensure distortionless transmission of communications signals – linear: constant amplitude, linear phase / constant group delay – nonlinear: harmonics, intermodulation, compression, AMto-PM conversion • Ensure good match when absorbing power (e.g., an antenna) F M 9 7 K P W R The Need for Both Magnitude and Phase S21 1. Complete characterization of linear networks S11 S22 S12 2. Complex impedance needed to design matching circuits 4. Time-domain characterization Mag 3. Complex values needed for device modeling High-frequency transistor model Time 5. Vector-error correction Error Base Collector Emitter Measured Actual Transmission Line Basics + I - Low frequencies wavelengths >> wire length current (I) travels down wires easily for efficient power transmission measured voltage and current not dependent on position along wire High frequencies wavelength or << length of transmission medium need transmission lines for efficient power transmission matching to characteristic impedance (Zo) is very important for low reflection and maximum power transfer measured envelope voltage dependent on position along line Transmission line Zo • • Zo determines relationship between voltage and current waves Zo is a function of physical dimensions and r Zo is usually a real impedance (e.g. 50 or 75 ohms) 1.5 attenuation is lowest at 77 ohms 1.4 1.3 1.2 normalized values • 1.1 50 ohm standard 1.0 0.9 0.8 0.7 power handling capacity peaks at 30 ohms 0.6 0.5 10 20 30 40 50 60 70 80 90 100 characteristic impedance for coaxial airlines (ohms) Power Transfer Efficiency RS For complex impedances, maximum power transfer occurs when ZL = ZS* (conjugate match) RL R s + j X Load Power (normalized) 1.2 1 j X 0.8 0.6 R L 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 RL / RS Maximum power is transferred when RL = RS Transmission Line Terminated with Zo Zs = Zo Zo = characteristic impedance of transmission line Zo Vinc Vrefl = 0! (all the incident power is absorbed in the load) For reflection, a transmission line terminated in Zo behaves like an infinitely long transmission line Transmission Line Terminated with Short, Open Zs = Zo Vinc Vrefl In-phase (0o) for open, out-of-phase (180o) for short For reflection, a transmission line terminated in a short or open reflects all power back to source. Transmission Line Terminated with 25 W Zs = Zo ZL = 25 W Vinc Vrefl Standing wave pattern does not go to zero as with short or open High-Frequency Device Characterization Incident Transmitted R B Reflected A TRANSMISSION REFLECTION Reflected Incident = SWR S-Parameters S11, S22 Reflection Coefficient G, r A Transmitted R Incident Return Loss Impedance, Admittance R+jX, G+jB = B R Group Delay Gain / Loss S-Parameters S21, S12 Transmission Coefficient T,t Insertion Phase Reflection Parameters Reflection Coefficient G Vreflected = = Vincident Return loss = -20 log(r), r r F = ZL - ZO Z L + ZO G = Emax Emin Voltage Standing Wave Ratio Emax VSWR = Emin = 1+r 1-r Full reflection (ZL = open, short) No reflection (ZL = Zo) 0 r 1 dB RL 0 dB 1 VSWR Transmission Parameters V Incident DUT Transmission Coefficient = T V Insertion Loss (dB) = - 20 Log V V Gain (dB) = 20 Log V Trans Inc = Trans V Transmitted V Transmitted V Incident = - 20 log Inc = 20 log t = t t