Keysight Technologies N5511A, User Manual

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220 N5511A Phase Noise Test System User’s Guide
FM Discriminator Fundamentals
The Frequency Discriminator Method
Figure 9-2 Nulls in sensitivity of delay line discriminator
To avoid having to compensate for sin (x)/x response, measurements are
typically made at offset frequencies (ƒ
m
) much less 1/2
τ
d . It is possible to
measure at offset frequencies out to and beyond the null by scaling the
measured results using the transfer equation. However, the sensitivity of the
system get very poor results near the nulls.
The transfer function shows that increasing the delay
τ d increases the
sensitivity of the system. However, increasing
τ
d also decreases the offset
frequencies (ƒ
m
) that can be measured without compensating for the sin(x)/x
response. For example, a 200 ns delay line will have better sensitivity close to
carrier than a 50 ns delay line., but will not be usable beyond 2.5 MHz offsets
without compensating for the sin(x)/x response; the 50 ns line is usable to
offsets of 10 MHz.
Increasing the delay
τ
d , also increases the attenuation of the line. While this
has no direct effect on the sensitivity provided by the delay line, it does reduce
the signal into the phase detector and can result in decreased K
Φ
and
decreased system sensitivity.
The phase detector constant
K Φ equals the slope of the mixer sine wave
output at the zero crossings. When the mixer is not in compression, K
Φ
equals
K
L
V
R
where K
L
is the mixer efficiency and V
R
is the voltage into the Signal Input
port (R port) of the mixer. V
R
is also the voltage available at the output of the
delay line.