Design engineers working on digital communications products often need to verify specifications stipulated in complex design criteria. Anyone who's ever designed a wireless modem, for example, knows it's not easy to test it for all the different reception failure modes before bringing it to market. With digital radios or other digital products required to work over long distances, it's difficult to simulate on the laboratory bench all the different scenarios that might occur in the real world. An arbitrary waveform generator (arb) and a digital storage oscilloscope (DSO) can be vital, then, to a company's R&D efforts.
An arb can output just about any signal pattern imaginable. So, virtually any form of signal "corruption" can be added to a starting signal that's known to be good. These modified signals can be saved and then used repeatedly at any time for product verification during the design phase.
Many digital communications products include software that monitors the received data, hunting for bit or packet errors while performing command and data processing. Some of these software systems are even capable of forward error correction. Test procedures can ensure that this software is, in fact, detecting bit and packet errors correctly. We'll cover five areas of error testing—signal-amplitude tests, asynchronous-data-timing errors, forced-bit errors in varied positions, noise testing, and command-spacing limits.
Our test setup consisted of an HP 54645D mixed-signal oscilloscope (MSO), an HP 33120A arbitrary waveform generator, and HP BenchLink software. Even though we chose the HP 54645D, just about any scope with a memory buffer large enough to store the 20 to 50 samples/bit contained in the analyzed data-packet structure will work.
The scope must have an IEEE-488 general-purpose interface bus (GPIB) or some other interface for easy uploading of data points to a computer. We chose the HP MSO because of its very large memory buffer (1 Msample/channel). And, it's available for under $5000. As a starting point, a scope with a 2- to 4-ksample buffer or more should be used.
The HP 33120A arb was selected for its features, including outputs of up to 40 Msamples/s with 12 bits of resolution. It also costs less than $2000. When simulating data packets over 2 Mbits/s or very large packets such as those used for computer local-area networks (LANs), an arb with faster sample rates or more memory might be required. These improvements, however, also will mean a dramatic jump in the arb's price.
Hewlett-Packard's BenchLink software is a dedicated Windows program meant only for use with HP scopes and the 33120 arb. It makes the cut, paste, and edit process extremely simple, but it doesn't lend itself to more advanced instrument control. Advanced automated instrument controls are best left to programs such as HP-Vee and LabView. While these programs are much more powerful than BenchLink, they require programming to use. Allowing for some programming time, though, both HP-Vee and LabView could do this job equally well.
The device under test (DUT) was LPA Design's FlashWizard II radio transceiver. This remote-control photographic-equipment system is used at major sporting events, such as professional basketball games. It can remotely synchronize up to 32 cameras to a single flash of light at a shutter speed of 1/250 of a second, using on/off-keyed (OOK) signaling at 68 kbits/s to send and receive commands.
The first step is capturing a "clean" data packet, typical of the command codes the radio normally receives, on the scope. To do this, we used HP's deep-memory MSO connected to the received-signal-strength-indicator (RSSI) signal in the FlashWizard receiver. The transmitter was located only a few feet away on the bench to achieve a good signal-to-noise ratio (SNR). Using a GPIB interface and the BenchLink software, this packet waveform was copied to the computer. The capture was completed using 2000 sample points to represent the packet. The packet contained under 100 bits of information, equating to more than 20 analog sample points per bit.
It's important to have a sufficient number of samples per bit to prevent the arb from adding too much distortion to the packet. As a guideline, about 20 to 50 samples should be used for each bit in the packet.
With a "clean" error-free packet now on the computer, we copied this waveform to the Windows clipboard. Then, we transferred it to the BenchLink/arb program for permanent safekeeping and future reuse (Fig. 1).
This error-free waveform was transferred into the arb using the GPIB bus and BenchLink/arb. The playback rate and amplitude were adjusted to match the original waveform, with the signal verified on the MSO. It's best to use the arb in the single-shot burst mode for this operation.
The next step was verifying that the error-free packet worked on the receiver, the DUT. This was done by connecting the arb to the DUT's baseband detector input (the RSSI line in Figure 2) and disconnecting the radio receiver subsection so it wouldn't conflict with the arb's simulated signal. After seeing error-free reception, fault testing followed.
The variations in signal amplitude are often very dependent on distance, battery levels, and the transmission medium. On the signal's receiving end, the circuit used to decode (quantitize) it back to digital logic levels may be sensitive to amplitude variations. In this particular application, a voltage comparator on the DUT was used to digitize the signal back to logic levels. Then, using the arb's front-panel control knob, the amplitude was ramped up and down to find the upper and lower signal-level limits of operation for the DUT.
The basic front-panel controls and one-line alphanumeric displays on low-cost arbs definitely aren't very useful for downloading waveforms by hand. But they are quite user-friendly for operation once the waveform is in the arb's memory.
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