Abnormalities ensuing from back-tracing and probe selection or what can we do to mitigate diagnostic problems

Fault localization should always be a top priority with TPS re-host or new development. In fact, diagnostics should be a first step and the most critical step. Fault isolation should be a conscience effort as the main-line program is developed. During main-line program development an engineer could perceive fault isolation problems that might arise and then adjust the main-line to refine the testing such that smaller or more precise sections of a circuit will be tested so that fewer components come into consideration. Current flow problems can arise that don't appear obvious at the time Test Requirements Documents (TRD) are prepared and thus the circuit test can be weak or somewhat incomplete. We need to augment or improve the test requirements through-out all phases of circuit design and testing. A probe can be any conductor used to establish a connection between the circuit under test and the measuring instrument¹. Standard probing techniques like the back-trace probe have proven effective. However, there are shortcomings with the standard back-trace probe. At times, the standard back-trace probe can cause diagnostic problems which can be deceiving. These deceiving problems tend to contribute to wrong callouts and mis-information. If the probing is defective then the diagnostics is defective. A “perfect” probe is nice to dream about. But in reality, no probe is perfect. Even a simple length of wire is still potentially a very complex circuit. For dc signals, a probe appears as a simple conductor pair with some series resistance and a terminating resistance. However, for ac signals, the picture changes dramatically as signal frequencies increase. Any length of wire has distributed inductance (L), and any wire pair has distributed capacitance (C). The distributed inductance reacts to ac signals by increasingly impeding ac current flow as signal frequency increases. The distributed capacitance reacts to ac signals with decreasing impedance to ac current flow as signal frequency increases. The interaction of these reactive elements (L and C), along with resistive elements (R), produces a total probe impedance that varies with signal frequency. Through good probe design, the R, L, and C elements of a probe can be controlled to provide desired degrees of signal fidelity, attenuation, and source loading over specified frequency ranges4. There are probing techniques that must be considered under certain diagnostic tests. There are circuit configurations that just don't respond well to the standard back-trace probe. During integration of a Test Program Set (TPS), whether a re-host or a new development,when fault insertion is being performed, problems with the standard back-trace probe might arise. If the engineer becomes aware of probing problems there needs to be a plan of action to resolve the issue. We should never dismiss any issues encountered in TPS development. The worn out comment “it can't be diagnosed” should never be a part of TPS development. Voltage probes are intended to measure or display voltages on the Unit Under Test (UUT). Ideally, the test instrument and its probe will not affect the voltage being measured. Practically, that translates into the test instrument and its probe presenting a high impedance that will not load the UUT. In many situations, an impedance with a resistive component of a meg ohm is adequate. For AC measurements, the reactive component of impedance may be more important than the resistive. Because of the high frequencies often involved, oscilloscopes do not normally use simple wires to connect to the UUT. Instead, a specific scope probe is used. Scope probes use a coaxial cable to transmit the signal from the tip of the probe to the oscilloscope, preserving high frequencies for more accurate oscilloscope operation. Scope probes fall into two main categories: passive and active. Passive scope probes contain no active electronic parts, such as transistors, so they require no external power. There is an alternate technique which can be deployed when backtracing in certain situations that seems to be effective. This technique involves the use of a little known EME probing technology. Properly configured EME equipment can detect and analyze signals without all of the short-comings associated with standard back-tracing. It is imperative the user have an understanding of problems that could arise from back-tracing and what action they can take to derive a proper diagnostic analysis. This paper will discuss probing problems and fault isolation techniques. Also, this paper will discuss ways to mitigate probing problems and address various diagnostic techniques the TPS developer or the test technician can use to diagnose a fault. Diagnostics and probing techniques can be adjusted to produce the best possible result. The EME technique will be explained with its' various requirements.