Faint 1 Hz "tick" heard on v5 uBITX, seems to be related to I2C #ubitx

Rob French (KC4UPR)

Hey there,

When I turned up the volume on my v5 uBITX relatively high, I realized that I was hearing a 1 Hz, somewhat high-pitched "ticking".  Now that I'm aware of it, I can hear it at lower volume settings too.  It appears to be related to I2C (I have an I2C 20x4 LCD display), since I get the exact same sound more rapidly if I turn the encoder (which obviously changes frequency on the display, requiring a display update as well as synthesizer update, and hence I2C noise).  I haven't looked into the KD8CEC source code, but I'm guessing there's a nominal 1 Hz message being sent on I2C in the absence of any other I2C traffic.  

Turning down my RF gain (I have the kit-projects.com ND6T AGC board installed, with a 1K audio taper pot for RF gain) makes the I2C noise more pronounced relatively to noise, i.e. turning down the RF gain makes my noise generally go down, but the I2C noise volume remains constant--so I'm assuming that means the I2C noise is coupling in downstream of the AGC, but that obviously isn't hard since the AGC is prior to the first RF.  I'll be honest, I probably didn't do enough testing/notetaking before installing the AGC to be able to say whether or not that is a cause... I know kit-projects recently removed the RF gain from the kit as offered, citing user challenges.  I attached an RF gain using two lengths of RG-174, with the shield of each grounded at the pot but not at the PCB, as I saw recommended on ND6T's website.  I do know that I had discovered the I2C "tuning noise" prior to the AGC installation, I just hadn't noticed the 1 Hz "tick".

So, anyway... I guess I'm talking myself out of the idea that I am picking up extra noise now with my install of the AGC.  Just trying to figure out how to alleviate it now.  Shield the I2C cable between the Raduino and the LCD?  Any thoughts on how to do that?  So far I haven't managed to find any shielded I2C cables.  I was thinking of taking the braid from some coax, inserting my leads through it, and then putting some heat shrink tubing on it.  Seems complicated... would definitely like to have a good vector on reducing the I2C noise before I expend too much effort!  Appreciate any suggestions...


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