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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:
I'm in need of oscilloscope with 50 ohm input impedance on the vertical channels.

My HP 1725A has this feature, but it's got an unrelated problem I'm not wishing to fix right now, and for bonus points, I'd really like to have four vertical channels (all the same.)

I've got a Tektronix 7000 mainframe with a pair of (200 MHz) 7A26 vertical plug-ins which are 1M ohm input impedance. I'd like to find a pair of working (400 MHz) 7A24 plug-ins (50 ohm specific) but nothing seen (that's offered as working).

My working vertical bandwidth requirements are 200-400 MHz (this is a first foray into high speed digital circuitry.)

The question is this (which isn't really specific to the Tektronix 7000/7A26, but I use that as an example to ground the conversation):

Can I use a BNC TEE (male-female-female), male to the scope input, terminate one of the two female sides in a (known good) 50 ohm terminator, and use the other female side as a 50-ohm input?

If this is completely no-go idea, why?

If this is possible but with conditions, what would be the trade-offs (for sake of discussion, assume the TEE is 50 ohm BNC type and 50-ohm termination is "perfect" (no spurious reflections, no attenuation, termination has adequate power handling capacity, connectors are free of impairments, etc.) to separate the architectural question from the specifics of the devices that would provide the external 50-ohm termination.

If this is obviously easy, then why (other than convenience) would my HP 1725A (for example) have 50 ohm input impedance selection?

Happy to answer any requests for clarification or any other way to improve statement-of-problem!

Thanks very much for any thoughts (and would welcome any pointers as to where I could scare up a pair of working 7A24's)...

Dave
The Doktor:
Get a 50Ω pass-through terminator. https://www.amazon.com/Copper-Adapter-38-5mm-Through-Terminator/dp/B07G566JC7/
Wrenches of Death:

The now overpriced programable single channel 7A16P also supports 50 ohm and 1 Meg input. I wouldn't use more than one in a regular 7000 series scope though. Their current draw is higher than usual.

I've got a number 7704A's that I've been using since the 1990's. My preferred vertical amp setup is a single channel 7A16P and a dual channel 7A24 in each.

Watch your input voltage on the 7A24 and the 7A16P in 50 ohm mode.

WoD

bdunham7:

--- Quote from: wb0gaz on October 24, 2022, 02:26:23 am ---Can I use a BNC TEE (male-female-female), male to the scope input, terminate one of the two female sides in a (known good) 50 ohm terminator, and use the other female side as a 50-ohm input?

If this is completely no-go idea, why?

If this is possible but with conditions, what would be the trade-offs (for sake of discussion, assume the TEE is 50 ohm BNC type and 50-ohm termination is "perfect" (no spurious reflections, no attenuation, termination has adequate power handling capacity, connectors are free of impairments, etc.) to separate the architectural question from the specifics of the devices that would provide the external 50-ohm termination.

If this is obviously easy, then why (other than convenience) would my HP 1725A (for example) have 50 ohm input impedance selection?

--- End quote ---

Yes, you can do exactly that, but unless you use very high quality parts you may be a bit disappointed, especially at 400MHz.  The reason the internal 50R impedance works better is partly because it eliminates the two connections and the short section between the terminator and the input, also with some designs the 50R input selection will reduce or eliminate (almost) your input capacitance.  You'll probably see observable attenuation at 400MHz even with good parts.
wb0gaz:
Thanks very much - very helpful comments indeed - covers several topics of direct interest right away, so much appreciated.

I'm going to take a crack at pass-through attenuator (I'd never heard of such a device) as an initial solution, and keep open search for 7A24's (and yes, limited input voltage range is fully expected!)
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