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BNC Adapter: 0 ohm vs 50 ohm Waveform Generator


Question

The output impedance of the waveform generator (of the AD2) can be chosen to be zero or 50 ohms when using the BNC adapter... can you explain reasons (situations) for using the 50 ohm over the 0 ohm option and vice versa? 

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Hi,

generally, the problem is that a 0 ohms output has theoretically an infinite power delivery capability, which is physically impossible. It may work correctly with some loads but not with others, and the transition area is ill-defined (e.g. peaks are clipped).

A 50 ohms output has a controlled behavior for all (passive) loads.

From a RF point-of-view, above some frequency the signal changes so rapidly that a reflection coming back from the other end of the (typically 50 ohms) cable is slightly delayed, compared to the generator output. Their interaction will cause unwanted RF effects such as an uneven frequency response. With a 50 ohms cable and 50 ohms source impedance, the reflection disappears once it reaches the source and the frequency response remains flat. This works even if the 50 ohms cable is not terminated at the other end (but only at the end!)

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28 minutes ago, xc6lx45 said:

Hi,

generally, the problem is that a 0 ohms output has theoretically an infinite power delivery capability, which is physically impossible. It may work correctly with some loads but not with others, and the transition area is ill-defined (e.g. peaks are clipped).

A 50 ohms output has a controlled behavior for all (passive) loads.

From a RF point-of-view, above some frequency the signal changes so rapidly that a reflection coming back from the other end of the (typically 50 ohms) cable is slightly delayed, compared to the generator output. Their interaction will cause unwanted RF effects such as an uneven frequency response. With a 50 ohms cable and 50 ohms source impedance, the reflection disappears once it reaches the source and the frequency response remains flat. This works even if the 50 ohms cable is not terminated at the other end (but only at the end!)

So then, in your opinion, 0 ohm shouldn't be used?

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They probably put the option there for a reason, try to find one for yourself.
It's easy to come up with relevant use cases, such as "drive a given voltage across some load". What will happen with a 50 ohms source?

 

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Just now, xc6lx45 said:

They probably put the option there for a reason, try to find one for yourself.
It's easy to come up with relevant use cases, such as "drive a given voltage across some load". What will happen with a 50 ohms source?

 

I appreciate that, but it would also be nice to know Digilent's reasons.  Thanks for you input :)

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Hi @Shalom,

 In case you want to match the output you'll use the 50Ohm jumper position in order to have adaptation. In this case your amplitude will be divided by 2. In the case you want to use the Gavegen just as a voltage source and don't want to reduce the amplitude,  you can leave it to 0V. 

Regards,

Bianca 

 

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