ETS-Lindgren 7405 E & H Near Field Probe Set - Manual (Page 30)

30 Common Diagnostic Techniques www.ets-lindgren.com
3. Obtain a clear picture of the signal produced on the oscilloscope. You
now have a good representation of what you are looking for when you
start sniffing with the probe.
Produce scope photos of the demodulated trouble frequencies and
then use the sniffer probes to look for similar signals in your
equipment. Locate close matches to the demodulated signals for clues
to the source of these signals. When you find the sources you will
determine the subassemblies, circuits, or gates that need work.
There are several physical phenomena that cause lower frequency signals to
modulate and radiate as high frequency signals. A working knowledge of FM,
AM, audio rectification, and other phenomena provides greater ability to
understand and interpret the data revealed by demodulated signals. This
understanding gives insight into the kind of radiating structure that must be
present to produce the observed event, and also allows greater facility in
recognizing the original signal from the altered and often distorted, modulated
representation.
Frequently the demodulated picture will contain just the transitions of a digital
signal. At times, only the rising or falling edge will be present in a high frequency
signal. Understanding the radiation physics allows the appearance of the original
signal to be surmised. Often all that will be present in the photograph from the
oscilloscope presentation is the high frequency components of a signal. These
waveform components are the source of the radiation.
E XAMPLES
Getting an idea of what the waveform may look like through demodulation is not
the only use for the time domain-frequency domain transform. Analysis can
reveal the component of the waveform that is causing the problem.
Example: If you have a 16 MHz clock and you have a 16 MHz problem,
then you know that the base signal is causing the problem.
More typically, your probing may lead you to the 16 MHz clock
when trying to find the 208 MHz problem. Remember a
208 MHz signal has a wavelength of 1/13 of 16 MHz.