Electronic Design

Back-Drive CT For Balanced Source

When conventionally applied, current transformers (CTs) measure ac currents. What may not be so obvious is that a CT can be back-driven to create a balanced wideband ac signal source from any 50-Ω generator.

One popular CT design will yield a unity conversion factor (Fig. 1a). The measured signal is looped through the CT toroid and becomes a 1-turn primary. A 25-turn secondary is internally terminated by 50-Ω and is connected to a 50-Ω measurement instrument. The 25-Ω combined load paired with the 1:25 turns ratio leads to a 1-mV/mA conversion factor. For instance, the Tektronix CT-2 uses this approach, and it provides a 1.2-kHz to 200-MHz passband.

In the back-driven application (Fig. 1b), the CT's internal 50-Ω termination matches the 50-Ω generator source impedance. However, the output voltage is 1/25th (-28 dB) the input voltage because of the turns ratio. The load R can be quite small since it's reflected to the primary by 625 times (i.e., the square of the turns ratio). The dc path to ground is broken and the CT output can simulate ac differential sources, such as disk-drive heads, or twisted-pair communication links.

In Figure 2, the upper waveform shows a disk-head signal from a 50-Ω generator; the lower waveform is the CT output, which is 25 times smaller but has excellent fidelity.

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