User`s guide

Agilent Technologies E5500 Phase Noise Measurement System 10-5
FM Discriminator Fundamentals
The Frequency Discriminator Method
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 response can be
reduced to
where is the discriminator constant.
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.
The reduced transfer equation implies that a frequency discriminator’s
system sensitivity can be increased simply by increasing the delay or by
increasing the phase detector constant . This assumption is not
completely correct. 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. Increasing
the delay will 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
L
V
in
LX(10)
−
LZ/20
Where K
L
is the phase detector efficiency, V
in
is the signal voltage 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.
If the amplifier signal puts the phase detector into compression, is at its
maximum and system sensitivity is now dependent on the length of the delay
. 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.
f
m
πτ
d
∆Vf
m
() K
d
∆ff
m
() K
φ
πτ
d
∆ff
m
()==
K
d
τ
d
K
φ
K
φ
τ
d
K
φ
τ
d