Sound card to the rescue

Cables in a secure phone server used in military communications. The server consists of a 20-slot VME chassis that contains a system controller and 19 phone-switching cards. Each card connects to a cable consisting of 20-to-30 sets of twisted-pair wires. Each pair carries a voice conversation that requires a 3.4-kHz bandwidth.

Find a new method for measuring the crosstalk in adjacent twisted pairs at 300 Hz, 1 kHz, and 3.4 kHz. (The product specification called for measuring crosstalk with an obsolete $80,000 spectrum analyzer. It also called for equipment capable of measuring crosstalk at –100 dB from the signal source.)

• Creative Labs Sound Blaster Audigy 2 Platinum PCI sound card. www.creative.com.
• True Audio TrueRTA Real Time Audio Spectrum Analyzer software, Level 4. www.trueaudio.com.
• B&K Precision 3011B function generator. www.bkprecision.com.
• Fluke 8060A 4.5-digit handheld DMM. www.fluke.com.

Sometimes, consumer products can perform the measurements you need at a fraction of the cost of industrial test equipment. When Richard Schrader, electrical project engineer at Tracewell Systems (Westerville, OH, www.tracewell.com), learned that a cable-test project called for an obsolete spectrum analyzer, he looked for a substitute. His research uncovered a high-end consumer-grade sound card ($199.99) and inexpensive audio software ($99.95). A standard sound card's 16-bit resolution was insufficient for the measurements, but a high-end sound card's 24-bit analog-to-digital converter (ADC) had the resolution he needed.

A high-end consumer sound card measures crosstalk in audio cables.

The sound card achieves its high SNR because its ADC resides in a module that mounts into a PC's drive bay. The cable that connects the converter to the sound card carries digital signals, which provides immunity from the PC's internal noise.

To measure crosstalk in adjacent twisted pairs, Schrader built a system (see the figure) that uses a function generator to create a sine wave at the three specified frequencies (300 Hz, 1 kHz, and 3.4 kHz). The function generator drives a twisted pair terminated with a 600-Ω resistor, chosen to match the impedance of a telephone line. An adjacent twisted pair, also terminated with 600 Ω, connects to the sound card, which digitizes the crosstalk signal. The software performs a fast Fourier transform on the captured signal and produces a frequency plot.

Schrader calibrates the system by driving a pair with a known signal. He measures the drive signal's rms voltage with a DMM and enters that value into the TrueRTA software. Shrader chose the TrueRTA Level-4 software because it measures audio frequency with 1/24 octave bins, or 240 bins across the 10-Hz to 22 kHz range.

The sound card's 24-bit resolution and 110-dB SNR let Schrader measure crosstalk on signals 100 dB below the function generator's output signal. His $300 investment in a sound card and software proves that, sometimes, you can use commercial-grade products to make engineering-grade measurements.

Schrader also used the sound card to prove that grounding helps reduce noise. The cable shields connect to the chassis through connectors. With an ungrounded chassis, the crosstalk levels increased by 20 dB to 30 dB compared to a grounded chassis.