Transcript
13. OSCILLOSCOPES AND OTHER MEASURING INSTRUMENTS analog oscilloscope (basic parameters, block schematic diagram, triggered time base – principle of synchronization, passive oscilloscope probe, dual-channel oscilloscope – block schematic diagram, two modes of operation and their use) digital storage oscilloscope – (basic parameters, principle, block schematic diagram, modes of sampling, pre-trigger and delay modes) spectrum analyzers waveform generators used in measurements sine-wave generators function generators arbitrary waveform generators optoelecrical sensors – incremental sensors, digital encoders, CCD and PSD sensors
A1B38EMA – P13
1
Y
AC
u1 DC
Analog oscilloscope
u2 VA
PA GND EXT.
u1
u2
Trigger level.
INT.
EXT. TRIG. AUTO
GTP X
u3
u5 u3
TB
u 4 TB HA X
Triggered timebase – setting trigger point: - level - slope (rising, falling) - trigger source: internalí, external, power line; - coupling of trigger signal (DC, AC)
A1B38EMA – P13
u4 x 10
u5
Hold-off regime
2
Dual-channel analog oscilloscope
Y1
PA1 DC
Y2
Alternate mode (Alt) u 2,1
AC
u1
u 2,1
GND AC
u1
u 2,2 PA2
DC
Alt
u3 Chop
GND EXT.
EXT. TRIG.
u 2,2
VA AM
u4
u6 CH1
Chopped mode (Chop)
CH2
u 2,1 u5 u 2,2
GSP
u3
TB
u 4 ČZ HA X
AUTO
X A1B38EMA – P13
u6
Čas. lupa x 10 3
Passive probe 1:10 a) equivalent circuit, b) equivalent circuit as frequency-compensated volarge divider5 PROBE TIP PROBE
OSCILOSCOPE
R1
C1
Ci
Ri
CK
R1
C1
Ri
Ci + CK
C C – cable
capacity
b)
a)
Probe calibration (setting capacity C 1 ) using periodic rectangular pulse train u
u t a) R 1 C 1 < R i (C K +C i )
A1B38EMA – L13
u t
b) R 1 C 1 = R i (C K +C i ) (correct compensattion)
t c) R 1 C 1 > R i (C K +C i )
Digital storage oscilloscope
INPUT (2)
CHANNEL 2 (IA, S/H, ADC, MEM) Micro comp.
INPUT (1)
Input ampl.
S/H circuit
ADC
RAM
FIFO Standard interface
Ext. TRIG
Gen. of “trigger“ pulse
TIMER
CLOCK
IEEE 488 RS-232 USB
Videoproc. Storing of samples into memory: Memories in channel are FIFO types (first in first out) – in regime ON filled with signal samples; After generation of „trigger“ pulse – memory filling stopped a) immediately (negative delay – pre-trigger mode) b) after delay corresponding to filling FIFO memory („normal mode”) c) after delay surpassing the filling FIFO memory (delay mod AE1B38EMA – L13
5
DIGITAL STORAGE OSCILLOSCOPE - PRE-TRIGGER MODE, „NORMAL MODE“, DELAY MODE TP – trigger point, by DSOs it is in fact “stopping point” p – pretrigger, c – memory capacity of any channel, d - delay MODE:
PRETRIGGER
p TP c
trigger level
c-p
trigger level
TP
„NORMAL“ (p = 0)
c TP
DELAY
trigger level d
c d+c
Sampling modes used in digital oscilloscopes A1B38EMA – L13
6
1. REAL-TIME SAMPLING 4 to 10 samples per period of the highest-frequency component allows pre-trigger mode allows capturing transients 2. SEQUENTIAL SAMPLING IN EQUIVALENT TIME (STROBOSCOPIC SAMPLING) for periodic signals only in each period one sample only shifted by t equivalent sampling frequency f S.EQ. =1/(t)
T T+t
t T+t
3. RANDOM SAMPLING IN EQUIVALENT TIME for periodic signals only sampling permanently with maximum sampling frequency (several samples during period of highest-frequency component) each set of samples delayed by random but known time faster reconstruction than in point 2)
A1B38EMA – L13
7
Spectrum analyzer Nonharmonic periodic signal sum of harmonic components (Fourier series) Harmonic components – sequence of complex numbers frequency spectrum of periodic signal Amplitude (magnitude) frequency spectrum: absolute values of harmonic components Phase frequency spectrum: phases of harmonic components
MEASUREMENT OF MAGNITUDE SPECTRUM: SELECTIVE VOLTMETER (HF selective voltmeter - heterodyne principle) HETERODYNE SPECTRUM ANALYZOR – analog signal processing – frequency band: tens of kHz up to units of GHz
MEASUREMENT OF BOTH SPECTRAL COMPONENTS FFT SPEKTRUM ANALYZOR - DFT (Diskrete Fourier Transform) of samples of digitized signal – frequency band from very low frequencies to hundreds of kHz
A1B38EMA – L13
8
HETERODYNE SPECTRUM ANALYZOR x(t)
IA
fX
M
IFF
fO SG
D
A
f M = konst.
VCO
input amplifier, mixer, intermediat frequency filter (bandpass filter set to a fixed frequency f I , the so-called intermediate frequency) D detector (rectifier), A amplifier, SG sawtooth waveform generator, VCO voltage-controlled amplifier IA M IFF
Low frequency (LF) generators of signals used in measurement A1B38EMA – L13
9
LOW-FREQUENCY ANALOG SINEWAVE GENERATORS (RC - GENERATORS) RC OSCILLATOR
AMPLIFIER (setting gain)
OUTPUT ATTENUATOR LEVEL MEASUREMENT
Output voltage defined:
Disadvantages: Advantages:
A1B38EMA – L13
50 (600 )
a) for high impedance (open circuit) b) for the defined loading impedance (usually 50 ) (If the loading impedance is high in this case, the output voltage is twice the set value. This is valid generally – also in other types of generators)
low stability of both frequency and amplitude Low distortion, low DC component
10
ANALOG FUNCTION GENERATORS C1
R
R
AMPLIFIER OZ2
OZ1 R1
+
u 2 (t)
+
u 3 (t)
OUTPUT DIVIDER
u 1 (t)
SHAPER
OUTPUT
t GENERATING SINUSOIDAL VOLTAGE FROM u 1 A SAWTOOTH WAVE USING SHAPER: t u3 u2
u2 t
A1B38EMA – L13
u3 11
GENERATORS OF ARBITRARY WAVEFORM (ARBITRARY GENERATOR) DIGITAL INPUT (N x k bits)
memory Nxk
DAC1 (k bits)
FILTER OUTPUT
UA COUNTER to N
DAC2
f S = N/T
AMPLITUDE (DIG. INPUT)
UN
T – PERIOD OF GENERATED SIGNAL SIGNAL SHAPE IN ONE PERIOD (N of k-bit sample values) STORED IN GENERATOR MEMORY, SUCCESSIVE CYCLIC SELECTION OF INDIVIDUAL SAMPLES VITH FREQUENCY f S OUTPUT AMPLITUDE OF DAC1 SET BY DAC2
FREQUENCY OF GENERATED SIGNAL:
A1B38EMA – L13
f SIG = f S /N
12
Optoelectrial sensors INCREMENTAL SENSORS LIGHTING
DIRECTION
u1
LS1
MOVABLE PART
LS2
u1
t u2´
FIXED SHADE
t Q
u2
t u1
+
DIRECTION
u1
D
u1
Q
t
U C
Q
u2´
FOR.
u2
+
u2´
REV. CNT
BACK
t Q t
Advantage: - simplicity; - principally infinite range. Disadvantages:- lost of information by power supply dropout, possible error by disturbances A1B38EMA – L13
13
DIGITAL ENCODERS Binary code: possibility of false reading údaje 63
Code with change in one bit
0
Advantage: - no lost of information by power supply drop out. Disadvantage: - more com[plicated optical system (bit number = light sensor number); - limited resolution (usually 10 – 12 bits).
A1B38EMA – L13
14
CCD SENSORS (Charge-coupled device) Principle: FOTOELEMENTs
uF
TRANSFER GATES
u
CD
u1
uO
TRANSPORT REGISTER
u2
Linear (1D) sensor – length 4 to 60 mm, až 6000 fotoelements, reading rate tens of MHz Usage: LIGHT SOURCE
MEASURED OBJECT
CCD SENSOR
OPTICAL SYSTEM
2D sensors – matrix structure – up to 3000 x 2000 fotoelements (reading by rows A1B38EMA – L13
15
PSD SENSORS (Position sensing device) Principle (linear sensor): L IA
LIGHT BEAM
A +
-
IO Použití: DIFUSIVE SURFACE LED
IB
P-LAYER INTRINSIC Si LAYER
L-x
x
B
I A RL x IB Rx
R L-x or R x is resistance of P-layer
between light beam position and elektrode A or B
N-LAYER
IA L x IB x
COMMON ELECTRODE (CATHODE) MEASURED OBJECT POSITION
Poznámka: 2D (area) PSD sensors are often used (based on the same principle)
PSD SENSOR AE1B38EMA – L13
16
