Low cost system for phase noise measurements of microwave oscillators

Every phase noise measuring system ends in very sensitive low frequency spectrum analyser. It is rather rarely met in a microwave laboratory. Here a cheaper solution is proposed. Instead of the low frequency spectrum analyser, PC computer with AD/DA card is proposed. The output noise signal from the frequency discriminator is gained, passed through the low pass filter and sampled by AD/DA card. The written computer program controls the sampling process and further mathematical calculations (Fast Fourier Transform with data windowing, gain characteristics correction of the low frequency signal path and averaging process). You can choose many known windows to the sampled data set to smooth the spectral samples. Besides, the discriminator sensitivity and the system noise floor can be taken into account. The power spectral densities Sf, S¿, ¿[dBc/Hz] can be displayed on a computer screen. These charts can be exported in the HPGL vector format and later imported into word processing programs to prepare documentations.