Low Frequency
Modulation
FFT 1-clk view
To Measure:
For more
information on this
tool, refer to:
VISI Online Help
Continue analysis of Periodic Jitter (PJ); Identify possible Low
Frequency Jitter content
Often it is not necessary to identify jitter at very low frequencies (in this tool <100kHz).
Low
frequency modulation
can
be analyzed in the
High
Frequency Modulation tool, but
the acquisition time becomes very long for lower “HPF –3dB freq” settings. So, the Low
Frequency modulation tool acquires measurements in a different way allowing faster
acquisition when looking for low frequency jitter or wander on a high speed clock.
•
From the main menu, press Clock, then Low Frequency Modulation to open the tool.
•
Choose the “Maximum Frequency”, which must be 100kHz or less.
•
“Minimum Data Points” determines the number of time measurements in the record.
Increasing this value will reduce the possibility of aliasing, increase FFT resolution
and allow lower frequency components to be acquired.
•
Press the SINGLE/STOP button on the front panel or the top menu.
Figure 6 - FFT shows jitter frequencies and amplitudes. Analysis of 2GHz
clock from Figure 3. Shows jitter at ~62kHz.
This 1-clk FFT view shows jitter components lower than 100kHz. This tool is useful when
very low frequency jitter or wander may be suspected. The example in Figure 6 shows
that we can see a Periodic jitter component at ~62kHz on the 2GHz clock signal; this
jitter was added to the clock signal for the purposes of displaying the capabilities of the
tool.
We have now identified low frequency jitter, but there may be other factors that affect the
timing stability such as drift due to temperature or voltage changes.
Clock Tools Overview
Page 7 of 13