Pulsed signals in reverberation chambers: Modulating the input signal to reduce the rise-time

Electromagnetic compatibility (EMC) measurements using reverberation chambers (RC) are well documented in the IEC 61000-4-21 standard. One of the main benefits is the reduced power necessary to achieve the required field strength for testing in comparison to anechoic chambers. This can be a great advantage for testing with radar signals where high field strength are common. However the high quality factor Q in a highly efficient chamber leads to a long chamber time-constant τ. This becomes a problem when testing with 3 μs-pulses is concerned. The IEC 61000-4-21 standard requires a time constant lower than 0.4 * pulse-width. If this limit is exceeded the chamber must be loaded in order to reduce the time-constant. Chamber loading reduces the field strength and the input power has to be increased to maintain the test level. This approach is a direct trade-off between power efficiency and chamber response timing. Additionally loading affects the field homogeneity and the statistical requirements for testing can be harder to fulfill. This paper presents IQ-modulation as a new approach to reduce the rise-time without the need for chamber loading. Arbitrary pulse shapes are used instead of standard rectangular pulses. With this approach the measurement setup is not changed, therefore not affecting field homogeneity at all. In order to reduce the rise-time a higher input power is still needed in comparison to the unmodulated operation. Yet this increase is significantly lower in comparison to the loaded chamber.