Noise Reduction Techniques. INC 336 Industrial Process Measurement Assist. Prof. Pakorn Kaewtrakulpong,, Ph.D. INC, KMUTT

Transcription

1 Noise Reduction Techniques INC 336 Industrial Process Measurement Assist. Prof. Pakorn Kaewtrakulpong,, Ph.D. INC, KMUTT

2 Intrinsic Noise Sources Thermal Noise or Johnson Noise Shot Noise Contact Noise Popcorn Noise

3 Thermal Noise J.B. Johnson discovered in From thermal agitation of electrons within a resistance. Nyquist formed rms voltage for thermal noise as Vt = 4kTBR k: Boltsmann constant, 1.38 x J/K T: : temperature, K B: : equivalent noise bandwidth, Hz R: : resistance

4 Equivalent Noise Bandwidth For any network transfer function A(f), there is an equivalent noise bandwidth of constant magnitude of transmission A 0 and bandwidth of 1 = 2 A0 0 B A( f) df 2 Normally greater than filtering bandwidth

5 Shot Noise Vacuum tubes and semiconductors Associated with current loss across a barrier. W. Schottky (1918) shows rms current as I sh = 2qI B dc q: : electron charge, 1.6 x C I dc : average current, A B: : equivalent noise bandwidth, Hz

6 Contact Noise Imperfection of contact (usually in switch and relay) changes in conductance 1/f noise I f = KI dc B f K: : constant dependent on contact material I dc : average current, A f: : frequency, Hz B: : equivalent noise bandwidth, Hz

7 Popcorn Noise Burst Noise Due to manufacturer imperfection of semiconductor devices especially at the junction due to metallic impurity

8 Active Device Noise Noise Factor F = Noise Power at Output of Actual Device Noise Power at Output of Ideal Device F F 1 = 1; if actual device is an ideal device F = ( S/ N) ( S/ N) i/ p o/ p S/ N = Power of Signal Power of Noise Noise Figure, NF = 10log F

9 Measurement of Noise Factor Two methods Single frequency method Noise diode or white noise method For both methods, the noise power at output of an ideal device is v ideal, no = Av t

10 Single Frequency Method R s v s Device or network with voltage gain A R L With generator turned off, measure v no = ( Av ) + (device noise) t 2 2

11 Single Frequency Method Next, generator is turned on and its magnitude is increased until output power doubles (increases by 3dB over that previously measured), then 2 v = ( Av ) + v Av no s no s = v no F F = v 2 no ( Av ) v t s = vt 2 2

12 Single Frequency Method (cont.) Adv. Any value of R L may be used. Disadv. At 290K, v = F v = t 2 s 20 BR s Noise bandwidth of the device must be known. 20 BR s

13 Noise Diode or White Noise Method I dc Noise diode Blocking capacitor Rs Device or network with voltage gain A RL With no diode current, the rms noise voltage is measured. v no = ( Av ) + (device noise) t 2 2

14 Noise Diode or White Noise Method (cont.) Next, the diode current is increased until output noise power doubles (increases by 3dB). The shot noise generator is i v sh = = i R sh sh s 19 I B dc 2 v = ( Av ) + v no sh no v = Av = Ai R no sh sh s

15 Noise Diode or White Noise Method (cont.) F F = ( i R ) sh s 2 vt 2 = 20I R dc s The noise factor is a function of R s but in this method the other influencing factor is the direct current through the diode. Adv The method is frequency independent. Both R s and I dc are easily measured.

16 Noise Factor in Cascade Circuit Rs Gain, G 1 Noise Factor, F 1 Gain, G 2 Noise Factor, F 2 Gain, G 3 Noise Factor, F 3 Gain, G N Noise Factor, F N RL F F 1 F 1 F2 1 3 N = F L+ G1 GG 1 2 GG 1 2LGN 1

17 Basic Noise Reduction Techniques Definition of Interference Grounding Shielding

18 Interference Sheingold (1980) classifies interference problem into three areas: 1. Problems generated locally by the materials used in the signal path (e.g., unwanted thermocouples, ohmic contact of switches and terminals) 2. Problems within a subsystem (e.g., grounds) 3. Problems originating in the outside world [e.g., electric, magnetic, and RF (radio-frequency) interference] Webster (1977) classifies interference into three types of coupling 1. capacitive (electric fields) 2. inductive (magnetic fields) 3. resistive (ohmic( voltages in ground conductors).

19 Grounding Ground = a reference connection (reference potential) Earth = a connection to earth Point of interest: interference created by resistive coupling in ground conductors

20 Grounding: Analog Circuits Interference voltages may develop on ground lines. R x = nonzero resistivity of the wire (in this case, 3 mωm per 15 cm of No. 18 copper wire) Parallel Distribution of Power

21 Grounding: Analog Circuits (cont.) Radial or star distribution minimizes voltage drops in both hot and ground wires. Circuit 3 has no difference between the parallel and radial distributions. Radial or Star Distribution of Power

22 Grounding: Analog Circuits Better solution: connect circuit 3 (higher current) closer to the e power supply, if possible. Circuit 3 should be connected to an extra power supply, if available, able, to avoid the resistance of a long wire while sharing the same power supply. If the voltage drop on the power supply path does not affect the operation of the circuits, a combination of parallel and radial distribution could be used (star connection for the ground wire).

23 Grounding: Analog-Digital Circuits Digital signals large current spikes along ground paths interference in analog circuits When analog and digital circuits sharing one power supply, the ground wire each must be different, with only one common point. to minimize common impedances between digital and analog circuits. One power supply, one common point

24 Grounding: Analog-Digital Circuits (cont.) When using separate analog and digital power supplies, each circuit is connected to its ground and both grounds are tied to a single point.

25 Magnetic Field Large current magnetic field Magnetic field cuts a conductor current is induced in that conductor AC currents DC currents via switches, relays, electronics, and brushes Sources Rf signals e.g. digital signal of processor, CATV, broadband, or baseband data communications cables (high frequency noise) and the high current signal of the power output stages produces significant magnetic fields within the electronic chassis

26 Protection against Magnetic Field Separate sensitive input signal conditioning from the other portions of the electronics. Put small signal analog circuitry on a separate card, covered by a magnetic shield, from the computer and power electronics. If not possible, group sensitive analog processing components together and as far away from sources of magnetic fields as possible. (may be covered by a small magnetic shield box)

27 Protection against Magnetic Field (cont.) Never run ac power line in the same raceway or conduit Shielding Using ferromagnetic conduit for power lines or for low-level level analog signal Spray some coatings to shield against high frequency magnetic field.

28 Shielding Magnetic absorptive loss depends on Material Thickness Frequency of the magnetic field

29 Electric Field Difference in potentials electric field (free charges, primarily electrons in conductors respond to this field)

30 Shielding The shield must be tied to an infinite source/sink of charge. Not only must the signal high be shielded, but the signal common must be as well. Do not use the shield as the signal common. Otherwise, external electric fields can float the ground up and down.

31 Ground Looping

32 Single Grounding

33 Grounded Devices

34 Isolation

35 Power Supply Grounding

36 Power Supply Grounding (cont.)