| Electronics > Projects, Designs, and Technical Stuff |
| Usefulness of different TDR designs? |
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| Per Hansson:
Maybe I can ask a bit off topic question in my own thread then ;) The 1502B contains a Lithium battery, the service manual mentions nothing about what is lost if that fails. Only that it has an expected lifetime of 7 years, well, the 1502B I bought has a build date of 1994 so I think it's a bit past-due ;) Battery still measures 3.7v though, it is a "Keeper" cell called BT1010 in the schematics, with Tek partno: 146–0049–00 and manufacturer partno: LTC–7P texaspyro also mentions the bad LCD's, I also saw that you got the same advice that I have seen here on tekscopes rhb and that is that an updated LCD exists. Does anyone know about part numbers or any other details to help locate such a panel? It has been discussed here in the past: https://www.eevblog.com/forum/repair/tek-1502b-tdr-restoration-project-and-more/msg1305389/#msg1305389 |
| rhb:
FWIW the lithium cells in my 30 year old 11801 are still at 3.1 V. At the time no one knew the shelf life. |
| David Hess:
--- Quote from: Per Hansson on April 19, 2019, 09:41:14 pm ---Thanks for the insight David, yea the 1502B seems to be a good fit for my uses. It provides almost an order of magnitude better rise time than a traditional schmitt trigger. And it does not require a scope with a very high bandwidth, that ideally should offer a 50Ω input. I think I have a keeper ;) --- End quote --- If I wanted something which is higher performance, I would look for a Tektronix 7S12 with S-6 loop-through sampling head (and a 7000 mainframe for it) or a mainframe for a SD-24 sampling/TDR head. There are a bunch of HP alternatives but I am not familiar with them. Making something better than a 1502 would be a challenge but could be done. 200 picoseconds is feasible with a good avalanche pulser but there are some suitable integrated parts which can be faster. I think you can get to 200 picoseconds with SMD transistors as well. |
| rhb:
Leo Bodnar uses a Maxim 3949 laser diode driver with a 22 ps rise and fall time in his excellent pulsers. My BNC version has a 36 ps rise time according to the CSA803 printout provided. Unless you just want to see if you can beat Leo, I recommend buying one. The 3.5 mm and 2.4 mm versions are faster. But more costly as the connectors cost as much as all the rest. Leo will provide 1 MHz square wave units on request for an extra 10 pounds. I have ordered two for TDR with a DSO. The Tek 11801 calibrator is ~20 ps rise time. The SD-24 heads have a pair of the same hybrid module. It's only a 250 mV step, so it won't drive a long cable. |
| David Hess:
I did not suggest using a part like the Maxim 3949 laser diode driver because without an accompanying sampling gate, extra performance from the faster edge will no be available. Analog Devices has some fast comparators with similar performance. A 4 GHz sampling gate is feasible with a surface mount design and does not require an edge that fast. |
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