Keysight Technologies N5511A, User Manual

Page: 221 / 369

N5511A Phase Noise Test System User’s Guide 221
FM Discriminator Fundamentals
The Frequency Discriminator Method
Optimum sensitivity
If measurements are made such that the offset frequency of interest (ƒ
m
) is
<1/2 π
τ
d
the sin(x)/x term can be ignored and the transfer response can be
reduced to
Δ
V (ƒ
m
) = K
d
Δ
ƒ(ƒ
m
) = K
Φ
π
τ
d
Δ
ƒ(ƒ
m
)
where K
d
is the discriminator constant.
The reduced transfer equation implies that a frequency discriminator’s system
sensitivity can be increased simply by increasing the delay τ
d
or by increasing
the phase detector constant K
Φ
. This assumption is not completely correct. K
Φ
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
1
. Increasing the delay
τ
d
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,
K Φ is at its
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.
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.