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USB Test Instrument Interface For PCs

With this USB interface you can turn your desktop or laptop PC into a whole suite of test instruments - a 2-channel digital scope, spectrum analyser, AC DMM and frequency counter plus a 2-channel audio signal/function/arbitrary waveform generator. Interested? Read on.

By Jim Rowe

Back in the October 2011 issue we presented an article on testing audio gear using PC-based sound card instrumentation and featured the TrueRTA software package. This is capable of quite respectable results but does have a few limitations, mainly due to those of the sound card.

In this article, we step up to a much more advanced set-up with a USB interface and a Windows-based audio testing package called Multi-Instrument 3.2, developed by Singapore-based firm Virtins Technology. This is a very professional software package and is reviewed elsewhere in this issue.

The interface described here is a development of the USB Recording and Replay Interface described in June 2011 and uses the same USB CODEC. For those familiar with that design, the input channel circuitry has been changed to be more similar to that of an oscilloscope/analyser and the output channel circuitry changed to be more like that of an AF signal/function/
arbitrary waveform generator. Both the input and output channels have also been improved in terms of bandwidth, noise floor and crosstalk.

Features & Specifications

A 2-channel virtual test instrument USB interface to suit to any Windows-based PC, powered from the PC’s USB port. The two input channels and two output channels can all operate simultaneously. Also provided is an S/PDIF input and output.

When used in conjunction with a suitable software package the interface allows the PC to be used as a 2-channel audio DSO and spectrum analyser combined with an AC DMM and a frequency counter, plus a 2-channel AF signal and function generator able to provide low-distortion sinewaves, a number of standard waveforms, white and pink noise, arbitrary waveforms and even DTMF signals and musical tones. Features of the Interface include:

• Input channels provide three switched sensitivity levels – x1.0, x0.1 and x0.01

• Nominal input sensitivity (x1.0 range) is 500mV RMS (1.414Vp-p/-3.8dBu).

• Maximum input level (x1.0 range) before clipping is 600mV RMS
   (1.70Vp-p/-2.2dBu).

• Input impedance (both channels) is 1MΩ//20pF.

• Effective noise floor of the input channels is at -99dBu (2.5µV).

• Two high-quality 16-bit ADCs capable of operating at sampling rates of 8, 11.025, 16, 22.05, 32, 44.1 and 48ksamples/s.

• Frequency response of input channels is as follows:

21Hz – 8kHz +0/-0.15dB

12Hz – 12.6kHz +0/-0.5dB

6Hz – 16.3kHz +0/-1.0dB

1.5Hz – 20kHz +0/-2.0dB

<1Hz – 22kHz +0/-3.0dB

• Output channels provide three switched output levels – x1.0, x0.1 and x0.01.

• Nominal output level on the x1.0 range is 500mV RMS (1.414Vp-p/-3.8dBu).

• Maximum output level (x1.0 range) before clipping is 750mV RMS
              (2.12Vp-p/-0.28dBu).

• Output impedance (both channels) is 675Ω.

• Two high quality 16-bit DACs capable of operating at sampling rates of 32,                             44.1 and 48ksamples/s.

• Frequency response of the output channels is as follows:

4.6Hz – 17.0kHz +0.15dB/-0.5dB

3.1Hz – 18.7kHz +0.15dB/-1.0dB

1.2Hz – 20.5kHz +0.15dB/-2.0dB

<1Hz – 22.0kHz +0.15dB/-3.0dB

• Crosstalk between channels, overall:

              below -62dB from 20Hz – 5kHz

              below -59dB from 1Hz – 10kHz

              below -56dB from 10kHz – 20kHz

• Crosstalk between output and input channels:

              as for between channels shown above

• THD+N for both channels, overall (ie, output-> input) for output/input levels of                             0.5V RMS: at 100Hz, 0.075%; at 1kHz, 0.075%; at 5kHz, 0.1%

• S/PDIF input and output both handle 16-bit stereo signals at sampling rates of 32, 44.1 and 48ks/s.

• Fully compliant with the USB 1.1 specification

• Installs automatically on Windows XP SP3 and later Windows operating systems (plus Mac and Linux systems) using the standard USBaudio.sys drivers – no special or custom drivers required.

• Fully compatible with Windows-based Virtual Instrument software such as Virtins MI 3.2 (standard and Pro versions).

• Low current drain from PC via USB cable: less than 65mA

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