Extending the range for precision AM noise measurements

Developed as an extension of the existing NIST phase noise measurement system, the AM measurement system presently covers carrier frequencies from 5 MHz to 70 GHz. Our new technique relies on a wideband AM modulator to calibrate frequency dependent errors. The system, utilizing a double-balanced mixer as the detector, works in two passes. In the first pass, the modulator is used to sweep a known modulation level across the Fourier frequencies of interest. This level is detected by three different spectrum analyzers that cover Fourier frequencies from 1 Hz to 1 GHz. The data is then compiled into a calibration curve that is used to correct frequency dependent errors in the noise data measured in the second pass. The entire system is computer controlled via the GPIB bus enabling measurements to be made with minimal user intervention