ETS-Lindgren 7405 E & H Near Field Probe Set - Manual (Page 31)

www.ets-lindgren.com Common Diagnostic Techniques 31
If the problem is caused by a rise or fall time, you may be looking for a waveform
component which is between a wavelength and 1/8 of a wavelength of the
radiating frequency.
Example: In the 208 MHz example a wavelength is 1/13 of the 16 MHz
clock; 1/8 of a wavelength is 1/104 of a 16 MHz pulse width.
Look at the oscilloscope picture for waveform components on
the 16 MHz clock that are 1/13 –1/104 of the 16 MHz
wavelength. You can then begin to zero in on undershoot and
overshoot or other parasitic components. You may not have to
quiet the entire circuit, but rather roll off the offending
components. What you have done is mentally transform a
frequency domain failure to a time domain picture that you can
work on.
After identifying what the signal of interest looks like on the oscilloscope, it must
be located within the equipment. At times this will have already been
accomplished during the demodulation process.
Example: As you demodulated a 5 MHz signal, maybe it became clear
that the 50 MHz was pulsing on at a 40 kHz rate. You may know
that the only 40 kHz source in your unit is the switching rate in
the power supply. If nothing else in the unit operates at that
frequency, you have identified your source. Thus, the first step
in identifying a signal source is to review what subassemblies in
the unit may produce a signal similar to the one you are seeing
radiated.