Transcript
1. MEASUREMENT USING ANALOGUE OSCILLOSCOPE
1.1. Tasks of the measurement 1.1.1. After studying the block diagram of the oscilloscope (Fig. 1.1) and being acquainted with function of the analogue laboratory dual-channel oscilloscope display the voltage waveforms in the measurement points of the astable oscillator. For each waveform, evaluate all the relevant waveform parameters: frequency, duty factor, U mean , U max , U min , rise time and fall time. 1.1.2. Check the X-Y mode by displaying Lissajouss patterns. Use LF generator and 6 V/50 Hz power line signal sources.
1.2. Schematic diagram AC
Channel 1
DC
CH1 (X)
GND
S1 ID
IAA S3
x5
AC
Channel 2
DC
CH2 (Y)
GND
VARIABLE
OVA
DL
S2 ID
IAB
*) x5
VARIABLE ALTERNATE MODE CHOPPED MODE
FO CH1 CH2 VERT MODE LINE
EXT TRIG
S4
x10
TC
TB
x1
HA LEVEL
VARIABLE
X-Y
Fig. 1.1 Block schematic diagram of dual channel oscilloscope (ID - input dividers, S1, S2 - input switches, IAA, IAB - input amplifiers, S3 - mode switch (in single channel mode - manual switching, in dual-channel mode - automatic switching; note: *) some oscilloscope types control the switching mode automatically in accordance with the chosen TB), S4 synchronization switch (SOURCE), OVA - output vertical amplifier, DL - delay line, EXT TRIG external signal trigger, FO - fixed frequency oscillator (for switching by fixed frequency), TB - time base, TC - trigger circuit of TB, HA - horizontal amplifier) Page 1 of 3
+5 V 5k1
M1
51k
5k1 K2
K1
6n8
6n8
T1
T2
BC237
B1
B2
BC237 0
Fig. 1.2 Schematic diagram of astable oscillator (Object) with marked measurement points
1.3. List of the equipment used OSC - oscilloscope, type ... Object - astable oscillator G - RC generator, type ...
1.4. Teoretical background A detailed description of the analogue oscilloscope functions can be found in [1], pp. 90-103. a) Hints for oscilloscope operation
Connect the measured signals to input BNC-type connectors (Channel A, Channel B - CH1, CH2 or Y1, Y2). Connect the outer wires to the oscilloscope chasis and consequently to the ground. This fact must be considered in case of two-channel measurements and also in cases when more grounded devices are used in the measured circuit. Zero level on the oscilloscope screen can be evaluated by setting the input switch to the position “GND” (0). In this case the input signal is disconnected and the pre-amplifier input is grounded. Time base (TB) must be set to AUTO (automatic trigger) in this case. Trace position can be set by the potentiometer POSITION of the respective channel. Synchronization of the time base with the measured waveform can be set by the potentiometer LEVEL. When the signal level achieves the trigger level, the trigger pulse is generated to the time base circuit. One period of ramp of the time base driving signal is generated - in the “direct run” part the electron beam moves over the screen (in the left-right direction) and in the “backward run” part the dimmed beam goes back to the left. If this process repeats periodically, all the traces are overlaid and the appearance is a static picture on the screen. If the triggering level is set outside the signal level range no synchronization is established; in the AUTO mode the picture will be unstable, in the NORM mode the screen remains dark. The synchronization source must be chosen by the switch TRIGGER SOURCE (CH1, CH2, LINE - power line, EXT TRIG - external signal, eventually VERT). Page 2 of 3
Note: In the synchronization mode „VERT“ (or „VERT MODE“) the time base (in case of alternating mode of switching between channels) is synchronized by the actually displayed channel. It allows the parallel display of two signals of different frequency (it is not common for two-channel oscilloscopes). It is not possible to use this mode for phase shift measurements between signals on Channels 1 and 2.
It is possible to set either DC (direct) or AC (via a capacitor) coupling of the input signal. The scale of the displayed signal is set by the input divider (ID) VOLTS/DIV (V/division) for each channel individually. The label value is valid only in case when the fine regulation is set to CAL position (right end position of the potentiometer). The switch of the time base speed TIME/DIV (time/division) allows the horizontal scale setting. The time scale is disabled by switching to X-Y mode. b) Rise time t r (duration of signal rising edge) is duration of the time interval between 10 % and 90 % of the amplitude (Fig. 1.3). Similarly, fall time t f (duration of signal falling edge) is duration of the time interval between 90 % and 10 % of the signal amplitude.
u(t) Umax 0,9U max
0,1U max tr
t
Fig. 1.3 Rise time
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