Tektronix RSA6100A Series, Help Manual

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RF Measurements > Phase Noise Phase Noise Display
if they were noise, coherent signal measurement results can contain errors in level. This is a key difference
between an SA-based measurement and a dedicated phase noise tester.
The largest difference between operation of a swept spectrum analyzer and a real-time spectrum analyzer
when measuring phase noise is a result of the wide acquisition bandwidths found in the real-time spectrum
analyzer. The real-time spectrum analyzer can select between 1 MHz, 40 MHz and 110 MHz IF
fi lters
when performing acquisitions. By contrast, a traditional spectrum analyzer uses an IF bandwidth that can
range from a few kHz wide up to 10 MHz wide, depending upon the selected resolution bandwidth.
When the IF bandwidth is small it is able to reject out-of-band carriers so they don’t overload the
fi nal
IF and digitizers in the instrument. This allows the spectrum analyzer to begin to reduce attenuation as
the phase noise measurement tunes farther away from the carrier, improving dynamic range. The same
thing is done in the real-time spectrum analyzer, but the 1 MHz
fi nal IF section in the real-time spectrum
analyzer can’t reject signal within about 1 MHz whereas a conventional spectrum analyzer can reject
signals within a few kHz of the desired carrier. This is the reason for the shape of the phase noise plot seen
in the following illustration.
In the previous illustration, the effect of choosing ‘optimize dynamic range’ is seen. In the offsets between
10 Hz and 1 MHz, the 40 MHz acquisition bandwidth is used, and the RF attenuator is set based on the
reference level of the instrument to prevent overload of the RF/IF signal path.
At an offset of >1 MHz, a 1 MHz IF bandwidth is applied, 5 dB of RF attenuation is removed from the
signal path and the noise ‘
fl oor’ of the measurement is reduced. At 2 MHz offset from carrier, 20 dB of
attenuation is removed with a resultant increase in dynamic range. Finally, at 20 MHz offsets, the 40 MHz
fi lter is used again with 20 dB attenuation removed. In order to be able to use the 40 MHz BW fi lter at a
20 MHz offset, the measurement frequency is moved so that the carrier is 30 MHz from the measurement
point, and well down the
fi lter rejection skirt.
RSA6100A Series Real-Time Spectrum Analyzers Help
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