| 摘要 |
The Figure shows a system for non-destructive detection of current restricting defects, such as cracks, narrowing, intermittent opens etc, in a conductor, e.g. a printed conductor. A composite test signal is applied to the test conductor 3 via probes 1 and 2. The signal is produced by a generator circuit 4 comprising a frequency (f0) oscillator 5, amplifier 6 and D.C. source 7. The defects in the conductor 3 cause its resistance to vary at the same frequency as the oscillator signal and cause current variations through the conductor. The signal at node 4A comprises a component Vf0 at frequency f0, a component 2f0GV at twice the frequency f0 due to the resistive variation of the restricting defect and a component 2f0CV at two the frequency f0 due to the resistance heating of the whole conductor, The f0 component is rejected by filter 8 and the 2f0 components amplified (9) and filtered by band pass filter 10. The amplified filtered signal is supplied as one input to a phase detector 12, the other input being derived from node 40, frequency doubler 14, band pass filter 15 and phase shifter 16. The detector 12 operates as a phase, sensitive demodulator to suppress the 2f0CV component and to compare the phase of the 2f0GV component with the 2f0 component from phase shifter 16. The result of the comparison is manifested as a DC voltage which is logarithmically amplified and applied to indicator meter 18 and, via threshold detector 19, to defect indicator 20. |
