Probing analog and digital signals with a mixed signal oscilloscope

Many microcontroller-based systems have 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 usually necessary to hold both analog and digital perspectives for the signals in the system. This is where a mixed signal oscilloscope (MSO) can help you.

Mixed signal oscilloscope has the functions of an oscilloscope and part of the functions of a logic analyzer. Common mixed-signal oscilloscopes are configured with 4 analog channels and 16 digital channels, which are suitable for troubleshooting on embedded microprocessor boards.

The processor board block diagram shown in Figure 1 contains analog signals such as power, clock, analog-to-digital converter (ADC) input and digital-to-analog converter (DAC) output, as well as parallel and serial digital signals. Parallel digital signals include the digital and address lines of 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 the two domains are synchronized in time, which helps to find problems. Triggering from analog, digital, or a combination of both also aids in diagnosis. These collection resources are supplemented by a complete set of measurement and analysis tools. These tools can handle the data in any domain. In addition, you can easily use the search function to locate serial or parallel digitized data patterns.



Figure 1: An example of an embedded microprocessor board containing analog (green), digital (red) and serial data (blue) signals. The mixed signal oscilloscope provides a single instrument to measure and troubleshoot all these types of signals.

compare analog and digital

The analog waveform in a digital oscilloscope represents the collected signal as a series of sampling points. These sampling points are acquired at the sampling rate of the oscilloscope and digitized with the amplitude resolution set by the number of bits of 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, which is sampled at a digital sampling rate. The amplitude basically changes from 0 to 1, depending on whether it is higher or lower than the preset logic threshold (many mixed signal oscilloscopes provide preset logic levels for various series 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 digital trace (top) and analog waveform. The digital track amplitude is represented by 1 or 0, and the judgment is based on whether the voltage at the digital input terminal is higher or lower than the logic threshold set by the user. The analog trajectory is decomposed into any one of 4096 (12-bit) amplitude levels.

The analog trace can show the small changes in voltage over time. You can see phenomena such as pulse upshoot and ringing. The cursor amplitude reading function visible in the C1 description block can read amplitudes as low as mV. The reading function of digital track cursor (in the description block of number 1) is the amplitude of 0 and 1. Remember, the digital trace only displays the status of the digital line, with only two values, 0 and 1.

When multiple digital lines are displayed, you can usually choose to observe with one line alone, bundle them into a bus, or conduct both observations at the same time, as shown in Figure 3. In Figure 3, 8 digital lines (D0 to D7) are simultaneously displayed on the screen in the form of a bus (bottom trace), which displays the total value of all digital lines in hexadecimal counting mode. Note that D7 is a bit (MSB) and D0 is a bit (LSB).



Figure 3: D0 to D7 multiple digital lines displayed in the form of a single line and a bus. The bus form shows the total number of all 8 wires counted in hexadecimal. D0 is a bit, D7 is a bit. Typical measurement tools include cursors and timing parameters with digital lines as the source, as shown in the figure.

You can apply the parameter measurement tool of the oscilloscope to any signal type, but the measurement of the digital trace is limited to time-related measurements, such as period, width, duty cycle and delay. These parameters, like the more common analog waveform parameters, can be used as the basis for trending (drawing parameter values in sequential order), tracking (drawing parameter values synchronized with the source trajectory in time), and histogram analysis tools. Figure 3 shows the 8 parameters (P1-P8) based on the digital line shown.

Error checking of digital design

The following example shows some basic diagnostic methods that can be implemented with a mixed signal oscilloscope. The circuit under study is a simple D flip-flop, which is triggered by the rising edge of the clock. The digital line D0 is connected to the data input terminal (D) of the trigger. D1 shows the clock and D2 shows the Q output. At the same time, analog channels C1, C3, and C4 are respectively connected to the same point. These waveforms are shown on the left side of Figure 4. The period and width of Q output (D2) are measured with parameters P1 and P2. The time base of the oscilloscope is set to collect approximately 5000 clock pulses.



Figure 4: Use the trace of the D2 cycle to locate the long period in the acquisition record. The zoom period can easily observe the details in the digital and analog traces.

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

represents the long period of the data signal triggered by the wrong clock is displayed in the digital trace in the upper right grid. Trace Z4 also shows the zoom result of simulated trace C4. Parameter P3 measures the setup time between data C1 and clock C3. The statistical results again show that the setup time is 20% shorter than the nominal value. The setup time trace in F2 shows that this shortened setup time occurs synchronously with the extended period.

This is a way to find such problems. Another method is to use the oscilloscope 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: Use WaveScan and search for the setting of abnormal points by searching the period measurement value exceeding the nominal 208ns on D2.

Search tool can search in very long records, looking for edges, unstable edges, ultra-short frames, serial data patterns, parallel (bus) data patterns or measurement data. In this example, we search for a period over 250ns measured on D3. When this condition is met, it will stop collecting, display the digital source trajectory, and zoom the source trajectory. The abnormal situation is highlighted in red, and the measured abnormal value is displayed in the adjacent table. Once the problem is found, the simulation trace will be opened to observe the physical layer problem that caused the problem, just as we did before.

Mixed signal oscilloscope allows you to observe up to 16 digital traces, which is more than analog channels. In Figure 6, 8 digital tracks record the working process of two cascaded 8-bit shift registers. These shift registers are the circuits of pseudo-random binary sequence generators. First of all, it needs to be noted that the trace label is customized to reflect the function in the circuit. We can see the clock and serial data input and the Q6, Q7 and Q8 outputs from the A and B parts of the shift register. We can think of it as a "long-short" pattern (the second from the top) propagating from left to right through all 16 levels of circuits starting from the serial input trace.

Parameter P1 uses the strobe delay parameter to measure the time from the start of the trigger to the falling edge of the pattern end on the serial input trace. Do a similar measurement for that edge on the Q6-A trace. The parameter formula is used for P3 to calculate the time difference between these two edges, and the result is 515.3 μs. 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 serial input to Q6-A. If it is 515.3 μs, it is the correct operation. Outputs Q7-A and Q8-A indicate that a delay of one clock cycle has been added. In a similar way, the correct propagation delay of all 16-level circuits can also be verified.



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

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



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

  to sum up

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 can be synchronized with up to 4 analog waveforms. The digital track can be measured with cursors or selected measurement parameters. Analysis functions and decoding operations can also be applied to digital lines.

From a functional point of view, the establishment of the digital state analysis function in a mixed-signal oscilloscope is simpler than that of a logic analyzer, and no additional platform space is required. The analog channels in the same instrument can be used for detailed physical layer analysis when problems are encountered.