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Audio.TST June 2009: Survey, Test System Grounding

By Bruce Hofer, Chairman & Co-Founder, Audio Precision

It’s been a busy 5 weeks! In May I attended the Munich AES convention, and I just returned from an 8-day trip in early June to Japan to visit one of our oldest sales partners, Toyo. 

My Japan trip was particularly interesting because I serve as a technical advisor to Toyo’s calibration department. Toyo happens to be accredited by A2LA, the same firm that accredits Audio Precision to perform certified calibrations. Toyo was scheduled to undergo a periodic audit by an A2LA representative, and I was asked to be present to answer any questions related to the audio section of their cal lab. Toyo successfully completed their audit on Friday June 5. Now it would not surprise me if Toyo had made previous arrangements, but the very next day (Saturday) I was treated to a moderate earthquake as I sat at my computer in my 15th floor hotel room.  I guess AP and Toyo just rock…

To help justify the expense of my trip, Toyo had also asked me to conduct a pair of technical seminars on the subject of “Calibration and Audio Test Equipment”. These technical seminars have become an almost annual event with many of our Japanese customers and friends. However, this time the topic was quite different from the usual new product introduction or lecture on digital interface testing, FFTs, or grounding.  On Monday June 8 I met with a fairly large group in Toyo’s central Tokyo conference facility (one of the best I have ever seen in the world), took an evening Shinkansen to Osaka (Japan’s “bullet” train), and then met again with a smaller group in their Osaka offices on Tuesday.  

Seminars such as the two I recently gave in Japan have convinced me that more and more engineers are discovering the subject of calibration is actually quite interesting and useful. Speaking from my own experience here at AP, we have learned many important lessons that have significantly improved our quality since beginning our endeavor to develop our own internal calibration capability nearly 5 years ago. I encourage all readers, but especially those involved in analog hardware design, to become more familiar with the basic concepts of calibration. You might just learn something useful and truly worthwhile… and it might just be an earth shaking experience.

Bruce

As promised, Audio Precision is conducting our third “Future of Audio Test” survey.

One of the challenges of test & measurement is staying ahead of the game. Your insights today help us provide the tools you’ll need to tackle tomorrow’s audio test challenges.

We will present highlights of the survey in next month’s Audio.TST (note: we’re only looking at general trends, we will not share individual results with anyone, including sales partners).

Start Survey

Thank you for your input!

Question:
What kind of grounding is best to use between the various devices in my test system?

Answer:
When integrating instruments, accessories, and DUTs (devices under test) into a test and measurement system, observing good grounding practice is always important in achieving optimal measurement results. Small ground potential differences between devices in the test system (such as switchers, accessories, the DUT, and the test instrument) can couple into the signal path and cause undesirable interference or noise due to the inherent stray capacitance between signal conductors and the chassis. To prevent this problem, Audio Precision strongly recommends connecting the chassis ground of each device directly to the ground of the test instrument via wires having as low an impedance as possible. This technique is often referred to as “ground bonding” or “chassis bonding.”

The most effective grounding arrangement for multiple devices is “star grounding,” where each device in the system is connected to the measurement instrument ground via a separate grounding wire.

We do not recommend bus grounding (daisy chaining), where several devices are serially connected to a low-impedance conductor called a “ground bus.” The resistance in each leg of the chain puts the devices at different ground potentials, and is not as effective as star grounding.

When the serial links are very short, the combination star/bus grounding configuration can simplify connections while providing good grounding performance.

The CAB-BOND kit, available from Audio Precision, provides the parts required for a star ground connection to one device, or a combination star/bus ground connection to two devices. For more devices, additional CAB-BOND kits may be ordered. Included are two ground cables, 20.5” (52 cm) and 4.25” (11 cm) in length, sized to connect one or two SWR-2755 switchers to an AP analyzer.

Alternatively, you can make your own ground cables. These must be very low-impedance, heavy gauge copper wire, as short as possible for the application, and terminated with large surface area low-impedance spade lugs. If the lugs are a crimp type, make sure to use the proper crimping tool to ensure a secure, gas-tight connection. Fasten the one end of the lug to the switcher with a large, truss head screw. Attach the other end to one of the ground posts on the front of the analyzer.

Our older SWR-2122 and SWR-122 switchers do not have a place for a grounding screw on the chassis. These units, however, are grounded through the power line, which is normally sufficient. If they are rack mounted, you can provide additional grounding through the rack. Make sure that the entire ground path is continuous by removing paint under the rack screws and rack ears as needed—the larger the contact surface, the better. If a good rack ground is not available, and additional grounding is needed, a hole may be added on the side of the switcher to add a grounding screw. Before doing this however, hold the loose end of a ground cable against the chassis and measure the noise. Sometimes having multiple grounds can cause ground loops that make noise worse instead of better.

• Setting up 1/3 or 1/6 octave sweeps for 2700 Series analyzers
• Portable One GPIB programming with VEE

AP has enjoyed some excellent press recently:

Software upgrade brings logic-analyzer-style insights to audio testing
EDN (June 9, 2009):
Covers the new metadata logic analyzer view in APx v2.4.
http://www.edn.com/article/CA6663971.html

“What is that noise?”
Electronic Design, June 09:
Cover story on causes of noise. Includes quotes from Bruce Hofer, Chairman & Co-Founder of AP.
http://electronicdesign.com/Articles/ArticleID/21258/21258.html

Audio Measurements the AP Way
Voice Coil, June 09:
A 3 page review of APx. “With their latest slew of instruments, AP demonstrates that they are able not only to follow, but if necessary to anticipate the fast-paced technology changes in the industry… AP remains at the cutting edge of audio measurement.”
(sorry, print only)

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