Persistent CM108 whine

Hello - I’ve got a horrible high pitched whine in my node that i cannot seem to beat Originally, I had everything kind of haphazardly assembled, and the whine would vary greatly if I moved any component of the system, so in an effort to eliminate it, I built a shielded enclosure, and added all kinda of filtering nad shielding, and it won’t go away.

Currently, I have the PI sitting a few feet away from everything else. The fob is connected via a shielded USB cable that runs into an enclosure made of PCB blanks soldered together, and surrounding it completely, except fro the hole where the cable enters. The cable itself has a choke on all of the leads inside of the enclosure. The cables going from the enclosure to the radio are shielded, and have chokes. The power into the radios has chokes, and I added decoupling caps at the radios. The radios are also in shielded cases. The fob is also isolated from the radios via isolation transformers on both audio lines, and optos on the PTT and COS lines. The only electrical connection between the radios and the fob is via ground since they share a common power source, but even if I separate the power supplies I get the whine.

If I disconnect the RX radio, and key the TX radio, the sound is there until I unplug the fob from the PI. Moving literally anything, changing anything about the antenna output, or touching anything changes the volume of the noise.

I’ve seen other people alluding to this issue, the only one who fixed it fixed it by adding a choke on the USB, which i have done. Someone else said it was due to the buck converter for the 3v3 on the CM108, in which case I don’t know how it would be possible to eliminate.

Any ideas? I’m at my wits end with this.

Try moving your HT further away from node. Possible RF interference. Disconnect lines to fob one at a time. Process of elmination.

I’ve moved the antenna (doesn’t help), but moving the radio further isn’t really feasible. There aren’t any lines I can disconnect from the fob. The sound is definitely on the audio output of the fob, which is electrically isolated, the PTT is not electrically connected to anything, and the only other connection is USB, and I have the issue whether directly connected, or connnected through a cable with filtering added, whether or not I am using the opto and the isolation transformers, and whether or not I have the other radio connected.

I misunderstood, Thought you were getting noise when transmitting to the node.

If this noise is there when the fob is only connected to the high side of volume control. 1) audio line to radio wrong connection or 2) power supply problem. Do you have a scope to check how clean the DC line is? Don’t try to use a desktop charger for a power supply - very dirty DC.

Sorry, just to clarify, this noise happens when the node is transmitting. It’s setup as a duplex node, but I eliminated the second radio while testing for simplicity. With everything disconnected other than the fob to the pi and the audio from the fob to the radio (via isolation transformer), if I manually trigger PTT, the noise is present in the signal transmitted. The audio connection is to the correct place (per the numerous bf888s tutorials out there as well as my own validation), and it happens regardless of the power supply used, I have tried several.

I haven’t hooked up headphones yet, but I would almost certainly bet that it’s present on the audio line.

What do you use as the power supply for the pi?

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Le mar. 21 juil. 2020 à 15:09, Derek Chauran via AllStarLink Discussion Groups <noreply@community.allstarlink.org> a écrit :

af7ux
July 21

Sorry, just to clarify, this noise happens when the node is transmitting. It’s setup as a duplex node, but I eliminated the second radio while testing for simplicity. With everything disconnected other than the fob to the pi and the audio from the fob to the radio (via isolation transformer), if I manually trigger PTT, the noise is present in the signal transmitted. The audio connection is to the correct place (per the numerous bf888s tutorials out there as well as my own validation), and it happens regardless of the power supply used, I have tried several.

I haven’t hooked up headphones yet, but I would almost certainly bet that it’s present on the audio line.


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I’ve had this in the past. It has always been bad grounding/shield and/or RF field high around the FOB (CM108) . Also it seems to appear only close by the transmitter, have you tried moving some diatance away?
GeorgeC W2DB

The power supply is a Canakits PI specific power supply.

I have tried moving away, approximately 60 feet away doesn’t change the whine. If you’ve been able to fix it in the past, would you mind sharing what additional shielding/grounding you have done? I added a ton of shielding, grounding, and filtering and it didn’t go away.

Have you try to change that power supply just in case this have a large ripple on it?

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Le ven. 24 juil. 2020 à 00:26, Derek Chauran via AllStarLink Discussion Groups <noreply@community.allstarlink.org> a écrit :

af7ux
July 24

The power supply is a Canakits PI specific power supply.


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In Reply To

Pierre_Martel
July 22

What do you use as the power supply for the pi? ··· (click for more details)


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Yes, I have tried a few different power supplies and nothing changes

Maybe it will help if we can hear it. Could you make a short recording and post the mp3 here?

I never got around to recording, but I finally seem to have sufficiently mitigated the issue by soldering the antenna “ground” to the case. It was in contact before, but was only held in place with tension, and was not a good electrical connection.

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