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Need 50 ohm input on an (old) oscilloscope that has only 1M ohm inputs: BNC TEE?

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wb0gaz:
The continuing discussion and observations are appreciated. While I have now ordered the inline terminator devices cited early in the thread (ETA sometime late this week), the details about measuring RF reflection properties (vs. onboard 50 ohm mode) is particularly valuable. The two product options I found for BNC inline terminator are typical "no name auction site" type things, so RF properties of the device itself will be evaluated (nanovna will probably come in handy for this.)

As to why not use regular probe (was asked a few posts back) - a couple of issues drive this application - I'm seeking to characterize waveform of a CMOS-output device that also claims 50 ohm drive capability, and probe attachment is really easy if I just solder ends of coax to board points very close to the device, and really awkward (three-handed operation at least) using a conventional probe (there are currently two test points that need simultaneous examination, and in the next step, four test points.)

I'm sure there are excellent solutions for this type of probing task in a commercial context, but as was noted, I am a ham without commercial resources, so I am "making do" (the discussion here really helps making up for resource constraints!)

bdunham7:

--- Quote from: wb0gaz on October 24, 2022, 02:14:42 pm ---As to why not use regular probe (was asked a few posts back) - a couple of issues drive this application - I'm seeking to characterize waveform of a CMOS-output device that also claims 50 ohm drive capability, and probe attachment is really easy if I just solder ends of coax to board points very close to the device, and really awkward (three-handed operation at least) using a conventional probe (there are currently two test points that need simultaneous examination, and in the next step, four test points.)

--- End quote ---

If your source is 50R, then in many instances you don't need to worry too much about smallish reflections at the scope termination.  However, if your source is not 50R, as appears to be the case here, the most effective way I can think of for testing it while driving 50R would be to solder in an appropriate 50R resistor near the source and then put a good (low capacitance) x10/10M probe across that.  But go ahead with your current plans because that may work fine as well.

tggzzz:

--- Quote from: edavid on October 24, 2022, 01:56:58 pm ---
--- Quote from: tggzzz on October 24, 2022, 11:18:09 am ---Does it? My HP1740A and Tek 2465 and most other scopes don't. OTOH unusually my Tek 485 does. The key difference is the Tek 485 has two internal attenuators, one the usual 1Mohm, the other 50ohm.

--- End quote ---

Look again, the 2465 has 50 ohm termination available on channel 1 and 2.  I don't think you'll find many 300MHz scopes without 50 ohm termination.

--- End quote ---

I've just measured my 2465. The return loss is, to pick a single figure:

* 485 internal attenuator: -35dB (VSWR <1.07)
* 2465 internal attenuator: -18dB (VSWR <1.28, spec 1.3)
* 485 with inline terminator: -8dBSo the 2465's internal termination is better than the 485+external terminator, but worse than the 485's "proper" attenuator.

As before, it is up to the OP to determine the significance of those for his purposes.

tggzzz:

--- Quote from: bdunham7 on October 24, 2022, 02:33:23 pm ---
--- Quote from: wb0gaz on October 24, 2022, 02:14:42 pm ---As to why not use regular probe (was asked a few posts back) - a couple of issues drive this application - I'm seeking to characterize waveform of a CMOS-output device that also claims 50 ohm drive capability, and probe attachment is really easy if I just solder ends of coax to board points very close to the device, and really awkward (three-handed operation at least) using a conventional probe (there are currently two test points that need simultaneous examination, and in the next step, four test points.)

--- End quote ---

If your source is 50R, then in many instances you don't need to worry too much about smallish reflections at the scope termination.  However, if your source is not 50R, as appears to be the case here, the most effective way I can think of for testing it while driving 50R would be to solder in an appropriate 50R resistor near the source and then put a good (low capacitance) x10/10M probe across that.  But go ahead with your current plans because that may work fine as well.

--- End quote ---

I'd modify that slightly, based on the observation that the usual *10 passive probe's input capacitance isn't much different to the scope's input capacitance.

Like you I would have a 50ohm termination near the source, but then I would

* make a passive resistive divider Z0 probe from a 450ohm (or 470ohm//10k) resistor and 50ohm coax. A commercial Z0 probe can have <1pF at the tip, and be useful to 1.5GHz or more.
* have a 10dB attenuator at the scope, to reduce the scope+terminator VSWR.That overall attenuation is probably tolerable for this application.
Soldering the 450ohm+coax isn't much more difficult than soldering a coax.

As for CMOS outputs, jellybean parts can achieve this: https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

egonotto:
Hello,

I tested 50 MHz rectangle once with the built-in 50 Ohm and then with a tee with two different 50 Ohm terminations and once the tee alone with the built-in 50 Ohm.

Best Regards
egonotto

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