How to Probe Analog and Digital Signals with a Mixed Signal Oscilloscope

　　Many microcontroller-based systems have both analog and digital signals. Even systems that appear to be completely digital are not completely digital because of analog effects such as ringing and crosstalk. Therefore, it is often necessary to have both analog and digital perspectives on the signals in the system. This is where a mixed signal oscilloscope (MSO) can help.

　　Mixed signal oscilloscopes have both the functionality of an oscilloscope and some of the functionality of a logic analyzer. The most common configuration of a mixed signal oscilloscope has 4 analog channels and 16 digital channels, and they are best suited for debugging embedded microprocessor boards.

　　The processor board block diagram shown in Figure 1 contains analog signals such as power, clock, analog-to-digital converter (ADC) inputs and digital-to-analog converter (DAC) outputs, as well as parallel and serial digital signals. Parallel digital signals include digital and address lines for the CPU and GPIO interfaces. Interfaces such as Ethernet, SATA, PCIe, SPI, I2C and UART are high-speed and low-speed serial data signals. A mixed signal oscilloscope allows you to observe these signals simultaneously in the analog or digital domain. The displays in both domains are time synchronized, which helps to find problems. Triggering from analog, digital or a combination of both also helps with diagnosis. These acquisition resources are complemented by a full set of measurement and analysis tools. These tools can process data regardless of which domain it is in. In addition, the search function can be easily used to locate serial or parallel digitized data patterns.

　　Figure 1: Example of an embedded microprocessor board containing analog (green), digital (red), and serial data (blue) signals. A mixed signal oscilloscope provides a solution for measuring and troubleshooting all of these signal types with a single instrument.

　　Comparing Analog and Digital

　　The analog waveform in a digital oscilloscope represents the acquired signal as a series of sample points. These sample points are acquired at the oscilloscope's sampling rate and digitized with an amplitude resolution set by the number of bits in the analog-to-digital converter (ADC) in the oscilloscope. Modern high-frequency oscilloscopes have ADC resolutions ranging from 8 bits (256 levels) to 12 bits (4096 levels).

　　The digital trace in a mixed signal oscilloscope represents a bit, sampled at the digital sampling rate. The amplitude basically varies from 0 to 1, depending on whether it is above or below a preset logic threshold (many mixed signal oscilloscopes provide preset logic levels for various families of logic devices), and they represent the state of the digital input. Figure 2 shows a comparison of the analog trace (bottom) and the digital trace (top).

　　Figure 2: Comparison of the digital trace (top) and the analog waveform. The digital trace amplitude is represented by a 1 or 0, depending on whether the voltage at the digital input is above or below the user-set logic threshold. The analog trace is resolved into any of 4096 (12-bit) amplitude levels.

　　The analog trace shows small changes in voltage over time. You can see things like pulse overshoot and ringing. The cursor amplitude readouts visible in the C1 block read amplitudes down to mV. The digital trace cursor readouts (in the Digital 1 block) report amplitudes of 0 and 1. Remember, the digital trace only shows the state of the digital line, which has only two values: 0 and 1.

　　When displaying multiple digital lines, you can usually choose to view them individually, bundled into a bus, or both at the same time, as shown in Figure 3. In Figure 3, eight digital lines (D0 to D7) are displayed on the screen in bus form (bottom trace), which shows the total value of all digital lines in hexadecimal notation. Note that D7 is the most significant bit (MSB) and D0 is the least significant bit (LSB).

　　Figure 3: Multiple digital lines D0 to D7 shown in single-line and bus form. The bus form shows the total of all eight lines in hexadecimal count. D0 is the least significant bit and D7 is the most significant bit. Typical measurement tools include cursors and timing parameters with the digital lines as the source, as shown in the figure.

　　You can apply the oscilloscope's parametric measurement tools to any signal type, but measurements on digital traces are limited to time-related measurements such as period, width, duty cycle, and delay. These parameters, like the more common analog waveform parameters, can be the basis for trend (plot parameter values ​​sequentially), track (plot parameter values ​​synchronized in time with the source trace), and histogram analysis tools. Figure 3 shows eight parameters (P1-P8) based on the digital lines shown.

　　Digital Design Troubleshooting

　　The following examples demonstrate some basic diagnostic techniques that can be implemented with a mixed signal oscilloscope. The circuit studied in the first case study is a simple D flip-flop that is triggered on the rising edge of the clock. Digital line D0 is connected to the data input (D) of the flip-flop. D1 shows the clock and D2 shows the Q output. At the same time, analog channels C1, C3, and C4 are connected to the same points. These waveforms are shown on the left side of Figure 4. The period and width of the Q output (D2) are measured using parameters P1 and P2. The oscilloscope's time base is set to acquire approximately 5000 clock pulses.

　　Figure 4: Using the D2 period trace to locate long periods in the acquisition record. Zooming in on the maximum period makes it easy to observe details in both the digital and analog traces.
 

　　The parameter statistics show that the average value of the period is 208ns and the maximum value is 416ns, which means that the output does not maintain the expected period. Trace F1 is the trace of the period measurement and is displayed in the upper left grid below the digital display. This trace shows the period of D2 as a function synchronized in time with the source trace. The cursor marks the point where the trace indicates and the period value increases. All traces are zoomed at the location of the maximum Q output period and the zoomed traces are shown on the right side of the display.

　　The long period of the data signal, which represents the faulty clock triggering, is shown in the digital trace in the upper right grid. A zoomed-in version of the analog trace C4 is also shown in trace Z4. Parameter P3 measures the setup time between data C1 and clock C3. The statistical results again show that the minimum setup time is 20% shorter than the nominal value. The setup time trace in F2 shows that this shortened setup time occurs in sync with the extended period.

　　This is one way to find this type of problem. Another way is to use the oscilloscope's built-in search tool called WaveScan, as shown in Figure 5. Note that most mixed-signal oscilloscopes have some form of search tool.

　　Figure 5: Setup for using WaveScan and looking for outliers by searching for period measurements on D2 that exceed the nominal 208ns.

　　The search tool can search through very long records, looking for edges, unstable edges, very short frames, serial data patterns, parallel (bus) data patterns, or measured data. In this example, we search for periods longer than 250ns measured on the D3. When this condition is met, it stops the acquisition, displays the digital source trace, and zooms in on the source trace. Anomalies are highlighted in red, and the measured anomaly values ​​are displayed in the adjacent table. Once a problem is found, the analog trace is turned on to observe the physical layer problem that caused the problem, just as we did before.

　　A mixed signal oscilloscope lets you view up to 16 digital traces, more than there are analog channels. In Figure 6, eight digital traces record the operation of two cascaded 8-bit shift registers, which are the core circuitry of a pseudo-random binary sequence generator. First, notice that the trace labels are customized to reflect the functions in the circuit. We can see the clock and serial data inputs and the Q6, Q7, and Q8 outputs from the A and B sections of the shift register. We can see the "long-short" pattern propagating from left to right through all 16 stages of the circuitry starting with the serial input trace (second from the top).

　　Parameter P1 measures the time from the start of the flip-flop to the falling edge at the end of the pattern on the serial input trace using the strobe delay parameter. A similar measurement is made for that edge on the Q6-A trace. The parameter formula is used in P3 to calculate the time difference between these two edges, which is 515.3μs. Parameter P4 measures the clock period. The parameter formula in P5 is used to multiply the clock period by 6 to verify the expected delay from the serial input to Q6-A, which is 515.3μs for correct operation. The outputs Q7-A and Q8-A show the added delay of one clock period. The correct propagation delays for all 16 stages can be verified in a similar manner.

　　Figure 6: Verifying the correct propagation delay of a dual 8-bit serial shift register.

　　The digital trace function of a mixed signal oscilloscope can be used to acquire serial data from I2C, SPI, and other low-frequency serial standards, as shown in Figure 7. Here, D0 contains the SPI data and D1 is the SPI clock signal. The decoder uses these waveforms as source traces to decode the data content and displays it in the blue trace overlay and accompanying table. The decoded data can be displayed in ASCII, binary, or hexadecimal. The table also lists the packet position relative to the trigger and the bit rate of each decoded byte.

　　Figure 7: Using the digital trace as a source for the SPI decoder. The data content in hexadecimal format is shown in the blue overlay and accompanying table.

　　Summarize

　　Mixed signal oscilloscopes can provide users with more functions than traditional digital oscilloscopes. Users can observe up to 16 digital signal lines at the same time and synchronize them with up to 4 analog waveforms. Digital traces can be measured with cursors or selected measurement parameters. Analysis functions and decoding operations can also be applied to digital lines.

　　From a functional perspective, the digital state analysis capabilities in a mixed signal oscilloscope are simpler to build than a logic analyzer and do not require additional platform space. Analog channels in the same instrument can be used for detailed physical layer analysis when problems are encountered.