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Ground isolation loop problem ? Motorboat ticking sound
Ground isolation loop problem ? Motorboat ticking sound
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Old 15th April 2017, 02:00 AM   #1
rajkumar88 is offline rajkumar88
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Default Ground isolation loop problem ? Motorboat ticking sound

Hi I am using a bluetooth module and giving output of it via differential to single ended using TPA6138 and it output given to audio output via aux or RCA analog. And I am having a tick tick sound when we hit advertising mode on the Module.
Advertising mode is like discoverable mode. It is like getting motorboat sound and surprisingly it is synchronizing with the LED's. I also have turned off the LED's by giving LED turn off command. Still I hear the tick tick sound. I guess it is in the ground because of ground isolation. How do we get the noise off the ground loop. Or is it the module creating the noise. It is actually a high frequency noise from the output of the bluetooth module the differential output.
TIP: I guess the sound is in the ground. I have analog and digital ground separated via 0ohm. I replaced it with ferrite bead it reduced it very lightly. Its random high frequency sound which is not going. Any advice would be appreciated. I tried a lot like taking the output audio running it via LOW pass filter also tried a second order band pass nothing worked.
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Old 15th April 2017, 04:56 PM   #2
Fast Eddie D is offline Fast Eddie D  United States
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What are you using as an LED flasher circuit? That is the noise you're hearing - specifically, you are hearing the timer capacitor discharging or recharging.

You must isolate your logic from all audio circuitry. Logic grounds are dirty compared to audio grounds. The logic must have its own power supply filter capacitor. Isolate both sides of the logic power supply from the audio supply with resistors. Find the largest value resistors that will allow the logic to operate correctly.

The ideal solution is to have a separate power supply for the logic. You can use the same power transformer winding, but use a separate rectifier and filter capacitor for the logic. Do not connect the grounds together in any way. This works best.

Last edited by Fast Eddie D; 15th April 2017 at 04:59 PM.
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Old 15th April 2017, 05:04 PM   #3
rajkumar88 is offline rajkumar88
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It is actually an Bluetooth Module. BC127. It says in its data sheet that it has to be placed at the edge specially the antennae for maximum range connectivity. But main was not to place any copper layer or analog ground in the top nor bottom layer. So i left it with no grounds copper template in that particular region. The noise is coming from the bluetooth module itself.
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Old 16th April 2017, 03:40 PM   #4
Fast Eddie D is offline Fast Eddie D  United States
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Either it's hooked up wrong, or else the module is crap.

Your timer circuit cycles whether the LED flashes or not. That's why you hear it whether the LED is on or off.

Digital and audio grounds should be kept completely separate, if at all possible.

This is why I prefer to design my own stuff. It's easier than trying to work around the deficiencies of commercial designs. Hacking someone else's design is often harder than doing it from scratch.

I'm building a preamp right now that has modest digital control, including a flashing LED for standby mode. The digital circuit is noisy as hell, but nothing comes through the audio chain. That's because I did it right the first time, instead of trying to save a few pennies or using a cheap module from ebay.
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Old 16th April 2017, 07:26 PM   #5
rajkumar88 is offline rajkumar88
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I have kept digital and analog grounds separate only thing is how do i connect them. I have used 0ohm resistor. When I use 120ohm ferrite bead the noise lowers a little bit. Which one should I use ?or try more to reduce or filter the high parasitic noise.
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Old 16th April 2017, 08:13 PM   #6
Arty is offline Arty
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2 didoes anti-parallel to eatch other, bypasse with a 5 ohm 10 watt resistor should do the trick.
sortha.
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Old 16th April 2017, 08:56 PM   #7
rajkumar88 is offline rajkumar88
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What does it do ?
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Old 17th April 2017, 04:01 AM   #8
Fast Eddie D is offline Fast Eddie D  United States
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Isolates the grounds, somewhat.

Artie's right. The larger the resistor, the better.

Your "120 ohm ferrite bead" is 120 ohms at a frequency much higher than the time constant of your circuit. It won't filter out the lower frequency components of the noise. Look at the datasheet.

Can you use a relay to isolate the grounds? That's what I'm doing. I think it really makes it simpler, and offers better results.
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Old 17th April 2017, 04:25 AM   #9
rajkumar88 is offline rajkumar88
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Can you explain in detail how you using relay ?
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Old 18th April 2017, 04:01 PM   #10
Fast Eddie D is offline Fast Eddie D  United States
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Provide a schematic or block diagram of your setup and maybe I can help you.

As far as logic circuits, my designs only drive relays or LEDs. The audio and logic grounds are completely isolated. The audio circuits are switched by the relays.

I don't know why anyone would do it any other way. There is no reason, no excuse for an LED or logic chip to share a ground with the signal ground. That's just lazy.
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