Test of shielding effect of digital signal processor wiring harness

Shielding effectiveness testing for DSP wiring harnesses evaluates the assembly's ability to protect sensitive high-speed digital signals from external electromagnetic interference and to prevent the harness itself from radiating noise that could disrupt other electronics. This testing quantifies the performance of braided shields, foil wraps, and drain wires in real-world scenarios.

Transfer Impedance and Surface Transfer Impedance Measurement

Transfer impedance is a fundamental metric for quantifying a shield's performance at lower frequencies, where the primary coupling mechanism is diffusion through the shield material rather than radiation through apertures.

Triaxial Test Fixture Setup

The most accurate method for shielded cable assemblies uses a triaxial test fixture. The center conductor of the harness under test forms the inner conductor of the fixture. The harness's shield becomes the middle conductor. An external outer tube or chamber serves as the third, outer conductor. A known RF current is injected between the shield and the outer conductor. The voltage induced on the inner conductor due to this current is measured. Transfer impedance is calculated as the ratio of this induced voltage per unit length to the injected current. This measurement is performed across a frequency spectrum, typically from 1 MHz up to several hundred MHz, revealing how shield effectiveness degrades with increasing frequency.

Bulk Current Injection Correlation

While not a direct measurement of transfer impedance, Bulk Current Injection testing provides a correlated assessment of shield performance in a system context. A current probe clamps around the entire harness and injects a controlled RF disturbance current. The level of noise coupled onto the signal lines within the harness is measured at the receiver end. The required injection level to cause a specified bit error rate or functional upset is recorded. A harness with poor shielding will fail at lower injection currents. This method tests the entire shield system, including connectors and grounding, under conditions that simulate noise coupling from nearby cables or fields.

Radiated Emissions and Susceptibility Testing

These tests evaluate the shield's performance at higher frequencies where radiative coupling dominates, assessing both the harness's tendency to emit noise and its vulnerability to external fields.

Radiated Emissions Measurement in Anechoic Chamber

Place the DSP harness in a semi-anechoic chamber or on an open-area test site. Connect the harness to an active DSP board or a noise source that simulates typical switching activity. Use a calibrated antenna and a spectrum analyzer to measure the electric field strength radiated from the harness across a broad frequency range (e.g., 30 MHz to 1 GHz or higher). Position the antenna at various angles and polarizations to find the maximum emission. Compare the measured levels to relevant limits. Poor shield termination, gaps in braid coverage, or unshielded connector sections will show up as distinct peaks in the emission profile.

Radiated Susceptibility Field Exposure

Expose the powered DSP system with its harness to a controlled, uniform electromagnetic field inside a transverse electromagnetic cell or anechoic chamber. The field strength is calibrated to the required test level (e.g., 3 V/m, 10 V/m). While the field is applied, monitor the DSP system for functional errors, communication faults, or increased bit error rates on high-speed links. A well-shielded harness will prevent the external field from coupling onto the internal conductors and disrupting operation. This test is particularly important for harnesses used in environments with strong RF sources.

Shield Continuity and Grounding Integrity Verification

A shield is only as good as its electrical connection to ground. Testing the DC and low-frequency integrity of the shield's grounding path is essential for effective high-frequency performance.

End-to-End DC Resistance of the Shield

Using a four-wire low-resistance ohmmeter, measure the DC resistance of the shield from one connector end to the other. For a braided shield, this resistance should be very low (typically only a few milliohms per meter). A high or unstable reading indicates a broken strand, a poor crimp in the shield termination, or an incomplete shield construction. This low resistance path is critical for providing a return path for common-mode currents and for effective operation of the shield at lower frequencies.

Shield Grounding Impedance at High Frequency

While DC resistance is important, the impedance of the shield-to-ground connection at high frequencies is what ultimately determines effectiveness. Use a vector network analyzer with a fixture to measure the impedance from the shield at the connector to the system ground plane across a frequency range (e.g., 1 MHz to 1 GHz). The goal is a low impedance across the entire band. A connection that looks like a short at DC can become inductive at high frequencies, rendering the shield ineffective. This test often reveals issues with "pigtail" ground connections, which add inductance, versus 360-degree circumferential clamp connections, which provide a lower-impedance path.

Time Domain Reflectometry for Shield Defects

Use a time domain reflectometer to send a fast edge signal along the shield conductor. Discontinuities, such as a change in braid density, a crushed section, or a poor termination, will cause an impedance mismatch that reflects part of the signal back. The TDR display will show these reflections as peaks or dips at specific time delays, which correspond to physical locations along the harness. This allows for pinpointing manufacturing defects or damage in the shield that would degrade its high-frequency performance. A uniform shield will show a relatively flat impedance profile.