Electronics > Beginners

Troubleshooting; finding cause of 35MHz signal

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permal:
Back home again if either David or boB has any feedback on the placement of the caps. If an actual image of the PCB is desired, please let me know.

dmills:
I don't think this is the actual problem, but how big is the loop formed by the cap ground to get back to the chip ground pin?

A via or two to strap the cap ground pour down to the ground plane would really not hurt.

Regards, Dan.

dmills:
Also, why the bidirectional part?

Those things tend to be slightly iffy at the best of times (see the note about not using pull up resistors in the TXB0102 datasheet), they have their place but I would want to be VERY sure about exactly what was on the other end of the link before using one.

If I was doing that I would probably use either a part that could be explicitly direction switched from the micro (Some sort of level shifting tri state buffer, bound to exist), or a single direction part.

Regards, Dan.

permal:
I was recommended the part for this by someone, didn't think much about it being biderectional or that it would cause issues, esp. since only one side (the keypad) is driving the lines. (fwiw, the same part is functional in v2 of this circuit, although against another keypad and different PCB). I'll check the length of the ground-loop and get back to you.

permal:
The ground loop is about 10 mm. Can possibly be shortened using an extra via or two.

I added a opto-coupler between the keypad and the driver, but unfortunately it seems to slow for the 44us pulses generated by the keypad so I didn't get any wiser from that.

I'm going to take the advice from previous posts and redesign this with a one-way level converter - any suggestions for choice of component? Two channels, 5V to 3.3V and 44us pulses.

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