Comb generators test radiated noise

25th July 2017

Mick Elliott

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Datasheet

Two TekBox TBCGx Comb Generators for EMC testing are now available at Saelig. The TBCG1 is a radiating comb generator with an internal dipole antenna and a base frequency of 100MHz. It radiates a comb spectrum that is characterised up to 6GHz. The comb generator is intended as a quick reference for testing radiated noise measurements in anechoic chambers, TEM/GTEM cells, shielded chambers, or to gauge the effectiveness of cable shielding and other shielding materials.

With its built-in dipole antenna the TBCG1 can be used in three orientations to emit vertically- or horizontally-polarised signals. It is a compact and portable device, powered by a 9V alkaline cell. Being self-contained and battery-powered, there are no cables to disrupt the emitted field.The TBCG2 is a versatile, passive comb generator/frequency multiplier which takes a square-wave input and creates a wide spectrum of comb frequencies.

Based on a step-recovery diode design, this comb signal generator is optimised for an overall flat spectrum with an output frequency range from 1MHz to 2.5GHz.

The recommended drive power is between +10dBm and +20dBm with an input frequency of 1MHz to 350MHz.

The TBCG2's capability to work with such low drive frequencies makes it also suitable for the measurement of cable resonances.

Cables on a product can themselves act as an antenna, causing radiated emissions and possible compliance test failures.

They can resonate at their half-wave frequency and magnify any harmonic currents in the cable shield near that resonance.

The resonant frequency of cables and wires in a product or system can be characterised by injecting harmonic energy from a comb generator into the cable and observing the resultant resonances on a spectrum analyser; a primary peak will show where the harmonic content is resonant.

Every EMC test engineer should own a harmonic comb generator that produces a simultaneous set of harmonically related CW signals whose spacing is based on a fundamental oscillator frequency.

The harmonic amplitudes produced are consistent and can be used as a frequency and amplitude calibrator, insuring, for example, that a semi-anechoic test chamber is reading correctly from day to day.

Simply place the TBCG1 generator on a turntable and measure specific harmonics each day and record the trend data. Loose connectors or bad cables can be detected by comparing the current readings with past data.

This will fulfill the requirement for "equipment verification testing" as specified in ISO 17025.