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The Discriminator
Transfer Response
Document Part No. E5500-90024Ed. 1.0
change proportionally. The delay line converts the frequency change
at the line input to a phase change a the line output when compared
to the undelayed signal arriving at the mixer in the second path.
The double-balanced mixer, acting as a phase detector, transforms
the instantaneous phase fluctuations into voltage fluctuations
). With the two input signals 90 ° out of phase (phase
(
∆φ
→
∆ V
quadrature), the voltage out is proportional to the input phase
fluctuations. The voltage fluctuations can then be measured by the
baseband analyzer and converted to phase noise units.
The important equation is the final magnitude of the transfer
response.
∆ V f m
(
Where
represents the voltage fluctuations out of the
∆ V f m
(
)
discriminator and
∆ f fm
(
device under test (DUT).
voltage translation).
τ d
line and
is the frequency offset from the carrier that the phase
f m
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
the voltage fluctuations
measurable for even small fluctuations
∆ V f m
(
NOTE: The system sensitivity is independent of carrier frequency
E5500 Phase Noise Measurement System Version A.02.00
FM Discriminator Fundamentals
The Frequency Discriminator Method
(
sin
)
K φ 2 πτ d ∆ f f m
(
)
----------------------------- -
=
(
π f m τ d
represents the frequency fluctuations of the
)
is the phase detector constant (phase to
K φ
is the amount of delay provided by the delay
and the amount of delay
K φ
out of a frequency discriminator will be
∆ V
.
∆ f
(
π f m τ d
)
sin
)
K φ 2 πτ d
----------------------------- - ∆ f fm
=
(
π f m τ d
)
π f m τ d
)
)
large so that
τ d
(
(
)
)
f o
10-3
10
.
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