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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.
Document Part No. E5500-90024Ed. 1.0
The phase detector constant
wave output at the zero crossings. When the mixer is not in
compression,
equals
K φ
is the voltage into the Signal Input port (R port) of the mixer.
also the voltage available at the output of the delay line.
Optimum Sensitivity
If measurements are made such that the offset frequency of interest
( ) is <1/2
the sin(x)/x term can be ignored and the transfer
πτ d
f m
response can be reduced to
where
is the discriminator constant.
K d
The reduced transfer equation implies that a frequency
discriminator's system sensitivity can be increased simply by
increasing the delay
τ d
. This assumption is not completely correct.
K φ
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. Increasing the delay
reduce the 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.
Sensitivity = K
V
LX(10)
L
in
Where K
is the phase detector efficiency, V
L
into the 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
LZ = 8.7 dB of attenuation
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.
E5500 Phase Noise Measurement System Version A.02.00
FM Discriminator Fundamentals
The Frequency Discriminator Method
equals the slope of the mixer sine
K φ
where
is the mixer efficiency and
K L V R
KL
∆ V f m
(
)
K d ∆ f f m
(
=
or by increasing the phase detector constant
− LZ/20
in
10
VR
is
VR
)
K φ πτ d ∆ f f m
(
)
=
is dependent on the
K φ
will
τ d
is the signal voltage
10-5
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