MIL-STD-461 CS109 Structure Current Testing with the Tekbox TBMR-110M

MIL-STD-461 CS109 evaluates whether equipment remains functional when low-frequency current flows through its conductive enclosure or structure. The Tekbox TBMR-110M turns the measurement into a controlled sweep, using either a conventional amplifier-and-transformer path or direct current injection with the TBRFCI1.

The source application note covers 60 Hz to 100 kHz and a maximum test current of 120 dBµA, equivalent to 1 A RMS. The required limit and applicability must always come from the contractual standard edition and the approved test plan.

What CS109 is designed to reveal

Current flowing through a chassis can create small voltage differences across joints, shields and reference points. Sensitive low-frequency circuits may then respond to the resulting magnetic fields or common-impedance coupling. CS109 reproduces that condition in a controlled way so susceptibility can be identified before it becomes a system-level problem.

The source note summarises the method as applying to equipment and subsystems operating at 100 kHz or less with sensitivity of 1 µV or better, while identifying handheld equipment as exempt. Treat that as a guide to the note: applicability must be checked against the edition invoked by the programme.

The conventional transformer-coupled arrangement

The standard arrangement drives the EUT structure through a power amplifier, coupling transformer and 0.5 Ω load resistor. A current probe measures the actual injected current and returns it to the receiver. The EUT and test hardware share a carefully controlled single-point ground connection.

The current Tekbox TBJT04 uses a 2:1 turns ratio, producing the 4:1 impedance transformation required by this application. The distinction matters: the “4:1” figure describes impedance ratio rather than turns ratio.

Automating CS109 with the TBMR-110M

The TBMR-110M combines a low-frequency EMI receiver with a tracking generator. Its PC software includes a dedicated CS109 application, allowing the same instrument to create the swept drive signal, measure the current feedback and regulate the output at each frequency point.

Alternative direct injection with the TBRFCI1

The TBRFCI1 is a four-quadrant current injector that can drive the structure directly, up to the 1 A RMS maximum described in the note. Its current-monitor output provides the feedback signal, removing the separate coupling transformer, load resistor and current probe from this version of the setup.

Because the injection path is tied to the EUT structure, the application note calls for an additional isolation transformer for the PC, TBMR-110M and TBRFCI1 supply arrangement. Grounding and isolation should be reviewed before energising either configuration.

Software setup: the values that matter

Closed-loop levelling and the final trace

At each frequency, the software measures injected current and adjusts the tracking-generator drive until the selected limit is reached or an equipment limit intervenes. The result therefore shows the stress actually achieved, rather than assuming that a fixed source level produces the correct current across the complete sweep.

Working below the main tracking-generator range

The application note states that the TBMR tracking generator is accurate over a 50 dB range. Very low current levels can push the DDS attenuation beyond that main range. Its practical workaround is to pause the sweep at 70 dBµA, add a 10 dB attenuator at the tracking-generator output, then resume. The external attenuation moves the generator back into its more accurate operating region.

The useful result is not simply that a sweep completed. It is a traceable record of the current that actually flowed through the structure, together with the EUT behaviour at each frequency.

Source acknowledgement: method and figures adapted from the Tekbox application note “MIL-STD-461 CS109 Structure Currents”, version 1.0. Always use the applicable MIL-STD-461 edition, approved equipment test plan and equipment manuals as the authoritative requirements.