Using an Arbitrary Waveform Generator & a Digital Storage Oscilloscope to Create a Transistor Curve Tracer

You can use an arbitrary waveform generator and a digital storage oscilloscope (DSO) to create a transistor curve tracer that you can then use to test and characterize semiconductor devices. I will describe a method to produce current vs. voltage (I-V) curves of any semiconductor device using a Siglent SDG 1000 series waveform generator and a Hantek DSO 5000 series oscilloscope.

Set the waveform generator to generate a staircase waveform at a frequency of 120 Hz. This frequency coincides with the frequency produced by a full wave–rectified AC line voltage. The full wave–rectified signal provides the collector (drain) to emitter (source) power, which is displayed on the horizontal axis. The staircase waveform provides the base (gate) driving signal for the device under test.

The collector (drain) current is displayed on the vertical axis of the oscilloscope by developing a voltage across a current-sensing resistance. You will use the oscilloscope in X-Y mode rather than the traditional voltage-time (Y-T) display mode. You must also change the persistence of the display to provide an easier-to-view and more meaningful display.

curve_tracer_1 The schematic of the test setup and an example of the resulting display are shown below for an NPN or N-channel device. Simply invert the polarities for a PNP or P-channel device.

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This example demonstrates a method for generating and displaying the current vs. voltage (I-V) curves of a transistor (either BJT or FET). This task was performed using the Siglent SDG1025 25MHz arbitrary waveform generator and a Hantek DSO5202B 200MHz digital storage oscilloscope as well as a handful of readily available components.

Buyer Beware: Overseas Sellers

It is very tempting to buy an expensive piece of test equipment from an online dealer located in China, since the initial cost for a comparable piece of equipment is typically lower than in countries like the United States. However, it can cost you much more in the long run. Online shopping has become extremely popular in recent years, thanks in large part to convenience — but with convenience comes greater individual responsibility. Shoppers don’t always realize that a seller may be nothing more than one person with no knowledge of test equipment selling or reselling products. Sellers like these have no idea how to use the products they’re offering and, thus, cannot provide any technical support whatsoever.*

If you’re buying simple items like a cell phone case or a USB flash drive, it sometimes makes sense to buy the cheapest product you can find, typically from an overseas seller. If the product fails you can just buy another equally cheap one instead of sending it overseas for repair or replacement. You (hopefully) won’t need technical support for a cell phone case, so it sometimes makes financial sense to buy whatever’s cheapest and hope for the best. But complex, expensive pieces of test equipment built from thousands of discrete components are another story altogether.

You can buy, for example, a cheap digital storage oscilloscope directly from a seller in China. But do you really know who you’re buying from? How long have they been in business? What kind of support do they offer their customers?  The fact of the matter is you might be dealing with a single person who is buying a product directly from the manufacturer, marking up the price, and shipping it overseas. That person might even be reselling a used or refurbished product without your knowledge.

More importantly, what happens when you buy a cheap piece of test equipment from an overseas seller only to discover you need technical support? You may have out-of-the-box issues that you could easily address yourself with a little guidance, or you may run into a serious soft- or hardware issue that requires professional repair. Unfortunately, sometimes you’ll return an item only to find that you need to return the replacement as well. And let’s not forget how easily packages are damaged in transit. These kinds of things have happened to all of us at some point.

Consider the logistics of shipping a product back overseas for repair or replacement — that is if the vendor even provides repair or replacement service. Shipping delicate electronic equipment overseas is more complicated than you might imagine. In addition to overseas shipping costs, you may have to pay import charges and other fees. Most US-based businesses will at the very least cover return shipping costs, but an overseas seller may expect you to foot the bill for return shipping. Keep in mind that it takes a long time to return an item to a seller on another continent. Simply contacting the vendor and waiting for a response can take days or even weeks.

Also consider that if the product you ordered is defective or was damaged in transit, many overseas dealers will not take responsibility, forcing you to deal with the manufacturer (more on this below) or shipping company on your own. In contrast, most established businesses in the United States insure their shipments, so if something is damaged in transit it may be replaced at no charge to you. Some businesses will issue a call tag to pick up products that are dead on arrival and may ship a replacement unit to you, even before they receive the defective item.

Dealing with an established business in the United States is the way to go when purchasing products like electronic test equipment. First of all, you don’t have to worry about the language barrier. In addition, an established business will provide support for the products they sell. That support alone can be as valuable as the product itself, even if you never run into a problem with the product. An overseas seller who doesn’t know anything about the product will not be able to help you with even the simplest questions you may have.
Another thing you should consider is service on a unit that is no longer under warranty. Some companies will not even work on units that weren’t purchased from them, and some manufacturers do not offer service on units that were sold in another country; for instance, some North American brands do not recognize their own products when they’re sold from China, even though that product was manufactured on the same assembly line (this has been a common issue with camera equipment for many years).

Finally, consider this: Counterfeit goods are very common in China. Many of these products not only look and feel the same as their name-brand counterparts, some even have the same logo and model number on them as the actual product. However, most of these counterfeits are poorly built and may be very different inside than the piece of test equipment you thought you were buying. Reputable US businesses that import products from China do not sell counterfeit products and will stand behind their entire product line.

With all of this in mind, you should thoroughly consider the true cost of an item and its actual overall value when deciding whether or not to risk buying a cheaper product from an overseas seller. Regardless of the sticker price, you may end up paying much more in the long run. Know what you’re really buying and shop smart.

 

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*Furthermore, these sellers may not know much English, making it difficult to even find out that they can’t help you with any technical problems.

 

Ten Measurements Using an Oscilloscope

1) Measuring and Viewing Voltage Waveforms

You can measure and view either DC or AC signals up to the oscilloscope’s BW using the standard voltage vs time mode of operation. Adjust the vertical setting to display the complete waveform and determine the value by counting the vertical divisions and multiplying by the vertical scale factor. Most digital storage oscilloscopes have a measurement menu that directly displays values without forcing you to count graticules.

2) Measuring and Viewing Current Waveforms

You may view current values and waveforms using an external low-value shunt resistor. Use Ohm’s law* to determine the correct scale multiplier for the measurement. The current shunt should be connected to the “low” side of the power.

*I = E / R

3) Measuring Frequency

You can perform frequency measurements by displaying the signal waveform on the oscilloscope’s screen and adjusting the horizontal time base value until you see at least one full cycle on the display. Measure the time value for one cycle and determine the frequency using the equation Freq = 1 / time.

4) Measuring Rise Time of a Pulse

You can determine the rise time of a pulse signal in a manner similar to frequency. Adjust the horizontal time base to display the rising edge of the pulse. Rise time is defined as the time between 10% and 90% of the amplitude.

5) Measuring Capacitance

You can estimate capacitance using a simple RC circuit and noting the phase difference between applied and resultant voltage across the capacitor using both vertical channels of the oscilloscope. The phase difference is noted and the capacitance is calculated using the following formula (α is the phase angle and Z is the impedance):

Capacitance = -1 / (2 x π x frequency x Z x sin(α))

6) Measuring Amplifier Gain

You may measure the gain or amplification of a circuit using both channel one and channel two of the oscilloscope. You will monitor the input signal with one channel and the output signal with the other. The difference between the amplitudes of these two signals indicates the gain.

7) Measuring Cable Length (TDR)

You may use a simple Time Domain Reflectometer (TDR) to determine the approximate length of a cable. A single fast-rising pulse from a pulse generator is simultaneously applied to the cable and the oscilloscope’s vertical channel. The time required for the pulse to travel to the end of the cable and reflect back again is a factor of both the cable length and the cable’s dielectric constant. The formula for measuring cable length is:

Length = (velocity of propagation x time) / 2

8) Measuring Differential Signals

You can measure differential signals, such as on a twisted pair cable, by using both of the oscilloscope’s vertical channels simultaneously. Use the MATH operation CH1-CH2 if both vertical channels are set to the same scale factor.

9) Measuring Signal Spectrum (FFT)

You can use the MATH operation FFT to view a waveform in an amplitude vs frequency representation. This is a simplified spectrum analyzer-type measurement and is useful for determining the frequency components of a periodic signal.

10) Measuring Duty-Cycle of a PWM Signal

You can determine the duty cycle of a PWM signal by displaying one complete cycle on the oscilloscope’s screen, which will enable you to determine the width of the positive portion as well as the width of the negative portion. Then you calculate the duty cycle with the following formula:

Duty cycle = (pulse high / (pulse high + pulse low) x 100%

TIP: Using the VDV501-089 Distance meter with unknown wire types.

Technicians and installers are often faced with the question of `how long is this wire?’ If the wire is already installed or you only have a partial spool it can be a difficult task. Often times without the proper tools your `best guess’ is all you can do.

Now with the tools available today, such as the VDV501-089 Distance Meter from Klein Tools. The VDV501-089 is a handy digital tool that can measure many types of wire either in place or while still on the spool. Frovdv501-089-am Coax to network and telephone twisted pair cables, it is a simple job to connect the VDV501-089 to one end of the cable, set the meter to the cable type and read the cable length right on the device.

If you have a cable that is not one of the preset values on the VDV501-089 you can setup the meter in either of two ways. If you know the capacitance per foot value of the cable you can enter that into the USER setting of the meter and then measure the cable. You can also use a known length of the desired cable and set the capacitance value using the up/down arrow keys until the meter reads out to the known length of the test cable. Note the capacitance value for future uses and when you check the same type of wire it will give you an accurate length. vdv501-089-b

Another handy item to have with the VDV501-089 is a 6 pin modular coupler. This will allow you to test telephone cables that already have modular connectors installed. (an 8 pin coupler would be used to test many network cables.)

Ten Uses for an Autotransformer, or Variac

1) Testing a repaired electronic device.

After performing a repair on a piece of electronic equipment, it is best to slowly power up the device to avoid burning up the replaced parts. You will often discover multiple part failures one at a time. Using the Variac to slowly power up the repaired unit can prevent damaging the replacement parts.

2) Powering up an old radio or amplifier.

When powering up an old radio or amplifier that has not been used for an extended period of time, it is best to apply reduced voltage power to the unit in stages in order to re-condition the electrolytic capacitors.

3) Roasting coffee beans or nuts.

To obtain the freshest coffee possible, coffee connoisseurs prefer to roast and grind their own coffee beans. You can use a Variac to adjust the roasting temperature to the ideal value.

4) Adjusting the temperature of a resistance-type heater.

Resistance-type heating elements can be adjusted by varying the AC power applied.

5) Hot-wire cutting of Styrofoam.

Styrofoam and other types of foam are typically cut to the desired shape using a hot wire cutting system. You can use a Variac to adjust the temperature of the hot wire to the optimal value required for cutting.

6) Dimming incandescent lighting.

Incandescent lights may be dimmed by varying the applied AC voltage. This is not possible with newer LED or fluorescent lamps.

7) Changing fan speed over a narrow range.

The speed of some types of small AC motors may be varied over a narrow range by changing the applied AC voltage. A problem with this application is that the torque of the motor is also reduced which may result in the fan or motor stalling out. Also note that this technique cannot be used with all electric motors.

8) Compensating for line voltage drop in AC power applications.

Long power line runs can often result in voltage drops across the lines, especially in high current draw applications like hot tubs or spas. You can use a Variac to compensate for this voltage loss.

9) High-current unregulated DC power supply.

A high-current adjustable unregulated DC power supply can be constructed using a Variac, high current rectifier, and filter capacitor. Do not attempt this without being aware of the possible safety concerns when using a Variac. Since a Variac is not isolated from the AC power lines, the AC power is directly accessible. The ideal configuration would use an isolation transformer to isolate the AC power mains from the application.

10) Adjusting the voltage of an AC power supply.

The output of an AC power supply, such as an electric train transformer, can be adjusted with a Variac.

Getting Started: The Aardvark HD3M Wireless Inspection Camera

Unlike many IP-based inspection cameras that connect to a wireless access point, the Aardvark HD3M wireless inspection camera is its own access point to which your computer or mobile device directly connects. This allows you to use the HD3M in areas without wireless access, such as a construction site or even a campsite. Because it is its own network, the HD3M also provides greater security, so other people will not see what is on the inspection camera.

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You will need to download the Android or iPhone software from the Google Play or Apple App store. The Android version is used in the following example, but the iPhone version is quite similar. First you will go to the Google Play Store and search for the “WiFi Endoscope” app and then install the app to your device. (Note: Sample screens were taken from a Samsung Galaxy S-III running Android version 4.3.)

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Once the app is installed the WiFi Endoscope app icon should appear on your device’s homescreen.

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At this point you will need to disconnect from the access point or wireless data connection you are currently on and connect to the camera. This initially causes confusion as you must disconnect the computer or mobile device from the access point to which you usually connect and then reconnect it to the wireless inspection camera‘s “WiFi_Endoscope” access point for it to work.

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You should also disable your “Auto Network Switch” setting. This will help maintain the connection if the signal level varies as you move the camera around while in use.

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Remember that the default wireless SSID for the Aardvark HD3M is “WiFi_Endoscope” and the default password is “00000000” (eight zeroes). Both can be changed, if desired, from within the Windows software. (More on the Windows software below.)

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Once you are connected to the camera you will open the WiFi Endoscope application.

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The next screen shows the link to the camera. Click on “WiFi Endoscope” to select the camera you’ll be using.

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The main viewing screen will appear. From this screen you can choose to capture a photograph or record video.

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The app will save the photos or video files to your computer or mobile device. Depending on your device you can view, email, or transfer these files to another device.

Once you have finished using the HD3M you will want to reconnect to your normal access point or wireless data provider.

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The Windows-based software for the camera is called “Smart Camera” and is included on the CD that came with your inspection camera.

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As with the Android and iPhone software, you must disconnect your computer from your normal wireless access point and connect to the camera’s WiFi access point, “WiFi_Endoscope.”

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Once connected to the camera you will run the “Smart Camera” software.

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You can take still pictures or record video that has been saved to your computer with this software.

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You can also change the resolution of the camera as well as the camera’s WiFi SSID and access password.

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As with the Android & iPhone software, once you are finished using the camera you will need to disconnect from the camera’s access point and reconnect to your normal WiFi access point.

Another unique feature of the Aardvark HD3M is Bridge Mode. Once connected to the inspection camera you are able to “bridge” the inspection camera’s WiFi connection to another WiFi access point and then to wherever that access point is connected. You will select the access point to which you wish to connect. Click “Join Network” and follow the login prompts.

On the next screen you will click “Save & Apply” to establish the bridged connection. To connect through the Bridge Mode access a web browser on your computer or mobile device and then access the camera’s default address at “http://192.168.1.1″. Select “Network/Bridge Mode” in the top menu.

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Note: Be sure you select a unique network address in Bridge Mode. If the access point to which you are connecting uses the same address you can change the camera’s address in the “Network/LAN” menu. In this example the IP address has been changed to 192.168.2.1 because 192.168.1.1 was already in use.

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Once connected in Bridge Mode you can access the Internet or any functions of the access point to which you are connected, enabling you to browse the Internet, networked printers, or shared network hard drives.

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You can also access the video stream from the camera from other computers.

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There are many options to explore once the HD3M inspection camera is connected to your network in Bridge Mode.

Using a Variac to Power Up an Old Radio or Amplifier

It is best to monitor the AC current drawn by the unit as it is powered up, which indicates the “health” of the radio or amplifier. The easiest method is a “clamp-on” AC current meter. In general the AC current draw should be about 50 to 75 % of the rating of the fuse in the radio or amp.

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Bring the variac up slowly over a period of several minutes until you reach about 50 volts. This will pre-heat the tube filaments and begin to re-form the electrolytic capacitors. Keep an eye on the current value and be on the lookout for smoke or any malfunctions. The variac should be left running at this voltage level for about 30 minutes.

Next, slowly bring the variac up to around 75 volts. This will allow the power transformer windings in a transformer-equipped set to warm up a bit, driving out any accumulated moisture, and will continue to re-condition the electrolytic capacitors. Leave the variac at this voltage for at least another 30 minutes.

After waiting 30 minutes, raise the voltage to around 90 volts. At this point the radio or amplifier should be functioning. If the unit begins to hum, you will need to replace the electrolytic capacitors. You should probably consider replacing the capacitors on an old piece of equipment even if it doesn’t hum. Old filter caps will fail sooner or later, so you may save yourself some time and trouble later on.

After about 10 to 15 minutes you can raise the variac output to full voltage and verify everything is working correctly. If you notice a sudden increase in current or see or smell smoke, immediately turn everything off.

A variac is an excellent tool for safely powering up an old radio or amplifier that has not been used for an extended period of time. Using this low-priced piece of equipment could save you time and money as well as enable you to bring that old piece of equipment back to life with very little effort.

Measuring Power with a Digital Panel Meter (DPM)

Digital Panel Meters (DPMs) are voltage measuring devices which can be used to measure current using a shunt resistor or hall effect current sensor. When a display of Power (watts) is required, the voltage and current values must be multiplied to produce a voltage that is proportional to the product of these two quantities.

Power (watts) = Voltage X Current

This application note will describe a method to accurately display DC Power values on a DPM.

There are several methods that can be used to calculate power based on voltage and current values. Most methods require a micro-controller to perform the calculation and then display the power level on an LCD or similar indicating device. While this can be an extremely accurate way to display Power, it requires that the analog values of voltage and current be converted to digital values that can be processed by the micro-controller. Furthermore, a firmware program must be written to make it functional. The method described here, uses an analog multiplier IC to perform the calculation directly on the two analog quantities which then produces a third analog value suitable for display on a DPM. This is entirely a hardware solution and requires no software program.

An Analog Devices AD633 device was chosen as the analog multiplier IC due to its excellent accuracy and ease of use. Since it is a 4 quadrant multiplier it functions equally well with any voltage polarity and produces an output voltage that is proportional to the product of the X and Y input voltages. This is the DC Power level that can now be displayed on a DPM.

This application note has shown how a Digital Panel Meter may be used to measure and display DC Power values based on a DC voltage and a DC current applied to the inputs of the circuit. The connection diagram is shown below.

measuring-power-with-a-digital-panel-meter

 

Using FFT in Digital Storage Oscilloscopes (DSO)

Many modern Digital Storage Oscilloscopes (DSOs) have advanced mathematical operations available that typically include varying degrees of FFT analysis of the observed waveform. Fast Fourier Transform (FFT) allows the frequency content of a signal to be displayed along with the amplitude of these various frequency components.

Since the FFT operation is a mathematical operation, it requires a large amount of computing time to properly display the information. For these reasons, FFT operation in a DSO is not equivalent to a spectrum analyzer, but oftentimes it is sufficient for many applications.

FFT analysis of a signal is useful for tracking down EMI/RFI problems and for detecting spurious oscillations in circuitry. It is also useful for displaying the harmonic content of various waveforms and tracking down problems in switching power supplies. Distortion created by nonlinear operation of analog circuitry can easily be shown using FFT analysis. This phenomenon is essentially impossible to view using a standard time domain representation of the signal as is shown with a standard DSO display of the waveform.

FFT analysis is an extremely useful tool available to the technician or engineer. It can provide insight into the operation of a circuit and help determine which components or operating parameters are causing problems. Since this function is entirely a mathematical operation, it is implemented in the firmware of the DSO and therefore does not require additional hardware to implement, and therefore does not increase the cost of the unit.

Reasons for Microstepping a Stepper Motor (Part Two)

Microstepping can replace a mechanical gearbox in certain applications when you need small relative movements or greater step resolution. Even if you have to use a larger stepper motor, this is often a preferable solution in a number of applications. You need to carefully select the appropriate stepper motor to get the best possible results, and you should also consider developing customized sine/cosine profiles.

You can increase stepper motor position accuracy beyond the manufacturer’s specifications by microstepping. One way to accomplish this is by designing a microprocessor-based microstepping system using the motor at two-phase-on stop positions, which are typically the most accurate rotor stop positions. Use an automatic or manual factory calibration process to store a correction value for every stop position on each motor you use.

You use the correction value to send adjusted full-step positions to the stepper motor. These adjusted positions have slightly different current levels in the windings, which compensates for the deviations of the position at the original stop positions. This type of microstepping technique is ideal when optimal step accuracy is the most important design criteria. When you use this technique, the stepper motor system must use a rotor home position indicator for synchronizing the rotor and the compensation profile.

Although the electronics needed to generate microstepping are more complex than the electronics used in half- and full-stepping, the total system complexity — including gearbox, transmission, and stepper motor — is less complex and costly in most applications. In addition, microstepping can simplify or altogether replace gearboxes and mechanics for damping noise and vibrations. Moreover, stepper motor selection is easier and more flexible.

You can use software and PWM-timers or digital-to-analog converters in the microprocessor as a replacement for external stepper motor controllers in microprocessor-based microstepping applications in order to achieve the lowest possible hardware cost, which is to say you can obtain microstepping hardware for about the same price as half- and full-step systems for comparable stepper motor sizes.

Read Part One

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