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10
FM Discriminator Fundamentals
The Frequency Discriminator Method
10-6
E5500 Phase Noise Measurement System Version A.02.00
If the amplifier signal puts the phase detector into compression,
at its maximum and system sensitivity is now dependent on the
length of the delay
. For maximum sensitivity more delay can be
τ d
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
Table
10-1.
Table 10-1 Choosing a Delay Line
Parameters
Source signal level
Mixer compression point
Delay line attenuation at source carrier frequency
Highest offset frequency of interest
1
To avoid having to correct for the sin(x)/x response choose the
delay such that:
1
τ
<
------------------------------ -
d
6
2 π
×
×
5
10
A delay
of 32 ns or less can be used for offset frequencies out
τ
d
to 5 MHz.
2
The attenuation for 32 ns of delay is 30 dB x 32 ns/100 ns or 9.6
dB. The total signal attenuation through the splitter and the delay
line is 15.6 dB. The signal level out of the delay line is −8.6 dBm
which is 11.6 dB below the phase detector compression point.
Improved sensitivity can be achieved by reducing the length of
the delay or by using a more efficient line so that the signal level
out is − 5.7 dBm or 8.7 dB below the mixer compression point.
Careful delay line selection is crucial for good system sensitivity. In
cases where the phase detector is operating out of compression,
sensitivity can be increased by using a low loss delay line, or by
amplifying the signal from the DUT. Because attenuation in coaxial
lines is frequency dependent, optimum system sensitivity will be
achieved with different lengths of line for different carrier
frequencies.
+7dBm
+3 dBm
30 dB per 100 ns of
Delay
5 MHz
Document Part No. E5500-90024 Ed. 1.0
is
K φ
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