The Prism Sound Dscope III is an easy-to-learn and use analog and digital test set that works in conjunction with a user-supplied PC. The unit itself is 9" deep and 12" wide and therefore can easily have a laptop computer sitting directly on top of it. The Dscope and the computer communicate via a USB cable. All its inputs and outputs reside on front of the unit.
If you are familiar with the Windows operating system (I used XP for this evaluation) then learning how to use the Dscope will be no problem. A series of icons along the top of the display window lead quickly to the controls and test functions.
The "Quick Tour" icon takes the user through the basic functions of the unit. After studying the tour I was able make use of the equipment's basic functions in about 30 minutes.
The Trace Window is a display that is essentially like that of an oscilloscope, but better. The user can look at the scope trace (time domain vs. amplitude) while at the same time looking at an FFT trace (frequency domain vs. amplitude). See Figure 1. This enables the user to study individual distortion products in real-time. The trace window is also used to display the outputs of other swept measurement tests, such as THD vs. frequency and THD vs. level.
This product features an analog and a digital signal generator (AES format) that operate simultaneously. The output signal type includes everything from sine wave to square wave, ramp, burst, white noise, pink noise, simultaneous multiple frequencies and user-defined waveforms. Output levels on the analog generator can be referenced to dBfs or more conventional units such as dBu or dBM. Different signals can be generated in the A output vs. the B output. While the digital output follows the analog in terms of the type of waveform, the user can change the sample rate and word-length independently. As part of the suite of digital tests, the user can also add deviation to the sample frequency (up to ±1,500 ppm) and errors and noise to the digital carrier output itself, such as variations in the actual output level, the rise time, jitter, common-mode interference and differential interference. These parameters allow the user to test the effects of lossy cable (for example) on a device's ability to receive and decode an AES data stream.
Performance at a glance
Uses Windows as the GUI
Simultaneous digital (AES-3) and analog outputs
User-selectable digital and analog signal analyzer
Continuous Time Analyzer akin to standard distortion analyzer
FFT analyzer performs complex array of tests, including user-defined
The signal analyzer side is versatile. With mouse control, the user can switch back and forth between the analog input and the digital input. The most basic analyzer window displays the received level (displayed in dBfs or more conventional units such as dBu), frequency and inter-channel phase. The Dscope includes a Continuous Time Analyzer, which is more like the traditional distortion analyzer. It can display distortion measured in the received signal in terms of dB or a percent. The CTA window also allows the user to add a high-pass and low-pass filter, and band-pass or band-reject filters in the distortion measurement process. In addition to the CTA is the FFT analyzer. The FFT analyzer can perform more complex functions than the CTA, including user-defined measurements from Visual Basic scripts. It can calculate 40 two-channel measurements at once. The CTA window and the FFT window can be displayed at the same time, and look similar. Graphical results of swept measurements using either or both of the CTA and FFT are displayed on the trace window.
Figure 1. Screen traces can be captured and stored as image files. This is the Trace Window, which looks much like the screen of an oscilloscope.
The unit also provides a set of monitor outputs that allow the user to monitor the signal being generated or the signal being analyzed. The unit included a headphone output, and four BNC connectors that correspond to the A and B outputs and the A and B inputs.
I first used the Dscope to align the analog vs. digital levels of a Moseley digital receiver located at our transmitter site on Cougar Mountain. Because this receiver has two sets of outputs that need to match, the Dscope's ability to quickly switch between the analog and digital inputs, along with the dBfs reference, made the level setting that much easier. This was done as part of an HD Radio installation, and even with the distraction of that project I was able to plug-and-play and to get the Dscope going in about five minutes. Strong RF fields seemed to have no detrimental effect on its operation.
For purposes of this evaluation, I decided to do something just a bit more complicated. The device under test (DUT) in this case was a pair of transmit and receive PT353 and PR353 cards for an Intraplex multiplexer. The PT353 card has digital and analog inputs, and the PR353 has digital and analog outputs. I attached the units back-to-back on the test bench, and quickly matched the analog and digital output levels. Using the AES ins and outs, I was able to measure THD vs. amplitude sweep followed by a THD vs. frequency sweep at -3dBfs. The results are turned out fast and they look good; I was comfortable with the results.
Irwin is director of engineering for Clear Channel Radio in Seattle.
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