| Products > Test Equipment |
| making an 20:1 coax probe |
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| Zeyneb:
Hi there, I do have the picotech 3206MSO oscilloscope. This one has 200 MHz bandwidth. I would like to measure signals with short rise times. In the old book High-Speed Digital Design from Martin Graham and Howard Johnson they recommend to build an 20:1 attenuation transmission line probe. You know with a 950ohm resistor in series. I'm now looking what I would need to buy to make this. At least I should buy these feed-through 50ohm bnc terminators. Well at least Pico Technology offers these as TA051 for 1 GHz bandwidth, these are $21, significantly less expensive than the Pomona ones which are about $50. Then some RG174 or RG58 coax. Digikey offer the Huber+Suhner brand sold per meter. These are rated for RF. The question is would it be hard to put a bnc connector on the end of it and maintain the performance? Or would it be better to buy a coax that already has the bnc connector. If it is fine to do myself where can I find good instructions to get the expected performance? |
| PartialDischarge:
The problem with a 20:1 probe is that loading is high, better find a FET probe from eBay or similar |
| pcprogrammer:
To mount a BNC connector to a cable one needs a crimping tool to do a proper job. Otherwise it is not that simple to make a good lasting cable. I bought a toolkit for this a long time ago, back in the day that ethernet still used coax cable to make networks. |
| David Hess:
A crimping tool and crimp BNC connector for RG-174 sized cable works well; I have done it several times. I actually prefer clamp BNC connectors which are available for RG-174. Either will work to 1 GHz and beyond without issues. A low-z probe will work, but at only 200 MHz, an active probe is feasible for home construction. |
| G0HZU:
If you want to get the best from a passive z0 probe I'd recommend using decent cable and try and avoid cheap crimped BNC connectors. The VSWR of a basic crimped BNC can be quite poor up at UHF. To minimise mismatch uncertainty issues the VSWR of the cable has to be good, the connector at the far end of the cable has to have low VSWR and the termination at the far end also has to have low VSWR. If the test gear at the far end is a basic 200MHz scope with a 1M input then I'd recommend having a good 10dB attenuator inline before the external 50R termination at the scope input. This will prevent the poor VSWR of the terminated scope from introducing too much uncertainty. |
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