EMC test filters with switched common-mode impedance


Switched cable decoupler system features

See technical info for construction and application information.

In the Hi-Z - unpowered and Off state a switched cable decoupler (SCD) offers a substantially resistive common-mode impedance of more than 100ohms from 30MHz to 500MHz with a typical value of 200 ohms at 200MHz. In the Low-Z - powered - state its impedance is less than 10 ohms from 1MHz to 300MHz. If used with cables of less than half the quoted maximum diameter then lower-than-specified high-frequency Low-Z performance will result because of increased leakage inductance. In most applications this will have no significant effect.

The products currently available are suitable for emission testing: Devices suitable for immunity testing are under development and enquiries will be welcomed.

The SCD Head draws power from its internal battery only in the Lo-Z state and so the only power switch for the system is that on the Controller Unit. Communication is up to a distance of 10 metres via a 1000 micron acrylic polymer fibre with a minimum bend radius of 50mm.


switched emc filter 16DH-7








In the above picture the 16DH-7 impedance head is on the left. The coil of fibre-optic cable joins it to the controller on the right.

In pulsed mode the SCD head is energised (low impedance) for 2.75 milliseconds. This pulse is repeated every 9.1 milliseconds - a period chosen to be distinguishable from the customary power frequencies and their harmonics. The transition between impedance states occurs in less than 0.5 milliseconds. Pulsing also facilitates the use of several SCDs on multi-cable EUTs and maximises battery life.

When displayed with a slowly-scanning spectrum analyser or receiver using an average detector, this pulse pattern results in the simultaneous display of multiple traces. The short pulses define the EUT response in the low-Z mode and the long pulses define the Hi-Z response.

Both head and controller include LED indication of ON.


Series Specifications


16DH-6.5 Impedance Head for cables up to 6.5mm outside diameter.

Clamp construction.

16DH-7 Impedance Head for cables up to 7mm outside diameter.

Thread-through construction.


16DH-10 Impedance Head for cables up to 10mm outside diameter.

Clamp construction.


16DH-13 Impedance Head for cables up to 10mm outside diameter.

Clamp construction.


16DC-1 Controller for a single SCD Head.

Battery life 9 hours continuous low state; 27 hours in pulsed mode.


16DC-4 Controller for up to four SCD Heads.

This product is under development.


Simplified SCD filters for use at the test-chamber wall are under development and enquiries will be welcomed.


In the search for improved quality this specification is subject to change.

and Design rights reserved Richard Marshall Ltd 2000. rev. 1.0