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9
FM Discriminator Fundamentals
.
Table 39 Choosing a delay line
Parameters
Source signal level
Mixer compression point
Delay line attenuation at source carrier frequency
Highest offset frequency of interest
1
2
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.
248
To avoid having to correct for the sin(x)/x response choose the delay such
that:
1
- - -- - -- - -- - -- - -- - -- - -- - -- - -- - -
τ
<
d
×
×
2π
5 10
τ
A delay
of 32 ns or less can be used for offset frequencies out to 5 MHz.
d
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.
+7dBm
+3 dBm
30 dB per 100 ns of Delay
5 MHz
6
Agilent E5505A User's Guide
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