Monitor the CMOS level with 50 Ohm oscillograph

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chang830

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Hi,
My circuit oupts the TTL/CMOS level, but my oscillograph only have 50 Ohm interface. How can I use this oscillograph to monitor the CMOS output? Adding a series high resistor can work?

Thanks
 

Adding a higher value series resitor will further lower the signal amplitude due to the voltage divider action, and the mismatch may cause ringing on the monitored signal. I think you be better off using a series 50 ohm resistor, just keep in mind that the signal level will be half of the regular TLL/CMOS levels.
 

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    chang830

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If carefully constructed, a series resistor into 50 ohms makes an excellent wide-bandwidth low-capacitance probe - much better than typical 10X 10-megohm passive probes. However, the resistance probe will load your circuit somewhat, but that's usually not too important when probing RF and high-speed logic. You must decide if the load will affect your project. Also, it is an attenuator, so you need an oscilloscope with sufficient gain.

I have two multi-GHz Tektronix probes that contain little more than a series resistor. One is a 10X probe using a 450 ohm resistor, the other is a 100X probe using a 4950 ohm resistor.

You can build good resistance probes yourself. Here is one way to do it:
https://www.emcesd.com/1ghzprob.htm
 

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    chang830

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