Measurements using Digital Storage Oscilloscope, different modes of DSO, capturing transients and analysis of waveforms.

AIM:  Measurements using Digital Storage Oscilloscope, different modes of DSO,

            capturing transients and analysis of waveforms.

I) To study different modes of DSO such as Roll, Average, Peak Detection.

II)  FFT analysis using DSO.

III) Capture transients

IV) Various math operations.

EQUIPMENT:

1) DSO (GWINSTEK GDS-1102, 100MHz 250M Sa/s oscilloscope)

2)Function generator (Aplab 1MHz, Model no:FG1MD)

THEORY:

The main purpose of an oscilloscope is to give an accurate visual representation of electrical signals. An oscilloscope is a measurement and testing instrument used to display a certain variable as a function of another.

                                                              

Ø  Types of oscilloscopes

Analog oscilloscopes The first oscilloscopes were analog oscilloscopes, which use cathode-ray tubes to display a waveform. The downside of an analog oscilloscope is that it cannot “freeze” the display and keep the waveform for an extended period of time. Once the phosphorus substance deluminates, that part of the signal is lost. Also, you cannot perform measurements on the waveform automatically. Instead you have to make measurements by hand using the grid on the display. Analog oscilloscopes are also very limited in the types of signals they can display because there is an upper limit to how fast the horizontal and vertical sweeping of the electron beam can occur. While analog oscilloscopes are still used by many people today, they are not sold very often. Instead, digital oscilloscopes are the modern tool of choice.  

                                                                                                   

Digital storage oscilloscopes (DSOs)

Digital storage oscilloscopes (often referred to as DSOs) were invented to remedy many of the negative aspects of analog oscilloscopes. DSOs input a signal and then digitize it through the use of an analog-to-digital converter. Figure 1 shows an example of one DSO architecture used by Agilent digital oscilloscopes. The attenuator scales the waveform. The vertical amplifier provides additional scaling while passing the waveform to the analog-to-digital converter (ADC). The ADC samples and digitizes the incoming signal. It then stores this data in memory. The trigger looks for trigger events while the time base adjusts the time display for the oscilloscope. The microprocessor system performs any additional post processing you have specified before the signal is finally displayed on the oscilloscope. Having the data in digital form enables the oscilloscope to perform a variety of measurements on the waveform.

Signals can also be stored indefinitely in memory. The data can be printed or transferred to a computer via a flash drive, LAN, or DVD-RW. In fact, software now allows you to control and monitor your oscilloscope from a computer using a virtual front panel.

Ø  Trigger controls

As we mentioned earlier, triggering on your signal helps to provide a stable, usable display and allows you to see the part of the waveform you are interested in. The trigger controls let you pick your vertical trigger level (for example, the voltage at which you want your oscilloscope to trigger) and choose between various triggering capabilities. Examples of common triggering types include:

Edge triggering – Edge triggering is the most popular triggering mode. The trigger occurs when the voltage surpasses some set threshold value. You can choose between triggering on a rising or a falling edge. Figure 18 shows a graphical representation of triggering on a rising edge.

Glitch triggering – Glitch triggering mode enables you to trigger on an event or pulse whose width is greater than or less than some specified length of time. This capability is very useful for finding random glitches or errors. If these glitches do not occur very often, it may be very difficult to see them. However, glitch triggering allows you to catch many of these errors..

Pulse-width triggering – Pulse width triggering is similar to glitch triggering when you are looking for specific pulse widths. However, it is more general in that you can trigger on pulses of any specified width and you can choose the polarity (negative or positive) of the pulses you want to trigger on. You can also set the horizontal position of the trigger. This allows you to see what occurred pre-trigger or post-trigger. For instance, you can execute a glitch trigger, find the error, and then look at the signal pre-trigger to see what

caused the glitch. If you have the horizontal delay set to zero, your trigger event will be placed in the middle of the screen horizontally. Events that occur right before the trigger will be to the left of the screen and events that occur directly after the trigger will be to the right of the screen. You also can set the coupling of the trigger and set the input source you want to trigger on. You do not always have to trigger on your signal, but can instead trigger on a related signal. Figure 20 shows the trigger control section of an oscilloscope’s front panel.

Ø  Input controls

There are typically two or four analogue channels on an oscilloscope. They will be numbered and they will also usually have a button associated with each particular channel that enables you to turn them on or off. There may also be a selection that allows you to specify AC or DC coupling. If DC coupling is selected, the entire signal will be input. On the other hand, AC coupling blocks the DC component and centers the waveform about 0 volts (ground). In addition, you can specify the probe impedance for each channel through a selection button. The input controls also let you choose the type of sampling. There are two basic ways to sample the signal:

Real-time sampling – Real-time sampling samples the waveform often enough that it captures a complete image of the waveform with each sweep. This is useful if you are sampling low-frequency signals, as the oscilloscope has the required time to sample the waveform often enough in one sweep.

Equivalent-time sampling – Equivalent time sampling builds up the waveform over several sweeps. It samples part of the signal on the first sweep, then another part on the second sweep, and so on. It then laces all this information together to recreate the waveform. Equivalent time sampling is useful for high-frequency signals that are too fast for real-time sampling.

 

Fig.1 Block Diagram of DSO

DSO features:

a)      Pre- trigger function: (observation of waveforms before triggering)

The DSO is capable of recording the waveforms preceding the triggering point. It continuously stores data until a trigger occurs storing is stopped at the predefined no. Of sampling after the trigger &then the stored data is displayed with the trigger point as reference.

b)      Observation of single shot events: A DSO can capture single _shot events such as power supply, start up characteristics, power resets, power failure detection counter measures against noise & instantaneous waveforms for areas that include mechanical equipments such as motors. The DSO can be used for failure monitoring purposes such as storing waveforms.

c)      Large memory capacity: DSO stores the observed data in memory. Memory capacity is unlimited. With a large memory capacity, phenomenon can be recorder over a long period.

d)     Computations: Since the collected waveform data is expressed as digital values, sophisticated computation processing can be performed on the waveform data &the results are displayed on the screen in real time. This enables various functions such as auto set up function.

e)                  Data output: Digitization of waveforms data allows various forms of output. For eg. By incorporating a printer in the digital oscilloscope, the display on the screen can be immediately printed out and time consuming.

Ø  Application

                             

If a company is testing or using electronic signals, it is highly likely they have an oscilloscope. For this reason, oscilloscopes are prevalent in a wide variety of fields:

• Automotive technicians use oscilloscopes to diagnose electrical problems in cars.

• University labs use oscilloscopes to teach students about electronics.

• Research groups all over the world have oscilloscopes at their disposal.

• Cell phone manufacturers use oscilloscopes to test the integrity of their signals.

• The military and aviation industries use oscilloscopes to test radar communication systems.

• R&D engineers use oscilloscopes to test and design new technologies.

• Oscilloscopes are also used for compliance testing. Examples include USB and HDMI where the output must meet certain standards.

This is just a small subset of the possible uses of an oscilloscope. It truly is a versatile and powerful instrument.

                                                

Ø  Important Oscilloscope Performance Properties

Bandwidth and channels

it dictates the range of signals (in terms of frequency) that you are able to accurately display and test. Bandwidth is measured in Hertz. Without sufficient bandwidth, your oscilloscope will not display an accurate representation of the actual signal. A channel refers to an independent input to the oscilloscope. The number of oscilloscope channels varies between two and twenty. Most commonly, they have two or four channels.

Sample Rate

The sample rate of an oscilloscope is the number of samples the oscilloscope can acquire per second. It is recommended that your oscilloscope have a sample rate that is at a least 2.5 times greater than its bandwidth. However, ideally the sample rate should be 3 times the bandwidth or greater. You need to be careful when you evaluate an oscilloscope’s sample rate banner specifications. Manufactures typically specify the maximum sample rate an oscilloscope can attain, and often this maximum rate is possible only when one channel is being used. If more channels are used simultaneously, the sample rate may decrease.

                     

Memory depth

As we mentioned earlier, a digital oscilloscope uses an A/D (analog-to-digital) converter to digitize the input waveform. The digitized data is then stored in the oscilloscope’s high-speed memory. Memory depth refers to exactly how many records and, therefore, what length of time can be stored. Memory depth plays an important role in the sampling rate of an oscilloscope. In an ideal world, the sampling rate would remain constant no matter what the settings were on an oscilloscope. However, this kind of an oscilloscope would require a huge amount of memory at small time/division settings and would have a price that would severely limit the number of customers that could afford it. Instead, the sampling rate decreases as you increase the range of time. Memory depth is important because the more memory depth an oscilloscope has, the more time you can spend capturing waveforms at full sampling speed. Mathematically, this can be seen by:

Memory depth =(sample rate).(time across display)

So, if you are interested in looking at long periods of time with high resolution between points, you will need deep memory. It is also important to

PART I

PROCEDURE:

a)Modes of DSO:

1) Apply a low frequency signal from function generator as input to DSO.

2) Observe it using different modes.

To select normal mode.

1)      Press Acquire.

2)      In the Acquire menu, press Acquisition until “Normal” is selected.

To select the Average acquisition mode:

1)      Press Acquire

2)      In the Acquire menu, press Acquisition until “Average” is selected.

To select the peak detection mode.

1)      Press Acquire.

2)      In acquire menu, press Acquisition until “Peak Detect” is selected.

b) Capturing transients:

1) Construct series RLC circuit on breadboard.

2) Apply 1KHz square wave as input from function generator.

3) Observe the voltage across the capacitor on DSO.

4) Take readings for rise time, fall time, overshoot, undershoot using ‘automatic measurement’.

To display an automatic measurement

1)      Press Measure.

2)      In Measure menu, select source to select input channel or math waveform on which to make automatic measurement.

3)      Select voltage(for voltage measurement) or time(for time measurement).

4)      Then push the menu button for the measurement to add the button of the display.

To clear automatic measurement from the display

1)      Press Measure.

2)      In the Measure menu, select Clear to clear all the automatic measurements from the display.

c) FFT Analysis

1) Apply 1KHz square wave as input from function generator to channel 1 of DSO.

2) Switch DSO to math(FFT) mode

3) Using horizontal and vertical cursors measure amplitude and frequency components.

To display a waveform’s FFT:

1)      Press Math.

2)      In the math menu, press operate until “FFT” is selected.

3)      In the FFT menu, press source until the desired input channel is selected.

4)      Press window until desired window is selected:

There are 4 FFT windows. Each window has trade-off’s between frequency resolution and amplitude accuracy.

5)      Select display to toggle between “Split” screen display and a “Full Screen” display.

To use manually adjustable cursors:

For horizontal cursors

1)      Press cursor.

2)      Press X<->Y to select the horizontal cursor.

3)      Press Source repeatedly to select the source channel.

4)      To move left cursor, press X1 and then use variable knob.

5)      To move right cursor, press X2 and then use variable knob.

6)      To move both cursors at once, press X1X2 and then use variable knob.

For vertical cursors

1)      Press cursor.

2)      Press X<->Y to select the vertical cursor.

3)      Press source repeatedly to select the source channel.

4)      To move upper cursor, press Y1 and then use variable knob.

5)      To move lower cursor, press Y2 and then use variable knob.

6)      To move both cursors at once, press Y1Y2 and then use variable knob.

d) Math Operations:

1) Apply two different signals from function generator to two channels of DSO.

2) Perform all possible MATH operations.

To add, subtract waveform

1)      1+2

2)      1-2

Observation Table:

1. Transients in series RLC circuit.

Sr. No.	Parameter
1	Rise time
2	Fall time
3	Overshoot
4	Undershoot
5	Pulsewidth(+ve)
6	Pulsewidth(-ve)