USBee PC and USB Oscilloscope, Logic Analyzer, Signal Generator, Protocol Decoder and Analyzer

This section details the operation of the Mixed Signal Oscilloscope application that comes with the USBee AX. Below you see the application screen.

The USBee AX Mixed Signal Oscilloscope functions as a standard Digital Storage Oscilloscope combined with a Digital Logic Analyzer, which is a tool used to measure and display analog signals in a graphical format. It displays what the analog and digital input signals do over time. The digital and analog samples are taken at the same time and can be used to debug mixed signal systems.

Mixed Signal Oscilloscope/Logic Analyzer Specifications

Analog Inputs	2
Analog Channels	1
Maximum Analog Sample Rate [1]	16 Msps
Analog Bandwidth	3 MHz
Input Impedance	1M Ohm/30 pF
Analog Input Voltage Range	-10V to +10V
Analog Sensitivity	78mV
Analog Resolution	256 steps
Channel Buffer Depth [2]	>1 Million
Volts per Division Settings	100mV to 5V in 6 steps
Time per Division Settings	100ns to 2s in 23 steps
Trigger Modes	Auto, Single, Digital Triggers
Analog Trigger Voltage	Between -10V and +10V
Cursors	2 Time and 2 Voltage
Voltage Display Offset	Up to maximum inputs
Time Display Offset	Up to available buffer depth
Trigger Position Setting	10% to 90%
Measurements	Min, Max, Top Bottom, Freq, Period
Digital Channels	8
Maximum Digital Sample Rate [1]	16 Msps
Internal Clocking	Yes
External Clocking	No
Digital Trigger Levels	4
Digital Trigger Qualifiers	Rising Edge, Falling Edge, High,Low
Trigger Prestore	Yes
Trigger Poststore	Yes
Sample Clock Output	Yes
Maximum Digital Input Voltage	+5.5V
Digital Input Low Level	< 0.8V
Digital Input High Level	> 1.4V

[1] Maximum sample rate depends on your PC hardware CPU speed and USB 2.0 bus utilization. For the fastest possible sample rates, follow these simple steps:

Disconnect all other USB devices not needed from the PC

Do not run other applications while capturing or generating samples.

[2] Maximum buffer size depends on your PC available RAM at the time the application is started.

Features

Pod Status

The Mixed Signal Oscilloscope display shows a current USBee AX Pod Status by a red or green LED. When a USBee AX is connected to the computer, the Green LED shows and the list box shows the available Pod ID List for all of the USBee Ax’s that are connected. You can choose which one you want to use. The others will be unaffected. If a USBee AX is not connected, the LED will glow red and indicate that there is no pod attached.

If you run the software with no pod attached, it will run in demonstration mode and simulate data so that you can still see how the software functions.

Analog Channel Control

You can choose which channel will be captured and displayed by pressing the CH1 or CH2 button. The next trace shown will be from that new analog channel. All 8 digital lines are always sampled every trace.

Acquisition Control

The Mixed Signal Oscilloscope captures the behavior of analog and digital signals and displays them as a “trace” in the waveform window. The Acquisition Control section of the display lets you choose how the traces are captured. Below is the Acquisition Control section of the display.

The button is the Run/Stop control. When the mixed signal oscilloscope is first started, the Run button is not pressed and is waiting for you to start a capture. The Run button captures a single trace and stops. This mode is good for detailed analysis of a single event.

The Buffer Size lets you select the size of the Sample Buffer that is used. For each trace, the buffer is completely filled, and then the waveform is displayed. You can choose buffers that will capture the information that you want to see, but remember that the larger the buffer, the longer it will take to fill.

You can also choose the Sample Rate that you want samples taken. You can choose from 1 Msps (samples per second) to up to 16 Msps. The actual maximum sample rate depends on your PC configuration. You can run the menu item Setup | Sample Rate Test to determine the maximum sample rate for your system.

Trigger Settings

The Mixed Signal Oscilloscope uses a Trigger mechanism to allow you to capture just the data that you want to see. You can use either a digital channel trigger (as in the logic analyzer), or an analog trigger (as in the oscilloscope). You can not use a combination of analog and digital.

For an Analog trigger, you can specify the trigger voltage level (-10V to +10V) by using the slider on the left hand side of the analog waveform display. A red line that indicates the trigger level will momentarily be shown as you scroll this level. A small T will also be shown on the right hand side of the screen (in the cursors bar) that shows where this level is set to.

For an analog trigger, the trigger position is where the waveform crossed the Trigger Voltage level that you have set at the specified slope. To move the trigger voltage level, just move the slider on the left of the waveform. To change the slope, press the button to the right of the CH1 and CH2 buttons.

You can also specify if you want the oscilloscope to trigger on a Rising or Falling Edge. Below shows a trace captured on each of the edges.

The Trigger position is placed where the actual signal crosses the trigger voltage with the proper slope. The USBee AX allows for huge sample buffers, which means that you can capture much more data than can be shown on a single screen. Therefore you can scroll the waveform back and forth on the display to see what happened before or after the trigger.

For a Digital trigger, you can specify the digital states for any of the 8 signals that must be present on the digital lines before it will trigger. Below shows the trigger settings (to the right of the Signal labels). This example shows that we want to trigger on a falling edge of Signal 0, which is represented by a high level followed by a low level. To change the level of any of the trigger settings, just click the level button to change from don’t care to high to low.

The digital trigger condition is made up of up to 4 sequential states of any of the 8 signals. Each state for a single signal can be high, low or don’t care. This allows you to trigger on rising edges, falling edges, edges during another signals constant level, or one edge followed by another edge.

The waveforms are shown with a trigger position which represents where the trigger occurred. This sample point is marked on the waveform display with a Vertical red dotted line and a “T” in the horizontal cursors bar.

You can use the Trigger Position setting to specify how much of the data that is in the sample buffer comes before the actual trigger position. If you place the Trigger Position all the way to the left, most of the samples taken will be after the trigger sample. If you place Trigger Position all the way to the right, most of the samples taken will be before the Trigger sample. This control lets you see what actually happened way before or way after the trigger occurred.

The status box on the display will show red when the unit is not acquiring samples, flash blue when it is waiting for a trigger, and glow green when the trigger condition has been met. It will glow red again when the capture is completed.

Waveform Display and Zoom Settings

The Waveform display area is where the measured signal information is shown. It is displayed with time increasing from left to right and voltage increasing from bottom to top. The screen is divided into Divisions to help in measuring the waveforms.

The position of the waveform defaults to show the actual trigger position in the center of the screen after a capture. However, you can move the display to see what happened before or after the trigger position.

To Scroll the Waveforms in Time left and right, you can use the scroll bar at the top of the analog waveform display (between the analog and digital waveforms), or you can simply click and drag the waveform itself.

To Scroll the Analog Waveform in Voltage up and down, you can use the scroll bar at the right of the waveform display, or you can simply click and drag the waveform itself.

To change the number of Seconds per Division or the number of Volts per Division for the analog channel, use the knobs at the bottom of the display. Simply click the knob and drag to the desired setting. You can also zoom in and out in time by clicking on the waveform. To zoom in, click the left mouse on the waveform window. To zoom out in time, click the right mouse button on the waveform window.

Measurements and Cursors

The main reason for using an oscilloscope or logic analyzer is to measure the various parts of a waveform. The USBee AX uses cursors to help in these measurements.

The X1 and X2 Cursors are placed on any horizontal sample time. This lets you measure the time at a specific location or the time between the two cursors. To place the X cursors, move the mouse to the gray box just below the waveform. When you move the mouse in this window, you will see a temporary line that indicates where the cursors will be placed. Place the X1 cursor by left clicking the mouse at the current location. Place the X2 cursor by right clicking the mouse at the current location.

The Y1 and Y2 Cursors are placed on any vertical voltage level. This lets you measure the voltage at a specific location or the difference in voltage between the two cursors. To place the Y cursors, move the mouse to the gray box just to the right of the scroll bar to the right of the waveform. When you move the mouse in this window, you will see a temporary line that indicates where the cursors will be placed. Place the Y1 cursor by left clicking the mouse at the current location. Place the Y2 cursor by right clicking the mouse at the current location.

In the Measurement window, you will see the various measurements made off of these cursors.

There are also a set of automatic measurements that are made on the analog waveform for each trace. These are calculated without the use of the cursors. These are:

File Save, Open and Export

Using the File menu functions, you can save, open or export. a current set of configuration and trace sample data.

Choose the menu item File | Save As to save the current configuration and sample data to a binary ULC file.

To load a previously saved waveform and display it, choose File | Open and specify the filename to load. This waveform will then be displayed as it was saved.

You can also export a specific portion of the sample data by placing the X1 and X2 cursors. When you choose File | Export to Text the samples between the X1 and X2 cursors will be written to a file in comma delimited text format as shown in the Users Manual.

USB, I2C, Async, and SPI Decoders

Using these View menu functions, you can decode these serial busses and extract the actual data transmitted via the protocols. These features will only function on the USBee AX-Plus and USBee AX-Pro models. more...

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USBee AX Mixed Signal Oscilloscope