N5511A Phase Noise Test System User’s Guide 219FM Discriminator FundamentalsThe Frequency Discriminator MethodThe discriminator transfer responseThe important equation is the final magnitude of the transfer response.Where ΔV(ƒ m ) represents the voltage fluctuations out of the discriminator andΔƒ(ƒ m ) represents the frequency fluctuations of the DUT. KΦ is the phasedetector constant (phase to voltage translation). τd is the amount of delayprovided by the delay line and ƒm is the frequency offset from the carrier thatthe phase noise measurement is made.System sensitivityA frequency discriminator’s system sensitivity is determined by the transferresponse. As shown below, it is desirable to make both the phase detectorconstant KΦ and the amount of delay τd large so that the voltage fluctuationsΔV out of a frequency discriminator will be measurable for even smallfluctuations Δƒ.The magnitude of the sinusoidal output term or the frequency discriminator isproportional to . This implies that the output response will havepeaks and nulls, with the first null occurring at n = 1/τ. Increasing the rate of amodulation signal applied to the system will cause nulls to appear at frequencymultiples of 1/τd (Figure 9-2).The system sensitivity is independent of carrier frequency ƒo.
