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Troubleshooting; finding cause of 35MHz signal
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permal:
Hi,

TL;DR; how do you find the source of a 35 MHz interference/feedback signal, when it only appears when connecting two devices together? The devices works fine by themselves.

The longer story: I'm working on v3 of my I/O card (entire schematic attached). I've got everything working; SD Card (both via SPI and MMC ifc) and digital & analog I/O via I2C. My last hurdle is the Wiegand interface for connecting standard entry/exit keypads. The specific part of the circuitry is as in the Wiegand.png image (page 3 in the attached pdf) - D0 & D1 run through a TXB0102DCU to convert 5V to 3.3V for the MCU (ESP32 Wrover-B module). I've verified that the voltage translation works as expected; every negative edge on the 5V side on either D0 or D1 results in an interrupt in the MCU. I did simply by pulling D0/D1 high/low with a resistor.

The keypad I have is an sTouch Reader R-W. From what I can tell, it gives a fair output, as seen in the image "DS1Z_QuickPrint1.png"

The problem is that as soon as I connect the keypad to the 5V side of the TXB0102DCU on my I/O card, I get the signal seen in image "DS1Z_QuickPrint2.png" and "DS1Z_QuickPrint3.png" on D0 and D1 (though only one line is connected in those images). How do I figure out where this interference/feedback signal comes from? Secondly, what I can do about it?

The measurement tools I have available are an unlocked Rigol DS1054 and Dave's 121GW multimeter.

PS: I'm already aware of that the NPN transistors are meant to be on the low side so no need to point that out. If you have constructive feedback on other changes I should do for v4, please let me know.
David Hess:
It looks to me like the TXB0102 bidirectional level translator is oscillating after it receives its first pulse.  That suggests a lack of termination where the cable length is long.

How long is the keyboard cable?

I would try a series or AC shunt termination at the output of the TXB0102 to quash any reflections which could be causing it to oscillate.
permal:

--- Quote from: David Hess on December 27, 2018, 06:17:48 pm ---It looks to me like the TXB0102 bidirectional level translator is oscillating after it receives its first pulse.  That suggests a lack of termination where the cable length is long.

How long is the keyboard cable?

I would try a series or AC shunt termination at the output of the TXB0102 to quash any reflections which could be causing it to oscillate.


--- End quote ---
The wire from the keyboard to PCB is about 25cm, but it's supposed to allow extensions to a few meters.

Can you elaborate on what an AC shut termination is, I've never heard that term before?
David Hess:

--- Quote from: permal on December 27, 2018, 06:32:42 pm ---Can you elaborate on what an AC shut termination is, I've never heard that term before?
--- End quote ---

An AC shunt termination is just a resistor and capacitor connected in series between the signal and ground.  The capacitor blocks the DC voltage so that the DC operating point is not disturbed.
permal:
I see. I'll try to add something to the circuit and see what happens.
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