Question

10-

a) What must always be used with an Oscilloscope when making a measurement?

b) How do you read the amplitude (Frequency) of a signal from the waveform on an oscilloscope?

c) How do you calculate phase shift by measuring the time delay between zero crossings of two different signals?

Answer 1

a)

One of the most powerful features of digital oscilloscopes is its ability to perform calculations on measurements. By using this capability, we can use an oscilloscope to measure parameters such as power.power = voltage x currentIt really is this simple when the load is resistive or static. In fact, in these cases, you don’t even need an oscilloscope. All you need is a digital multimeter to make the voltage and current measurements.Unfortunately, it’s not always that easy. If a load is reactive, for example, there will be a phase difference between the voltage and current measurements, and that must be taken into consideration when making measurements.To make this measurement, you can use an oscilloscope. You use one channel to measure the voltage, a second channel to measure the current, and then program the scope to multiply the two measurements and create a third traceTo make this measurement, there are a few things to consider. First, the rise and fall times of the voltage waveform may be quite fast, even though the switching frequency may not be all that high. Because of this, the scope needs to have sufficient bandwidth to accurately measure that rise time.Secondly, you’ll want to use an isolated scope probe or isolated scope inputs to make these measurements. Because you’re connecting one channel across the switching transistor and the second perhaps across a current sensing resistor, using probes with a common ground won’t work.You could make the current measurement by including a very low value current-sensing resistor in the circuit. Another approach is to use a current clamp probe. These clamps eliminate the need to include a sensing resistor in the circuit or to open the circuit when making measurements. Using a current clamp also provides good isolation between any ‘live’ wiring and the test instrument.

b) Frequency Measurement:Frequency can be measured on an oscilloscope by .investigating the frequency spectrum of a signal on the screen and making a small calculation. Frequency is defined as the several times a cycle of an observed wave takes up in a second. The maximum frequency of a scope can measure may vary but it always in the 100’s of MHz range. To check the performance of response of signals in a circuit, scope measures the rise and fall time of the wave.

Method to Measure Frequency

1. Increase the vertical sensitivity to get the clear picture of the wave on the screen without chopping any of its amplitude off.
2. Now adjust the sweep rate in such a way that screen displays a more than one but less than two complete cycles of the wave.
3. Now count the number of divisions of one complete cycle on the graticule from start to end.
4. Now take horizontal sweep rate and multiply it with the number of units that you counted for a cycle. It will give you the period of the wave. The period is the number of seconds each repeating waveform takes. With the help of period, you can simply calculate the frequency in cycles per second (Hertz).

5. ans c) Electronic circuits will inevitably delay signals and, although it’s not always a bad thing, shift the phase of the signal. The amount of phase shift will be different for various circuits as a result of their unique designs. One thing you can do is choose a circuit design with minimal phase shift and then visually measure the phase shift with an oscilloscope. You can calculate the phase shift by measuring the circuit’s input signal with your oscilloscope’s first channel and the circuit’s output with your scope’s second channel.First off connect a BNC T-connector to the sine wave oscillator’s output, which will essentially give your oscillator two outputs. Connect a BNC cable from one end of the T-connector to the channel one input on your scope. Then connect another BNC cable from the other end of the T-connector to the input of the circuit you’re testing and attach an oscilloscope probe to the channel two input, connecting the probe to the circuit’s output.Turn on the oscillator, circuit, and oscilloscope and adjust the oscillator until its frequency and amplitude come into the circuit’s operating range. Be careful not to overdrive the circuit with the oscillator.Turn your oscilloscope’s horizontal sweep rate control until you eventually observe two or three sine wave cycles on your scope’s display, and then adjust the vertical position and sensitivity until you can see sine waves on both channels (channel one on top and channel two on the bottom). Set your oscilloscope’s sweep trigger to channel one to steady the sine wave if necessary.

   Find a peak on the channel one sine wave. Follow that point down to the horizontal time-division marks in the oscilloscope’s display’s center. Then find the corresponding peak on channel two and its time-division mark. Count the major divisions between the time-division marks for channel one and two (including any fractions of a division) — this is the phase shift of your two signals.Check the top sine wave again and count the major divisions for an entire cycle (including fractions again). You’ll then want to divide the difference between the signals for channels one and two by the divisions for an entire cycle, and multiply two time pi to find the phase shift in radians. Multiply by three hundred sixty instead of two times pi to get the phase shift in degree. Some digital storage oscilloscopes are able to measure and calculate the time difference between to points on the display automatically. In this case you would select the time measurement function and situate two vertical cursors at the starting and ending points for the interval you want to calculate. The oscilloscope will then display the time difference.