Fundamentals of 8 versus 12-bit oscilloscopes and how to use modern 12-bit scopes
resistance (Rds) of the power FET. This requires measuring a voltage on the order of one or two volts on a waveform with a peak-to-peak swing on the order of 400 volts. The WaveSurfer 4000HD series is compatible with all Teledyne LeCroy probes compatible with the scope’s bandwidth range (Figure 5). Wide range of applications set higher standard for ‘workhorse’ scope The WaveSurfer 4000HD series is not limited to only high dynamic range applications. It is an excellent scope in its own right and may set a higher standard for ‘workhorse’ scopes. It is a good choice for low- speed serial data troubleshooting, offering analysis packages and probes to support serial buses like SPI, I2C, UART-based links, as well as automotive buses like LIN, CAN, and FLEXRAY. Serial bus analysis requires the ability to acquire and decode the bus protocol and to read the data content (Figure 6). The color-coded overlay shows each packet. The red overlay indicates the address data while the blue overlays mark the data packets. The address and data content appear within the overlay. Decode information is available in binary, hex, or ASCII format. The table at the bottom of the display summarizes acquired transactions showing time relative to the trigger point, address length, address, direction (read or write), the
number of packets, and the data content. Triggering can be based on activity, address, data content, or a combination of address and data. The Teledyne LeCroy ZD 1000 active differential probe is a good choice for measuring serial data. This 10:1 probe has an input impedance of 1 Megaohm, has a bandwidth of 1 G Hz, and can handle differential voltages of up to 16 volts and common mode voltages of up to 2 0 volts. It is especially well matched to differential buses such as CAN.
HD and wide dynamic range of a true 12-bit oscilloscope, but their relatively high cost has kept them out of reach of many designers and test engineers. The Teledyne LeCroy WaveSurfer 4000HD series oscilloscopes go a long way toward addressing that problem with a much lower cost entry point. It provides HD measurements based on 12-bit vertical resolution, a 5 GS/s maximum sampling rate, and a low noise floor. It is also compatible with Teledyne LeCroy probes and analysis software packages. As such, the scopes open the door to cost effective high-dynamic-range measurements and move its availability from the research lab to the engineer’s bench or factory floor.
Teledyne LeCroy HVP1306 high voltage differential probe. This probe is rated for a maximum CATIII voltage of 1000 volts at a bandwidth of 120MHz. Both probes are recognized by the WaveSurfer scope, which automatically scales the measured waveforms to account for the probe’s gains and units of measure. The power measurement software automates the most common SMPC measurements. Figure 4 shows the calculation of device power dissipation as the yellow trace. This is calculated from the current and voltage waveforms for the whole switching cycle. Measurement parameters isolate and display turn-on, conduction, turnoff, and off-state losses based on the acquired waveforms, with each zone clearly delimited by a color overlay. It also shows the total loss from all zones as well as the switching frequency. Other available measurements, in addition to device measurements shown in the Figure, help characterize control loop dynamics, line power, and performance characteristics such as efficiency. The 12-bit resolution is also useful in power measurements when calculating the drain source Figure 4: Characterizing an SMPC’s losses involves measuring the voltage and current of the power switch devices and then calculating power loss in each phase of the power switching cycle. Image source: DigiKey
HD oscilloscopes can also handle higher voltage measurements like those encountered in switched mode power converters (SMPCs). SMPCs include power supplies, inverters, and industrial controllers. They control power by adjusting the duty cycle or frequency of a switched waveform. The main measurements involve the voltage across and the current through the power switching device(s), usually a field effect transistor (FET). To help developers with SMPC
measurements, Teledyne LeCroy provides application-specific software and voltage and current probes. A typical measurement is shown in Figure 4. The current, the pink trace, is measured with a Teledyne LeCroy model CP030B current probe. This clamp-on probe has a maximum current input of 30 amperes (A) and a bandwidth of 50MHz. The voltage waveform, shown as a beige trace, is measured using a
Conclusion
While 8-bit oscilloscopes will always have a place, there are many applications that could use the
Figure 6: Low-speed serial trigger and decode of the I2C bus includes the ability to read the data content of the bus. Shown is the acquisition and decoding of an I 2 C bus signal for both a read and a write operation. Image source: DigiKey
Figure 5: The Teledyne LeCroy WaveSurfer 4000HD oscilloscopes are compatible with the company’s extensive line of probes, including the power measurement related probes shown here. Image source: Teledyne LeCroy
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