Inject harmonics, test for EMI and resonance

September 28, 2015 // By Martin Rowe
Circuits behave differently depending on frequency and you often need to subject them a wide range of frequencies to see how they respond. For example, EMI emissions and susceptibility are frequency dependent. While you can use a swept-signal generator to produce test frequencies, you may not need one.

Comb generators such as the J2150A USB Harmonic Comb Injector from Picotest let you inject frequencies from 1 kHz to 1.5 GHz. It can generate impulse or square-wave signals whose frequencies are harmonics of a fundamental frequency. You can use the harmonic comb injector to look for frequencies that might interfere with your circuit's or system's operation. Furthermore, you can perform spectrum testing on clocks and power-distribution networks. The figure below shows how the comb generator identified a 7-MHz resonance in a circuit.

The harmonic comb generator is powered by its USB connection from a host computer. An SMA connector give you access to the output signal. The comb injector is compatible with 50Ω test instruments. Key specifications include:

-13 dBm typical 1 kHz and 100 kHz

-17 dBm typical 8 MHz

10 kHz square wave 13 dBm, typical duty cycle 50%

DC coupled 0 to +2.5 V pulse into 50Ω

Typical rise/fall time 470 psec/270 psec

The device is priced at $895.