Unstable Signature

An intermittent fault is one of the biggest problems in electronic repair. The fault comes and goes, and in most cases does not stay long enough for positive detection. Signature analysis can detect such faults if they occur within a measurement window. However, the operator may not receive the message if the measurement cycle time is too short.

The random-access memory (RAM) in the main assembly of the 5004A continuously writes and reads the display information from the display latch at the display scan rate. During each scan cycle, the signature comparator compares the signature stored in the RAM with the one in the display latch, and turns on the unstable signature light on the front panel when any difference exists. This light is stretched for 100 ms to allow recognition. The comparison is done on a

Fig. 7. Simplified schematic of one channel of the 5004A input pod.

sampled basis and not each time a new signature is developed, so the unstable signature detector works most of the time, but not 100%. Errors occurring in a very short measurement cycle may not always be detected by the relatively slow-scanning comparator.

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