Electronics > Beginners
Troubleshooting; finding cause of 35MHz signal
David Hess:
--- Quote from: permal on December 29, 2018, 04:48:22 pm ---Properly decoupled - what do you mean?
--- End quote ---
I mean the power and ground to the level shifter need to be low impedance and properly decoupled. This is especially important when driving a low impedance load like a cable.
permal:
Please excuse me if I'm not understanding you correctly, but level shifter is actually not driving the cable, the cable is on the outside, i.e. the keypad is what is driving the voltage in the cable.
Ground is connected to the ground plane, can't get lower impedance than that I think? I've learned that a star-configuration would have been better than what I used for my rails, they both are a few mm wide so it's not powered b y a regular 0.25mm trace. There is a 100nF cap just by the shifter for both 5 and 3.3V.
Does this answer your question?
David Hess:
--- Quote from: permal on December 29, 2018, 05:42:40 pm ---Please excuse me if I'm not understanding you correctly, but level shifter is actually not driving the cable, the cable is on the outside, i.e. the keypad is what is driving the voltage in the cable.
--- End quote ---
The TXB0102 is a bidirectional level translator so the keypad has to overcome a 4k load to drive it. When it switches states, internal one-shots drive *both* sides hard. It was not intended to be interfaced with a high capacitance line like a cable.
The TXB0102 also has a minimum input slew rate of 30ns/V so I wonder if the keypad is too slow to drive it without problems.
boB:
David is asking about C305 and C306 connected up to the bidirectional level shifter power lines to GND.
Those are the decoupling capacitors. How are they physically connected in the circuit ? Maybe a picture of the actual
circuit breadboard would be helpful.
permal:
Here's how the caps are placed in relation to the converter.
Thanks for all the help so far; I'm going a way for a week so I might not be able to reply until I'm back.
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