Tuesday, December 2, 2008
Product tryout: USB mixed-signal oscilloscope
Link Instruments MSO-19 USB mixed-signal oscilloscope. Price $249. www.linkinstruments.com.
The MSO-19 is a small single-channel, 200 Msample/s DSO with eight logic inputs that connects to any PC with a USB port. It’s logic outputs also function as a pattern generator. You should really consider it an eight-channel logic analyzer with an analog input. All other MSO’s that I’ve seen have two channels, but those are all benchtop, stand-alone instruments. Having just one channel lets you check any logic input for noise or anomalies that logic input won’t show.
The unit comes with one passive 1x/10x analog scope probe. A logic pod contains a single 16-pin connector provides the unit with eight color-coded leads and eight ground leads that let you connect the logic analyzer to pogo pins. Each colored lead comes with a matching color mini-grabber clip, although the kit of mini-grabbers have just two black clips. Even those probes are too small for grabbing today’s IC pins. I was able to grab corner pins only for the surface-mount devices on a board. You also get a mini-USB cable. The MSO-19 is bus powered and thus doesn’t need a separate power supply.
Installation
The MSO-19 software (version 1.0.9) installation was smooth. I received no Windows XP error messages although I had to install Microsoft .NET framework 3.5, but even that went smoothly.
When plugging in the MSO-19, I was greeted by another install window, which installed DLLs from the install disk. You can download Link Instruments’ software if you lose the install CD or need an update, which is a nice feature. All equipment manufacturers should provide free software upgrades for their hardware.
When you start the MSO-19 software, you get a waveform display grid, control buttons and knobs, and configuration buttons. The MSO-19 software has no drop-down menus, yet it makes all functions easily accessible. Clicking on the setup button opens a tabbed window that lets you configure the instrument. For example, you can select trace colors, choose among several measurements, and enter channel names for the logic analyzer traces.
You can get started by clicking the GO button or the Auto Set button to start capturing signals. I started with the scope channel connected to a demo board from Rigol when I tried that company’s benchtop MSO. The demo board has several basic signals available so I started with a sine wave. Figure 1 shows that you can measure a signal’s frequency and use cursors for amplitude measurements. You can make measurements between cursor A-B, A-T(rigger), or B-T(rigger). Just click on the setup button to select your measurements. You can also use cursors to measure time difference at two points in a waveform.
Figure 1. The MSO-19 software provides cursors for amplitude measurements and if lets you measure parameters such as frequency.
The MSO-19’s user interface includes buttons for selecting oscilloscope, logic analyzer, or MSO displays. An FFT button provides you with frequency domain plots. Other options include TDR, logic analyzer state list, I2C and SPI bus windows. There’s a small bug that occurs when you place the mouse over the SPI option: A balloon appears that says I2C, but that’s an easy fix for the manufacturer. [December 11, 2008: The manufacturer reports that the bug has been fixed] Other options include Autoset and Calibration.
The MSO-19’s screen-capture button is a nice feature. By default, the images are of the grid only; they don’t give you the vertical nor horizontal settings as the default. You can turn them on through the setup button as I did in Figure 1, though they should default to on.
A save button lets you save and recall setups when you need to repeat measurements or develop setups for others to use. It also lets you open a set of tabs that include measurements (frequency, cycles, period, pk-pk, mean, and median). Other tabs let you set colors, assign names to logic signals, and set up color and data range for printing.
Oscilloscope
The MSO-19 user interface has virtual knobs that you can “turn” with the mouse to change V/div and time/div settings. As you’d expect, you get Horiz/Vert settings, offset, and trigger. That makes basic functions easy and intuitive. You can also adjust screen settings with your keyboard’s tab key and with the up/down arrow keys if you don’t have a mouse. Finding which control is active is, however, hard to see so you might end up selecting the wrong control if you don’t use a mouse.
Oscilloscope triggers include rising and falling edges, pulse width (> or <) and pulse width for a rising or falling pulse. I used the pulse trigger to get a stable trigger on a pulse train with pulses of varying widths. Once I found the shortest pulse width (920 ns) with the cursors, I selected the best width setting (<1000 ns) and obtained a stable trigger (Figure 2). You can move the cursors from button controls or with the mouse. I easily found that the time between peaks of a sine wave was 4 µs of a 250-kHz sine wave.
Figure 2. Pulse-width triggers let you attain a stable waveform on a digital pulse stream.
The digital signal in Figure 2 produced by the demo board has some noise and overshoot on the rising edges. I tried to zoom in on the overshoot by first changing the time base to open up the spike, but that changed the pulse-width trigger setting and I could no longer trigger on the narrowest digital pulse. It held the trigger setting until I reached 1 µs/div, then the pulse-width setting changed. The workaround is to use a zoom function, which covers 1:1, 2:1, 5:1, and 10:1. Then, I was able to see the overshoot (Figure 3).
Figure 3. Zoom settings let you see details such as overshoot in digital signals.
Logic analyzer
The logic pod has eight colored signal leads and eight black ground leads. Each wire color matches a trace on the screen by default. You can change the colors of that logic traces as they appear on the screen. That’s helpful if you prefer certain colors. For example, you might want to use the yellow probe (normally channel 3) to display as channel 1, but that will cause a mismatch between the logic pod’s lead wires and the screen.
When I first tried to use the logic inputs, I connected the scope probe to the demo board’s digital outputs. But, I found what appeared to be loading problems that pulled down the digital signals from the demo board. Looking at the signal with the oscilloscope channel and no logic probe connected to it, I found that the amplitude of the signals from the demo board was about 4 Vpk-pk. Troublesheeting the problem uncovered two item worth noting.
* The MSO-19 comes with a 1x/10x passive probe and the user interface lets you select which setting to use. Just be careful to make sure that then you change setting on one, you should change the setting on the other.
* The vertical scale on the MSO-19 doesn’t follow the usual 1, 2, 5 pattern. Instead, changing the V/div virtual knob produces a more linear setting range. For example, set the oscilloscope probe to 1x and choose the 1x/10x setting on the user interface. The V/div knob will cover a range of 50 mV/div to 500 mV/div in 50-mVsteps, that is 50, 100, 150 etc. When you select 10x on the screen, the vertical sensitivity change to 500 mV/div to 5 V/div in 500-mV/div steps. That can take some getting used to, but it can provide you with better resolution that the standard steps.
After first dealing with the odd vertical scale settings, I discovered the loading problem. The 16-pin connector for the logic pod was installed upside down, which grounded some of the dem boards logic signals. It’s not enough to rely on a label to tell users how to orient the connector. There should be just one way to use the connector. Fortunately, grounding the pins didn’t cause any dmanage to the MSO-19 or the demo board.
Logic triggers
I wanted to set up a trigger on a logic channel. The MSO-19 lets you set up a trigger based on a pattern of the eight logic channels. To start, I set XXXXXXX1, false to true, which tells the instrument to trigger whenever D0 channel transitions from false (0) to true (1) without regard to the status of other logic channels. Next, I set XXXXXX11 so that it triggered at the point when D0 and D1 are both become logic 1. Then, I tried 11000010 false to true. Figure 4 shows an example of a digital trigger while lookinat one one signal with the oscilloscope input.
The MSO-19 also lets you trigger on SPI or I2C signals. Using these triggers, you get four patterns so you can trigger on a specific character in either serial bus.
Figure 4. The logic analyzer lets you trigger on an digital pattern while looking at one channel with the oscillscope.
The MSO-19 has an interesting feature where you can display a waveform and store it in memory, then display it along with a live waveform. Figure 5 shows two sine waves from the Rigol demo board. The sine wave has some periodic frequency modulation on it, which is hard to see. Using the store-to-memory features, I captured a waveform (blue trace) and displayed it while observing a live waveform (green trace). At one point, the two waveforms align in time, but the live waveform’s frequency decreases (period increases.
Figure 5. The MSO-19 lets you compare a live waveform (green trace) to a stored waveform (blue trace).
The MSO-19 is a versatile tool for field troubleshooting and bench testing. At just $249, you can get one to use at home, too. The engineers at Link Instruments need to fix the label on the I2C balloon. They also need to use the standard 1x, 2x, 5x sequence for setting a vertical scale. The company need to change its logic-pod connector so that you can’t accidentally plug it in upside down. Link’s engineers shouldn’t wait to make this change. Lastly, Link Instruments needs to develop a two-analog-channel version of the MSO-19.
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