HP HP-Bench-Briefs-1983-01-02 free download

File name HP-Bench-Briefs-1983-01-02.pdf

SERVICE INFORMATION FROM HEWLETT-PACKARD JANUARY-FEBRUARY 1983 Improving Bandwidth and Risetime in 174x Oscilloscopes Doug Moloney, Hewlett-Packard Low bandwidth and slow risetime problems i n your 174X series of oscilloscopes may be caused by cor- rosion of the delay line's braided shield. What HP has discovered is that under extreme operating condi- r ' tions (wide ambient temperature variations) the braid becomes cor- roded from gasses emitted by the outer PVC (polyvinyl chloride) jack- et of the delay line. This PVC outgassing is very similar to the vinyl dashboard and seats in a car emitting gasses to create the film on the inside of the car windows. ( I . Regular BNC cables with PVC jack- ets are susceptible to the same prob- I lem. However, due to the constant scope is operated in a constantly con- times while watching the CRT I I flexing in normal handling, the in- trolled environment, it may take for high-frequency response I dividual strands of the braided years for the contamination to be- improvement. shield rub against each other to keep come objectionable. the contact between them clean. 4. If response improves, keep squeezing the cable. You can ex- Since the delay line is rarely moved Identifying a faulty delay line is pect to see improvements of 1 to 3 once it is installed in the scope, the easy. nanoseconds or 10 to 60 MHz. If PVC contamination may continue to no improvement is observed, per- build until the braid's conductivity 1. Remove the scope's bottom cover. form the pulse response adjust- decreases to the point where high- ments and repeat step 3. frequency attenuation becomes sig- 2. Input a fast risetime pulse or 100 nificant. This process can occur in a MHz sine wave and note t h e 5 . Perform the pulse response ad- relatively short time if the scope is risetime or amplitude. justments and high-frequency f subjected to repeated and radical performanc