90 Chapter 3
Making Measurements
FM Deviation Measurement
High Pass Filter
Select between None (<20 Hz), 50 Hz, 300 Hz. If PSA firmware revision A.11.08
or above, or Option 23B is installed, 400 Hz is available.
Low Pass Filter
Select between 3 kHz, 15 kHz, 30 kHz, 300 kHz and None (>300 kHz). If PSA
firmware revision A.11.08 or above, or Option 23B is installed, 80 kHz is
available.
Band Pass
Selects between None and CCITT weighing, if PSA firmware revision A.11.08 or
above, or Option 23B is installed.
FM De-Emphasis
Select between None, 25
μs, 50 μs, 75 μs, 750 μs.
Auto Carrier Frequency
Requires PSA firmware revision A.11.08 or above, or Option 23A.
Accurate angle demodulation (FM or PM) depends on precisely identifying the
carrier frequency. Errors result in phase ramping. The arc tangent of the complex
time record is the basis of both FM and PM demodulation. Hence, correcting for
this phase ramp is the goal of auto carrier frequency.
The default is Auto Carrier frequency On, and in this state, the analyzer uses an
algorithm to estimate the carrier frequency. If you lock the analyzer to an external
reference which is coherent with your carrier, no carrier frequency estimation is
required, and you do not need to select auto carrier frequency. Without auto carrier
frequency, the analyzer uses its center frequency to determine the carrier frequency
of your signal.
Considerations When Using Auto Carrier Frequency:
The following situations can bias the phase ramp estimation and the Auto carrier
should be turned off:
• Low frequency modulation, such as a periodic signal with fewer than 10 cycles
over the time record.
• Phase discontinuities present in digital communications formats.
• Transients, such as carrier turn-on in the middle of the time record. Another
example is when measuring the settling time of a VCO or a synthesizer. This
causes a DC shift in the demodulators output if it is trying to estimate the
carrier frequency before it is stable.
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