Pulses in Transmission Lines
Professor Jeff Filippini Physics 401 Spring 2020 Key Concepts for this Lab 1. Networks with distributed parameters What if the R/L/C are spread out? Thevenin-equivalent networks
2. Pulse propagation in transmission lines When signals move like waves Reflections from resistive and reactive loads
3. Impedance matching Getting power where you want it to go
Physics 401 2 When do Wires Carry Waves?
Thus far we’ve implicitly assumed that V(t) and I(t) are synchronized throughout our circuits – but signals travel at finite speed (� = �/�)! • Speed of light: � = 3×10 m/s = 30 cm/ns = 1 ft/ns Real signals are typically slower by a factor (n) of order unity
Over distances that are a significant fraction of the wavelength, we’re better off thinking of wave propagation
Frequency Application nλ 60 Hz AC power lines 5000 km 580 kHz WILL-AM broadcast 500 m (0.3 mile) 2.4 GHz WiFi 12.5 cm (5 inches) 430 THz Red light 0.7 μm
Physics 401 3 Transmission Lines
Transmission line: a specialized cable (or other structure) designed to conduct alternating current of radio frequency (RF). • Limit wave reflection by maintaining uniform impedance (details below!) • Reduce power loss to radiation
Wikipedia: Transmission Line Design goal: Source Maximize power from the impedance source delivered to the load Load impedance “Line impedance”
Physics 401 4 Common Types of Transmission Lines
Twisted pair Twin lead Wikipedia Wikipedia
Analog Devices Coaxial cable
Physics 401 5 Coaxial Cable
Specification: Signal voltage between central conductor and Impedance: 53 Ω braid shield Capacitance: 83 pF/m Conductor: Bare Copper Wire (1/1.02mm) Shield reduces external dipole radiation (and response to RF interference)
Physics 401 6 Experimental Setup
Wavetek 81 Tektronix 3012B
Sync output Triggering input
RG8U Signal output
Load
Physics 401 7 Modeling a Transmission Line
Simplified I(t) Ideal equivalent case Model as distributed network circuit Li-1 Li Li+1 rather than lumped components Ci-1 Ci Ci+1 V(t) Ideal line has inductance / capacitance per unit length I(t) (lossless)
Ri Li Real lines have finite Real conductance � = between situation Gi+1 Ci conductors (i.e. loss)
Physics 401 8 Pulses in Transmission Lines
reflected
rout i(x,t)
V(t) V(x,t) ZL
x V 0 forward
Physics 401 9 The Telegrapher’s Equations
rout i(x,t)
Lossless V(t) V(x,t) ZL line (G=0) i(x,t)
x dx C = capacitance per unit length L = inductance per unit length
Distributed capacitance � �� � = −�� Distributed inductance �� �� �� � �� = − = −� �� = −(� ��) �� �� �� �� �� �� �� � = − = −� �� �� �� ��
Physics 401 10 The Wave Equation
Distributed capacitance Distributed inductance �� �� �� �� C = − = −� �� �� �� �� � � �� �� � � � � � � � � = −C = −� ���� �� �� ����
Combining
� � � � � � � � = LC = LC �� �� �� ��
Physics 401 11 Voltage and Current Waves
� � � � � � � � Waves of velocity v = LC = LC � �� �� �� �� � �, � = � sin � � − Seek solutions � of the form… � I �, � = � sin � � − Substitute � �, � , I �, � into… � C = − = −� … to find two key consequences: 2. � �, � � 1. 1 � ≡ = � = � �, � � �� Characteristic impedance of line Speed of wave propagation
Physics 401 12 Characteristic Impedance
� C = capacitance per unit length (F/m) � = L = inductance per unit length (H/m) � � = 8.854×10 F/m � = 4�×10 H/m 2πε ε � � = � C= 0r (F/m) d D æöD µµ0 r æöD lnç÷ L = lnç÷(H/m) èød 2p èød
er – dielectric permittivity µr-magnetic permeability ≈1
138 æöD Finally for coaxial cable: ZOhmsk = log10 ç÷ ( ) e r èød
Physics 401 13 Wave Propagation Speed 2πε0r ε µµ D � = 8.854×10 F/m 1 C= 0 r æö � = L = lnç÷ æöD 2p èød � = 4�×10 H/m �� lnç÷ èød � � = �
1 1 � � � = = = ≈ d D �� � � � � � � �