FAQs from Rohde & Schwarz

Noise Floor below 3 MHz


I have a question regarding the ESL test receiver. When I scan from 150 kHz to 30 MHz, the displayed noise drops down at the frequencies below 3MHz. I used a 9 kHz IF bandwidth and the RF input was terminated with 50 Ω. This behavior is exhibited in both the scan and sweep mode. My FSL does not exhibit this behavior. Is there an explanation for this?


The ESL pre-compliance test receiver combines two instruments in one, measuring EMC disturbances and also serving as a full-featured spectrum analyzer for diverse lab applications. For both operating modes, the ESL uses a direct path hardware concept for frequencies up to 3 MHz. This feature is a technological advance compared to the FSL or other general purpose spectrum analyzers.

The big advantage of the direct path is the lower noise floor, since the direct path does not have additional phase noise coming from the built-in local oscillator. RF signals up to 3 MHz are fed directly to the A/D converter without a mixer. A mixer and LO are only used in the first stage for higher frequencies. This hardware concept for lower frequencies gives you an unrivalled low noise for an instrument in this device class.