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A practical experiment  with the Oscilloscope

It is always considered so much easier to understand something if you do a practical experiment in a related item. This experiment was demonstrated at the recent 2004 BRATS Radio Rally.

Have a look at the diagram below and ensure you understand what is mentioned.

Looking at a tuned circuit.

Your task is to loosely couple a signal generator to a Parallel Tuned circuit ( series resistor or a few turns of wire) and connect the oscilloscope p[robe across the tuned circuit. The circuit is given in Diagram 1.

Now set the VOLTS/DIV to 0.5V per Centimetre and the TIME/DIV control to 0.1 Millisecond per Centimetre. Set the signal Generator (Sig Gen) output to MAXIMUM.

Now turn on or switch, the frequency controls on the Sig Gen until the display on the scope shows a vertical (VOLTAGE) deflection.

RESONANT FREQUENCY - Adjust the Sig Gen frequency until deflection on the scope is at maximum, reduce the Sig Gen volts if the display goes off the scale, then adjust TIME/DIV controls until a sinewave is displayed. Use the trigger control to stabilize and show about 5 cycles displayed, measure TIME for these 5 cycles to occur, and divide by 5 to obtain the waveform period. The FREQUENCY (Resonant) is  1/Period, Approximately !! Check the sig gen for the frequency.

BANDWIDTH   - Adjust the Sig Gen Output to obtain 8CM vertical deflection, swing Sig Gen frequency each side of the resonant frequency until the deflection falls  to 5.6 CM ( which is approx 0.707 of the max) read off total change in frequency from lower to highest this equals the BANDWIDTH, let's call it FB

APPROX Q -- Note the resonant frequency  = FR and calculate FR / FB  = approx Q of the tuned circuit.

DYNAMIC RESISTANCE - if a simple RF voltmeter is available, measure Sig Gen output volts VSIG and the voltage across the 100k resistor in diagram 1, VR

By ( and Dynamic resistance is a pure resistance) VR / r = I ( series) and V across tuned circuit = VSIG -  VR

Then the dynamic resistance is E/I  = (VSIG -  VR) / I

The less voltage across R the higher the dynamic resistance will be.

SHOCK EXCITATION and DAMPING - If you now reduce the Sig Gen frequency considerably and change the output to SQUARE WAVE we can excite the tuned circuit into oscillation, due to the sharp positive and negative edges of the square wave, careful  adjustment of the scope controls will produce a wave form as shown in Diagram 3. The oscillation dies away due to losses mainly in the inductor, in other words the oscillation is "DAMPED" by ( mainly) resistive losses, if we ad a series of parallel resistance to a tuned circuit we increase the amount lost each cycle and Q is reduced, Bandwidth is increased and dynamic resistance falls.

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