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Optimum sensitivity
If measurements are made such that the offset frequency of interest ( ) is
<1/2
reduced to
where
The reduced transfer equation implies that a frequency discriminator's system
sensitivity can be increased simply by increasing the delay
the phase detector constant
is dependent on the signal level provided by the delay line and cannot exceed a
device dependent maximum. This maximum is achieved when the phase
detector is operating in compression
signal level out of the delay line often reducing the sensitivity of the phase
detector. Optimum system sensitivity is obtained in a trade-off between delay
and attenuation.
Where K
delay line, LX (dB) is the sensitivity provided by the delay line and LZ is the
attenuation of the delay line. Taking the derivative with respect to the length L
to find the maximum of this equation results in
The optimum sensitivity of a system with the phase detector operating out of
results from using a length of coaxial line that has 8.7 dB of attenuation.
One way to increase the sensitivity of the discriminator when the phase
detector is out of compression is to increase the signal into the delay line. This
can be accomplished with an RF amplifier before the signal splitter. The noise
of the RF amplifier will not degrade the measurement if the two-port noise of
the amplifier is much less than the noise of the DUT. However, some
attenuation may be needed in the signal path to the reference input to the
double-balanced mixer (phase detector) to protect it from excessive power
levels.
If the amplifier signal puts the phase detector into compression,
maximum and system sensitivity is now dependent on the length of the delay
τ d
. For maximum sensitivity more delay can be added until the signal level out
of the delay line is 8.7 dB below the phase detector compression point.
The following example illustrates how to choose a delay line that provided the
optimum sensitivity given certain system parameters. (See
page 248).
1 Compression: The level of the output signal at which the gain of a device is reduced by a specific amount,
usually expressed in decibels (dB), as in the 1 dB compression point.
Agilent E5505A User's Guide
πτ d
the sin(x)/x term can be ignored and the transfer response can be
∆V f m
(
)
K d ∆f f m
=
is the discriminator constant.
K d
Sensitivity = K
V
LX(10)
L
in
is the phase detector efficiency, V
L
LZ = 8.7 dB of attenuation
(
)
K φ πτ d ∆f f m
(
)
=
. This assumption is not completely correct.
K φ
1
. Increasing the delay
−LZ/20
is the signal voltage into the
in
FM Discriminator Fundamentals
τ d
or by increasing
τ d
will reduce the
K φ
is at its
Table 39
on
9
f m
K φ
247
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