Keysight Technologies N5511A, User Manual

Page: 219 / 369

N5511A Phase Noise Test System User’s Guide 219
FM Discriminator Fundamentals
The Frequency Discriminator Method
The discriminator transfer response
The important equation is the final magnitude of the transfer response.
Where
Δ
V( ƒ
m
) represents the voltage fluctuations out of the discriminator and
Δ
ƒ ( ƒ
m
) represents the frequency fluctuations of the DUT. K
Φ
is the phase
detector constant (phase to voltage translation).
τ
d
is the amount of delay
provided by the delay line and ƒ
m
is the frequency offset from the carrier that
the phase noise measurement is made.
System sensitivity
A frequency discriminator’s system sensitivity is determined by the transfer
response. As shown below, it is desirable to make both the phase detector
constant K Φ and the amount of delay τ
d
large so that the voltage fluctuations
Δ
V out of a frequency discriminator will be measurable for even small
fluctuations Δ ƒ.
The magnitude of the sinusoidal output term or the frequency discriminator is
proportional to . This implies that the output response will have
peaks and nulls, with the first null occurring at n = 1/
τ
. Increasing the rate of a
modulation signal applied to the system will cause nulls to appear at frequency
multiples of 1/
τ
d
( Figure 9-2 ).
The system sensitivity is independent of carrier frequency ƒo.