Three Things You Can’t Measure if Your Oscilloscope Doesn’t Have Enough Memory
- Friday, 01 August 2014
Modern oscilloscopes come equipped with a host of different attributes, and many vendors tout their latest additions as “must have” features. With so many attributes and marketing messages, recalling the importance of a long-held attribute such as memory depth can become lost in the noise. However, any engineer who has grappled with shallow memory on an oscilloscope will be vocal about the frustration of the experience. Those who aren’t vocal simply haven’t stumbled on an issue that required it — yet.
Memory depth of an oscilloscope is the primary determinant of how much “time” you can record. It is often referred to as Record Length for this reason. Many modern oscilloscopes come standard with “megasamples” of memory, which, considering that 10 years ago this was “kilosamples,” may seem adequate. But as devices combine elements that operate in “human time” (seconds and milliseconds) such as power supplies or control buses, and devices that operate in “machine time” (nanoseconds and picoseconds) such as I/O drivers, you can quickly run out of memory or create an improper setup for your signal. Because memory depth is intimately tied to the sample rate of the oscilloscope (see Figure 1), the common action of turning the scale knob on your scope to see more time may be impacting your measurements in many ways, including lowering your sample rate.
Issues During Power Supply Boot-Up
The boot time of a power supply is a classic example of something that occurs in “human time.” A common source of frustration, project managers often add months to a project schedule to account for testing new or untested power supply used in a design. Deep memory in an oscilloscope can unravel problems faster when something taking place during the power supply turn-on impacts other parts of the system. Electromagnetic interference (EMI), crosstalk/cross-coupling, and related issues can plague many different parts of a design. Often, the device impacted is operating at full speed, but the source of the problem — the power supply — is operating in milliseconds or seconds. A boot sequence of a power supply shown in Figure 2 is taking place across a few milliseconds. With a shorter record length of 2M or 4M samples, you might be limited to 100 or 200 MSa to catch this same sequence of time, and the issue on an adjacent channel at higher speed could be obscured.
An example shown in Figure 2 is where noise was spotted on an I2S audio signal (in green at top). The signal displays a somewhat repetitive burst of noise that, when seen in a zoom window, appears to be high-frequency noise, which is causing an issue in the system. By investigating the surrounding signals with shallow memory, we are unable to locate the source of the high-frequency noise. But expanding the time record, while preserving the high sample rate, allows observation of the source of the noise. The nearby 5V to 1.8V Buck (step down) converter is emitting this high-frequency noise at the same interval as the noise on the I2S signal as it is going through a reboot sequence. The ability to cross-correlate the phase of power supply bring-up with high-speed channels can often be the debug method that saves the day (or the project schedule).
Debugging Low-Speed Serial Buses
Low serial buses such as I2C, SPI, CAN, or LIN are often used as the control elements for digital designs. In many cases, as these are the source of change or action on the system, they are used to troubleshoot and understand system behavior. While protocol triggers might assist, visibility across many bursts or packets of data is often required to gain insight. Two commonly used methods to view multiple bursts — reducing sample rate on the scope or viewing multiple packets with segmented memory — can be fraught with issues. These methods are often driven by a lack of deep enough memory on an oscilloscope (or user misunderstanding of how to use the deep memory).
Method 1: Reducing the sample rate on the scope to view a large sequence of time. Since all oscilloscope measurements use a single time base, compromising time base to view multiple serial packets will also compromise the accuracy of the very item you might need to debug. This mix of high-speed items and slower items again presents a problem that deep memory can solve. In Figure 3, the I2C bus is operating at 100 KHz. A packet of date is sent about every 12 ms. Because the data and clock rates are slower, we can utilize a lower sample rate, but we need to be careful not to slow it down too much. In this example, we are viewing 20+ packets of data. The two examples in Figures 3 and 4 expose a glitch that is seen only with a high enough sample rate. The first capture (Figure 3) is taken at a 10 MSa/s rate. We might be forced to use a slow rate like this due to shallow memory on our scope. Alternatively, we might find this as an automatic setting chosen using the Autoset (or Autoscale) button on the oscilloscope. Unfortunately, there is a glitch that is not visible since the sampling rate is too slow to catch it.