Modern Instrumentation PHYS 533/CHEM 620

1. Modern InstrumentationPHYS 533/CHEM 620 Lecture 9 Data Transmission & Data Acquisitions Amin Jazaeri Fall 2007

2. Transmission lines • Types of transmission lines • parallel conductors • coaxial cables • transmission line wave propagation • Losses • incident and reflected wave and impedance matching

3. Transmission Media • Guided • some form of conductor that provide conduit in which signals are contained • the conductor directs the signal • examples: copper wire, optical fiber, wave guides • Unguided • wireless systems – without physical conductor • signals are radiated through air or vacuum • direction – depends on which direction the signal is emitted • examples: air, free space

4. Transmission Media • Cable transmission media • guided transmission medium and can be any physical facility used to propagate EM signals between two locations • e.g.: metallic cables (open wire, twisted pair), optical cables (plastic, glass core)

5. Types of Transmission Lines • Balanced Transmission line • 2 wire balanced line. • both conductors carry current. But only one conductor carry signals.

6. Types of Transmission Lines

7. Types of Transmission Lines • Unbalanced Transmission line • One wire is at ground potential • the other wire is at signal potential • advantages – only one wire for each signal • disadvantages –reduced immunity to noises

8. Types of transmission lines

9. Types of Transmission Lines • Baluns • Balanced transmission lines connected to unbalanced transmission lines • e.g.: coaxial cable to be connected to an antenna

10. Metallic Transmission Lines • Parallel conductors • Coaxial cable

11. Parallel Conductors • Consists of two or more metallic conductors (copper) • Separated by an insulator – air, rubber etc. • Most common • Open Wire • Twin lead • Twisted Pair (UTP & STP)

12. Parallel Conductors • Open Wire • two-wire parallel conductors • Closely spaces by air • Non conductive spaces • support • constant distance between conductors (2-6 inches) • Pro – simple construction • Con – no shielding, high radiation loss, crosstalk • Application – standard voice grade telephone

13. Parallel Conductors • Twin lead • Spacers between the two conductor are replaced with continuous dielectric – uniform spacing • Application – to connect TV to rooftop antennas • Material used for dielectric – Teflon, Polyethylene

14. Parallel Conductors • Twisted pair • formed by twisting two insulated conductors around each other • Neighboring pairs is twisted each other to reduce EMI and RFI from external sources • reduce crosstalk between cable pairs

15. Parallel Conductors • Unsheilded Twisted Pair • two copper wire encapsulated in PVC • twisted to reduce crosstalk and interference • improve the bandwidth significantly • Used for telephone systems and local area network

16. Parallel Conductors • UTP – Cable Type • Category 1 • ordinary thin cables • for voice grade telephone and low speed data • Category 2 • Better than cat. 1 • For token ring LAN at tx. rate of 4 Mbps • Category 3 • more stringent requirement than level 1 and 2 • more immunity than crosstalk • for token ring (16Mbps), 10Base T Ethernet (10Mbps)

17. Parallel Conductors • UTP – Cable Type • Category 4 • upgrade version of cat. 3 • tighter constraints for attenuation and crosstalk • up to 100 Mbps • Category 5 • better attenuation and crosstalk characteristics • used in modern LAN. Data up to 100Mbps • Category 5e • enhanced category 5 • data speed up to 350 Mbps

18. parallel conductors • UTP – Cable Type • Category 6 • data speed up to 550 Mbps • fabricated with closer tolerances and use more advance connectors

19. Parallel Conductors • Sheilded Twisted Pair (STP) • wires and dielectric are enclosed in a conductive metal sleeve called foil or mesh called braid • the sleeve connected to ground acts as shield – prevent the signal radiating beyond the boundaries

20. Parallel Conductors • STP – Category • Category 7 • 4 pairs • surrounded by common metallic foil shield and shielded foil twisted pair • 1Gbps • Foil twisted pair • > 1Gbps • shielded-foil twisted pair • > 1Gbps

21. Coaxial Cable • used for high data transmission • coaxial – reduce losses and isolate transmission path • basics • center conductor surrounded by insulation • shielded by foil or braid

22. Transmission Line Wave Propagation • Velocity factor • The ratio of the actual velocity of propagation of EM wave through a given medium to the velocity of propagation through vacuum • Vf = velocity factor • Vp = actual velocity of propagation • c = velocity of propagation in vacuum

23. Transmission Line Wave Propagation • rearranged equation • the velocity of wave in transmission line depends on the dielectric constant of insulating material • ϵr = dielectric constant • The velocity along transmission line varies with inductance and capacitance of the cable

24. Transmission Line Wave Propagation • as • velocity x time = distance • therefore • normalized distance to 1 meter • Vp = velocity of propagation • √LC = seconds • L = inductance • C = capacitance

25. Losses • Conductor Losses • conductor heating loss - I2R power loss • the loss varies depends on the length of the transmission line. • Dielectric Heating Losses • difference of potential between two conductors of a metallic transmission lines • Negligible for air dielectric • increase with frequency for solid core transmission line

26. Losses • Radiation Losses • the energy of electrostatic and EM field radiated from the wire and transfer to the nearby conductive material • Reduced by shielding the cable

27. Losses • Coupling Losses • whenever connection is made between two tx line • discontinuities due to mechanical connection where dissimilar material meets • tend to heat up, radiate energy and dissipate power • Corona • luminous discharge that occurs between two conductors of transmission line • when the difference of potential between lines exceeds the breakdown voltage of dielectric insulator

28. Incident and Reflected Wave • Incident voltage • voltage that propagates from sources toward the load • Reflected wave • Voltage that propagates from the load toward the sources

29. Incident and Reflected Wave • Resonant and non resonant transmission line • Flat @ non-resonant line • Transmission line with no reflected power • Infinite length transmission line • terminated with a resistive load equal in ohmic value to the characteristic impedance of transmission line • Resonant transmission line • When the load is not equal to the characteristic impedance of the transmission line, some incident power reflects back towards the source • energy present on the line would reflect back and forth (oscillate) between the source and load

30. Incident and Reflected Wave • reflection coefficient • vector quantity that represents the ratio of reflected voltage to incident voltage (or current) • Γ = reflected coefficient • Ei/Ii= incident voltage/current • Er/Ir= reflected voltage/current

31. Data Acquisition Systems Fundamental Characteristics • Input properties • Gain and filtering capabilities • Sampling rate • Number of channels • Resolution of the digital converter

32. Data Acquisition Systems Input options Inputs Single-ended inputs Typically one channel is used for a single input line. The reference is a common ground to all the channels of the acquisition system Differential Inputs Two separate line are used, generally 2 channels. The Differential voltage is measured

33. Data Acquisition Systems Input buffering Buffer amplifier The buffer serves to present a specific impedance, generally In the range of 1 M. The buffer also serves to AC couple or DC couple the input. The buffer isolates the input from the data acquisition portion of the instruments

34. Data Acquisition Systems Gain and filtering Gain and filtering Gain is adjusted to optimize the range of the analog signal to the capability of the A/D converter. The filtering serves to remove frequencies above the Nyquist frequency This is the first stage of anti-aliasing

35. Data Acquisition Systems Sample and Hold Sample and Hold Sample and hold circuits sample the input voltage and holds for digital conversion. This technique is use when simultaneous data is required for multiple channels Sequential data acquisition generally bypasses the S/H circuitry (cost) to acquire data sequentially from channel to channel

36. Data Acquisition Systems A/D converter A/D converter The converter specifications include the resolution, bits, and the data rate. The rate of acquisition set the frequency range

37. Data Acquisition Systems Assume that we collect a block of data N samples in length This implies that T= N*t Also, the fundamental frequency, f0= The sample rate can be defined as S = Since two samples are require to define a harmonic function Shannon Sampling Theorem The maximum number of frequencies that can be calculated N/2 Fmax=f0 (N/2)= Nyquist frequency Also Fmax=S/2

38. Data Acquisition Systems Multiplexer Multiplexer The multiplexer serve to stream the digital data to the storage medium. The multiplexing operation is commonly the limiting component of speed in a data acquisition system

39. Data Acquisition Systems Multirate systems • Data acquired at the highest sample rate. • Filtering(analog) performed at this point for the fixed sample rate • Data is decimated and digitally filtered for all other (lower) sample rates

40. Common Implementations of Interfaces • Parallel port (8 bits per shot) • Serial (RS-232, RS-485) • usually asynchronous • GPIB (IEEE-488) parallel • General Purpose Interface (or Instrument) Bus • originally HPIB; Hewlett Packard • DAQ card (data acquisition) • like national instruments A/D, D/A, digital I/O • CAMAC • Computer Automated Measurement And Control • VME bus / VXI bus • modern CAMAC-like bus

41. A quick note on hexadecimal

42. Exchanging Data bit 0 Device A Device B • Parallel: Fast and expensive • devices A, B simple, but cabling harder • strobe alerts to “data valid” state bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 bit 7 strobe • Serial: Slow and cheap • but devices A and must convert between serial/parallel Device A Device B data slide courtesy E. Michelsen

43. The Parallel Port • Primarily a printer port on the PC • goes by name LPTx: line printer • usually LPT1 • 8 data bits • with strobe to signal valid data • can be fast (1 Mbit/sec) • Other control and status bits for (printer) communication data valid data held static for some interval see http://www.beyondlogic.org/index.html#PARALLEL

44. Parallel Port Pinout

45. Parallel Port Access serial port • Most PCs have a DB-25 female connector for the parallel port • Usually at memory address 0x378 • Windows 98 and before were easy to talk to • but after this, a hardware-abstraction layer (HAL) which makes access more difficult • one option is to fool computer into thinking you’re talking to a normal LPT (printer) device • involves tying pins 11 and 12 to ground • Straightforward on Linux • direct access to all pins parallel port

46. Serial Communications • Most PCs have a DB9 male plug for RS-232 serial asynchronous communications • we’ll get to these definitions later • often COM1 on a PC • In most cases, it is sufficient to use a 2- or 3-wire connection • ground (pin 5) and either or both receive and transmit (pins 2 and 3) • Other controls available, but seldom used • Data transmitted one bit at a time, with protocols establishing how one represents data • Slow-ish (most common is 9600 bits/sec)

47. Time Is of the Essence • With separate clock and data, the transmitter gives the receiver timing on one signal, and data on another • Requires two signals (clock and data): can be expensive • Data values are arbitrary (no restrictions) • Used by local interfaces: V.35, (synchronous) EIA-232, HSSI, etc. • As distance and/or speed increase, clock/data skew destroys timing sample on rising edge of clock clock sample times centered in data bits data time slide courtesy E. Michelsen

48. No Clock: Do You Know Where Your Data Is? • Most long-distance, high speed, or cheap signaling is self timed: it has no separate clock; the receiver recovers timing from the signal itself • Receiver knows the nominal data rate, but requires transitions in the signal to locate the bits, and interpolate to the sample points • Two General Methods: • Asynchronous: data sent in short blocks called frames • Synchronous: continuous stream of bits • Receiver tracks the timing continuously, to stay in synch • Tracking requires sufficient transition density throughout the data stream • Used in all DSLs, DS1 (T1), DS3, SONET, all Ethernets, etc. transitions locate data data time interpolated sample times (bit centers) slide courtesy E. Michelsen

49. Asynchronous: Up Close and Personal • Asynchronous • technical term meaning “whenever I feel like it” • Start bit is always 0. Stop bit is always 1. • The line “idles” between bytes in the “1” state. • This guarantees a 1 to 0 transition at the start of every byte • After the leading edge of the start bit, if you know the data rate, you can find all the bits in the byte transition locates data one byte idle idle 1 0 start bit 0 bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 bit 7 stop time interpolated sample times (bit centers) slide courtesy E. Michelsen

50. Can We Talk? ASCII “A” = 0x41 9600, 8N1 idle idle start bit 0 bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 bit 7 stop • If we agree on 4 asynchronous communication parameters: • Data rate: Speed at which bits are sent, in bits per seconds (bps) • Number of data bits: data bits in each byte; usually 8 • old stuff often used 7 • Parity: An error detecting method: None, Even, Odd, Mark, Space • Stop bits: number of stop bits on each byte; usually 1. • Rarely 2 or (more rarely) 1.5: just a minimum wait time: can be indefinite 1 bit @ 9600 bps = 1/9600th sec 9600, 7E2 idle idle start bit 0 bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 parity stop 1 stop 2 slide courtesy E. Michelsen