MEASUREMENTS Telephony

Virtual Instrumentation Performs dBrnC Measurements by Craig Rupp, Senior Electrical System involved the use of SCXI™-1120, coupled One common specification found in Engineer, Alliance Technologies Group with data acquisition. For the complex telephony applications is dBrnC – the dBrnC measurement, purchasing an amount of noise measured through this The Challenge: Performing expensive audio analyzer with excess C-message-weighted filter relative to a dBrnC noise measurements on functionality was unnecessary. reference noise level of 1 picoWatt. A several power supply test stations. simple approach to performing the dBrnC The Solution: Synthesizing the measurement is to purchase a filter digitally using the LabVIEW After reviewing the client’s test commercially available audio analyzer. ™ Advanced Analysis Toolkit, SCXI signal requirements, we found that the This audio analyzer performs the dBrnC conditioning, and data acquisition. most cost-effective solution for measurement but also includes functions unnecessary for PCP’s application. Introduction measuring the voltage regulation, Because PCP only required one function Power Conversion Products (PCP), a efficiency, and power factor involves from this instrument, that measurement leading manufacturer of high-power the use of SCXI-1120, coupled with had a high price tag. switchmode rectifiers for the data acquisition. Having unused channels and jumper- telecommunications industry, hired selectable gains, the SCXI-1120 and Alliance Technologies Group (ATG), a Because the human ear is not equally MIO DAQ board provided a method for National Instruments Alliance Program sensitive to all voice frequencies and obtaining an amplified, isolated, digital member, to develop an automated test because telephone handsets do not have a representation of the signal. Using the system for its Twinpack Plus 2.5 kW flat electrical-to-acoustical frequency LabVIEW Advanced Analysis Toolkit, we rectifiers. The test system needed to response, not all frequency components synthesized the C-message-weighted filter measure the performance of the rectifier of noise are equally objectionable. In the digitally and incorporated the dBrnC including voltage regulation, efficiency, 1960s, Bell Labs devised a noise- measurement with virtual instrumentation. power factor, and dBrnC noise over to correlate measurements various line and load conditions. with a listener’s subjective assessment. Measurement Process This frequency response, called a Simulations showed the most efficient and System Components C-message-weighted filter, has a 0 dB accurate method of implementing the After reviewing PCP’s test requirements, reference at 1 kHz with components filter involved first computing the power we found that the most cost-effective above and below this frequency of the input signal. solution for measuring the voltage attenuated to match the response of the Integrating the output over all frequencies regulation, efficiency, and power factor ear and telephone handsets. then derives the output power.

fs, Hz DBL df 2.0 X power, dBrn ti 1 1 x x(+)d± 1 Y to input samples X Y 1 10 x DBL Hanning Numeric Integral LOG [DBL] x Trapezoidal Rule 600.0 1E-12 10.00 Hanning Offset Gain

2 N []

x THRESH 0 [] C Message Filter Attenuation i 10 H(f) 0..N N..0 1 INTERP 10 X 1 1 -20.0

LabVIEW code implementing dBrnC measurements.

30 National Instruments Tel: (512) 794-0100 • Fax: (512) 683-9300 • [email protected] • www.ni.com/telecom MEASUREMENTS

Using a sampling rate of 40 kHz, a total 24 Vrms, a low-level signal. Maintaining of 8,192 input samples were windowed the maximum ± 5.0 V rails over the SCXI LabVIEW with a Hanning window. This reduces the interface and using the 12-bit MIO Evaluation spectral leakage that occurs when board, we computed that the Package computing the power spectral density quantization noise would be 10 dB below found in using a straight boxcar window. the lowest level signal with gains ranging The input samples are gain-corrected from 116 to 1479. By using a gain value because the Hanning filter attenuates the in the SCXI-1120 near the minimum, input samples by a factor of 1.64. The there would be plenty of headroom Hanning-filtered samples are then taken available for any high-level signals The LabVIEW Evaluation outside the C-message-weighted filters Package includes a free, fully- bandwidth but prior to the digital filter. functional LabVIEW Because the signal measured has an Evaluation CD with which The advanced analysis capabilities appreciable DC component, it was users can do tutorials, run of LabVIEW gave us the ability to necessary to add a blocking capacitor sample programs, and learn to implement the single dBrnC with a low leakage current. A large build virtual instrumentation measurement in a system that leakage current can cause an excessive applications for a variety of DC bias that takes away from the telecom applications. already contained DAQ products. headroom in the converter. We used the LabVIEW graphical 10 kHz filters on the SCXI-1120 for programming software is through the auto power spectrum VI. antialiasing and to help attenuate the available for Windows The C-message filter is frequency frequency components above 10 kHz. NT/95/3.1, Macintosh, Sun, interpolated to match the frequency bin With this network, an SCXI-1120 gain of HP-UX, and Concurrent size of the input power spectral 200, and the 12-bit MIO board, we found PowerMAX. density. The result is then squared and the noise floor of the measurement system multiplied by the input power spectral to be at –12 dBrnC. Order the evaluation package density and integrated. This result is then by calling (800) 258-7022 or referenced to 600 ohms and to the Results downloading the program reference noise of 1 picoWatt. The advanced analysis capabilities of online at PCP’s maximum and minimum noise LabVIEW gave us the ability to ni.com/labview/lv_dl.htm levels were 30 and 0 dBrnC respectively. implement the single dBrnC As a rule of thumb, we kept the measurement in a system that already quantization noise floor from the contained DAQ products. The resulting converters at least 10 dB below the product increased performance and smallest signal. This results in a required reduced costs.1 dynamic range of at least 40 dB. Given a 12-bit MIO board in the system, this gave For more information, contact Craig Rupp, a dynamic range of 72 dB, easily meeting Alliance Technologies Group, www.ni.com/labview the requirement. 1017 Butterfield Road, Vernon Hills, IL 60061, With a 0 dBrnC signal across a tel (847) 247-9284, fax (847) 247-9724, telephony-standard impedance of e-mail [email protected] 600 Ω, the minimum voltage would be at

National Instruments 31 Tel: (512) 794-0100 • Fax: (512) 683-9300 • [email protected] • www.ni.com/telecom