Look inside Tek’s Spiral Shunt design

-September 06, 2013

Editor's Note: I asked Ken Price to give us a behind-the-scenes view of how Tektronix designed its spiral shunt technology.--JSL

A key design challenge for power analyzer centers is how to provide a more stable and linear response over a wide range of input current levels, ambient temperatures, crest factors, and other variables. Further, the design should minimize inductance and offer high overload capability and thermal stability. Given the wide range of signal conditions common in today’s power conversion technologies, this required a new approach to shunt design.

In power analyzers, shunts play a central role in the instrument. In this application, the term shunt describes an impedance connected in series with a circuit in order to measure current flow. When a suitable shunt impedance is placed in parallel with a voltmeter, the voltmeter can be calibrated as an ampmeter, which measures the total current flowing in the circuit. For the Tektronix PA4000  power analyzer series, improving shunt performance was a major point of emphasis. This led our team to the development of what’s now called a Spiral Shunt.

Figure 1: The Tektronix PA4000 power analyzer incorporates a new shunt design to address the problem of how to maintain accuracy over varying temperature and humidity ranges.

The search for a better shunt pointed to the use of Manganin, an alloy of manganese, copper and nickel.  Manganin has attractive properties for this application, including stable resistance value over temperature. For the Spiral Shunt, Manganin is formed into a wide, flat ribbon shape which increases surface area. This ribbon is then insulated and formed into a loop with the material folded back upon itself, so that the input and output connections to the shunt are together on the same end.  An ideal shunt is purely resistive at all frequencies, but any real-world shunt material will have some inductance, however low. The Spiral Shunt design minimizes inductance by virtue of the loop construction – inductance is effectively cancelled by equal current flowing in both directions, outbound and returning.  Low inductance also contributes to higher bandwidth capability.  

Figure 2: The Spiral Shunt uses a wide, flat ribbon of Manganin that is insulated and formed into a loop. A Spiral Shunt for a 30 A input is shown.

Higher levels of current through a shunt will create heating, which is a source of non-linearity.  To reduce non-linearity caused by heating, a cooling fan at one end of the shunt moves air across the entire surface of the shunt. The Spiral Shunt’s cooling fan is variable-speed, with speed controlled according to the measured current.  In this way the fan’s cooling effect can be precisely controlled to maintain a relatively constant shunt temperature and thus maximize linearity. The remaining small errors are measured and stored as calibration constants within the analyzer to provide accurate measurements, compensated for amplitude and phase, over the specified 1MHz bandwidth.

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