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FM Discriminator Fundamentals
The Frequency Discriminator Method
The following example illustrates how to choose a delay line that provided
the optimum sensitivity given certain system parameters.
Table 10-1
Choosing a Delay Line
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:
τ
<
d
A delay
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.
10-6 Agilent Technologies E5500 Phase Noise Measurement System
Parameters
1
------------------------------ -
6
×
×
2π
5
10
τ
of 32 ns or less can be used for offset frequencies out to
d
+7dBm
+3 dBm
30 dB per 100 ns of
Delay
5 MHz
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