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uBITX + Teensy audio interface - ground loops (?), RX/TX audio bleed? #ubitx #teensy

Rob French (KC4UPR)
 

Alright!

So, I've put together a rudimentary audio interface and DSP filter using a Teensy 3.2 and the Teensy audio board.  Currently, it's on a breadboard.  I have three 1:1 audio transformers to interface the Teensy to the uBITX:

RX audio: Vol-H => 1:1 => Teensy audio in (L) => (DSP processing) => Teensy audio out (L) => 1:1 => high-side of volume pot
TX audio: Mic => preamp => Teensy audio in (R) => (DSP processing) => Teensy audio out (R) => 1:1 => 10uF blocking cap => uBITX mic in

(I actually haven't hooked up the mic/preamp yet, but I'm actually using USB audio.)

So that's the other piece--I've put together a sketch for the Teensy that allows switching between audio inputs/outputs, enabling/disabling filters, etc.  So I've been doing FT8, and the Teensy sketch actually takes USB audio in, and sends that to the Teensy audio out (R) for transmit.  Likewise, I take the audio in (L) and send that to the computer via the Teensy USB audio.

So, it IS working... I've just made a few FT8 contacts with my terrible attic antenna, and they seem like about what I've been getting when I was using a simple USB audio adaptor.  I will eventually need to do some kind of assessment of the transmitted audio, make sure that's up to snuff (especially make sure I'm not transmitting noise/hum).

Which brings me to my issues:

(1) When I just have the Teensy plugged into the computer via USB, I get terrible hum.  Nearly unusable.  But, when I also plug in the Raduino to the computer via USB for CAT control, the hum goes away!  So I'm guessing this is some kind of ground loop caused by the USB cables and the computer??? Why would it go away when I plug in the second USB device?  And why aren't my transformers "protecting" me from the hum?  As far as I can tell, the ground between Teensy and the computer is isolated from the input and output audio grounds.  (That said, I will note that for ease of use on my breadboard, the uBITX side of the transformers are all tied together, both the input transformers and the output... maybe that's an issue?)

(2) Whenever I transmit, I also get the transmitted audio in my uBITX headphones.  So, I'm FAIRLY certain I've got my sketch configured to use the left channel for all RX audio, and the right channel for all TX audio.  Likewise, I tell WSJTX to do input on the right channel, and output on the left channel.  This transmitted audio also has pretty significant hum on it.  I'm wondering if it's coupling between the left and right channel output transformers?  (They're about an inch apart.)  It is pretty loud... I wouldn't expect that if it were coupled, but I'm really not smart on this.  

Thanks,
Rob KC4UPR

(FYI, I've attached the audio layout of my sketch.  As noted, L channel is always intended to be RX, and R channel is always intended to be TX.  I use the rxAudioMixer and txAudioMixer as switches to select which input to pass through.  I probably need to eliminate some paths that I put in when I was testing the sketch... e.g. there will never be RX audio input coming from the PC, nor should there ever be TX audio output going to the PC (unless I want to do it for monitoring).  The txAudioSwitch and rxAudioSwitch are used to determine if I send unfiltered audio to the PC, or filter it first.)

Rob French (KC4UPR)
 

An additional note, I am powering the Teensy currently via USB.  My intent eventually is to power it off 5V internal to the uBITX.  So I don't know if that will help, or hurt.  (I'd like to end up with a single USB port, with the Teensy being both a USB Audio and a USB Serial device, and passing through CAT to the Raduino.)

Evan Hand
 

I believe that there is a grounding issue, though not a "loop".  When you have only the Teensy connected, there is not a reference to ground on the computer, so you get the hum.  This would be irrespective of the transformers.  When you connect the Raduino, you get the ground from the uBITX.

I would be curious if you are using a laptop or a desktop computer?

On the other issue, feedback into the uBITX headphones, I have had that issue, along with distortion in transmitted audio.  For me it was traced to RF getting back into the uBITX along the coax shield and the mic input.  I solved that with snap on ferrites at the entrance of the coax to the shack.  This is a trait of the uBITX as the audio is not completely muted to allow for the side tone in CW mode, at least that is what I surmized, as grounding that input removed the issue as well.  Since you are bread boarding, the total audio system could be an antenna feeding back into the uBITX.  Ferrites on the audio lines might help if you determine that it is RF causing the issue.  Try transmitting into a good dummy load that would have minimal radiated RF.  If the tx distortion goes away, then RF is the issue.

Above are just some ideas to try.

73
Evan
AC9TU

Rob French (KC4UPR)
 

Evan, thanks for the input.

Regarding the feedback into the headphones... that totally makes sense.  I will try the dummy load test.  My breadboard is a bit of a spider's nest, and even has a couple of wires floating in space at the moment (the mic input to the Teensy, which I don't actually have hooked up yet, since I'm using USB audio for my transmit audio).  I also noticed that moving my hand in various places around the system seems to impact the sound of the feedback audio--muffle it, change the tone a little, change the amount of hum, etc. 

I'm using a laptop computer.

I don't understand how I'd be missing a connection to ground, however.  USB carries a connection to ground, so the Teensy (and the Teensy side of the audio transformers) should be referenced to the same ground as the computer, via the USB cable, right?

I wonder if I should try switching USB ports?

Evan Hand
 

Most Laptops do not have an earth ground.  You do have the shield of the USB port, however that would not necessarily ground for AC interference.

It was just a suggestion for a possible cause for the hum disappearing when the uBITX is connected, as the shield of the coax is usually at earth ground.

Good luck and 
73
Evan
AC9TU

Jack, W8TEE
 

What happens when you connect a 12V battery?

Jack, W8TEE

On Friday, October 4, 2019, 9:28:51 AM EDT, Evan Hand <elhandjr@...> wrote:


Most Laptops do not have an earth ground.  You do have the shield of the USB port, however that would not necessarily ground for AC interference.

It was just a suggestion for a possible cause for the hum disappearing when the uBITX is connected, as the shield of the coax is usually at earth ground.

Good luck and 
73
Evan
AC9TU

--
Jack, W8TEE

Rob French (KC4UPR)
 

I have not tried that.  You are referring to 12V power of the system as a whole?

I will note that, I don't believe I've had any significant hum issues previously (i.e. prior to hooking up the Teensy) using my Samlex power supply.

Rob French (KC4UPR)
 

Trying to visualize this in my noggin... so you're saying, a possible source of the hum is based on the following:

- Teensy IS at ground potential of the laptop, which IS NOT (probably) at earth ground (I will note, I was using the laptop plugged in).
- Teensy and laptop are isolated from uBITX.
- So somewhere along the line, hum is getting into the Teensy/laptop combination, which is then transmitted as audio through the transformer into audio output circuit (I'm using the uBITX LM386 for output).
- Then, when I plug in the Raduino to USB, it brings the laptop, Teensy and uBITX all to the same ground potential, and my hum goes away.

Maybe? 

At some point, I'm going to have an internal (to the uBITX) 5v supply for the Teensy (I know there's a trace on the Teensy I need to cut when I do that), which will be tied to the incoming power DC ground for the uBITX.  Should I expect that to help?

I suppose another check I could do is to temporarily tie the Teensy and and uBITX grounds together on my breadboard, and see if that solves the problem... though, that makes my whole isolation transformer scheme seem a little useless...

Martin Potter
 

In general transformers are used to isolate one circuit from another, as well as to transform the impedance, etc etc. So it is quite possible, and sometimes desirable, to have different ground circuits on each side of the transformer. You could try rearranging your ground connections to take advantage of the isolation provided by the transformer.
Very 73,
... Martin VE3OAT

Rob French (KC4UPR)
 

So!  An update!

I have found 3 ways I can make the hum go away.  Any of these on their own appear to work: (1 and 2 are with just the Teensy USB plugged in)

(1) Unplug the laptop from it's power supply... so clearly, the laptop's AC power is a player here.  Hum goes away.

OR

(2) Short the ground side of the transformer input to the ground side of the transformer output.  I.e. tie my grounds together across the transformer.  Hum goes away (but, this doesn't seem like the right solution, because then why am I using the transformer, if the grounds aren't isolate?)

OR

(3) (What I observed previously) Plug in both the Teensy USB and also the Raduino USB.  So this is also a case of the two grounds being tied together.

I've got to hit the sack, but do these observations help at all?

Thanks!
-Rob