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10
FM Discriminator Fundamentals
The Frequency Discriminator Method
Figure 10-2 Nulls in Sensitivity of Delay Line Discriminator
10-4
E5500 Phase Noise Measurement System Version A.02.00
The magnitude of the sinusoidal output term or the frequency
discriminator is proportional to
output response will have peaks and nulls, with the first null
occurring at
. Increasing the rate of a modulation signal
1 τ d
⁄
fm
=
applied to the system will cause nulls to appear at frequency
multiples of
(Figure
1 τ d
⁄
To avoid having to compensate for sin (x)/x response, measurements
are typically made at offset frequencies ( ) much less
possible to measure at offset frequencies out to and beyond the null
by scaling the measured results using the transfer equation. However,
the sensitivity of the system get very poor results near the nulls.
The transfer function shows that increasing the delay
the sensitivity of the system. However, increasing
the offset frequencies ( ) that can be measured without
compensating for the sin(x)/x response. For example, a 200 ns delay
line will have better sensitivity close to carrier than a 50 ns delay
line., but will not be usable beyond 2.5 MHz offsets without
compensating for the sin(x)/x response; the 50 ns line is usable to
offsets of 10 MHz.
Increasing the delay
τ d
While this has no direct effect on the sensitivity provided by the
delay line, it does reduce the signal into the phase detector and can
result in decreased
K φ
(
π f m τ d
)
⁄
(
π f m τ d
sin
10-2).
f m
f m
, also increases the attenuation of the line.
and decreased system sensitivity.
Document Part No. E5500-90024 Ed. 1.0
. This implies that the
)
. It is
1 2 τ d
⁄
increases
τ d
also decreases
τ d
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