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Old 22-01-2014, 18:11   #1
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Pactor II, USB-Serial, Laptop - Grounding Problem?

I've managed to successfully connect the SEA 235 radio and Pactor II modem to my Windows 7 laptop (using a USB-Serial adapter) and send/receive email via Winlink 2000, Airmail and RMS Express! I'm finding when transmitting, there's high-voltage from radio traveling back to the laptop's mouse touchpad. The funny thing is I've noticed this when the laptop is running off battery and not connected to the boat's AC.

Do you think there's a ground problem between the radio and modem? modem and laptop? Or, is the laptop picking up the broadcast and manifesting that as stray current on the touchpad? I'm assuming the radio to modem is grounded and the interconnect cable has the proper ground, but I will double-check that. How does one ground a laptop, when running on laptop battery - seems counter intuitive.

I would like to figure this out, so not to fry the laptop or keep getting shocked.

Thanks (again) for all your help - bit by bit, I'm figuring this out!
Don
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Old 22-01-2014, 23:51   #2
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Re: Pactor II, USB-Serial, Laptop - Grounding Problem?

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I'm finding when transmitting, there's high-voltage from radio traveling back to the laptop's mouse touchpad.
How does one ground a laptop, when running on laptop battery - seems counter intuitive.Don
Go with the basic ohmic law.
Try to reduce the voltage which you find at the laptop.
When you are going to ground the laptop,
you will get a higher current that might distroy a USB output of the laptop.
With ferrits you can introduce more inductive resistance.
Wilhelm
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Old 23-01-2014, 02:41   #3
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Re: Pactor II, USB-Serial, Laptop - Grounding Problem?

It is likely that the issue is radio energy picked up by the serial/USB cable. Wilhelm is on the right track with the ferrite idea.

See Icom SSB Radio Kits & Components . Gary sells good Type 31 ferrites. Put one at each end of the serial cable and each end of the USB cable. Make sure the pieces of the ferrite are solidly in contact with one another before you snap the shell over them.
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Old 23-01-2014, 07:45   #4
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Re: Pactor II, USB-Serial, Laptop - Grounding Problem?

Great idea - I'll try the ferrites. I'll keep you posted - thanks,
Don
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Old 23-01-2014, 08:46   #5
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Re: Pactor II, USB-Serial, Laptop - Grounding Problem?

One pass through a ferrite may help, sometimes not enough though. The effectiveness of a ferrite is increased as the square of the number of turns through it. If you use a ferrite with a large enough inside diameter to take more than one loop of cable you dramatically increase its effect. So two passes has the same effect as four ferrites with one pass, and three passes is equivalent to nine ferrites with one pass. Just something to keep in mind. One complication is that DC current through the cable can saturate the ferrite and render it ineffective. This shouldnt be an issue in this application since DC current flow should be small.

Chip
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Old 02-04-2014, 17:34   #6
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Re: Pactor II, USB-Serial, Laptop - Grounding Problem?

To report back, I managed to block the RFI causing high-voltage across my laptop mouse pad. I tried several sizes of ferrites on various cables. The small beads, straight through didn't help. I ended up using two large 1/4" ferrites with 3 wraps of cable, close to both ends, on just the serial cable (from modem to the USB-Serial adapter plugged into the laptop). I did experiment with other cables, etc., and reduced it down to just needing the ferrites on the serial cable.

A few other things I've learned. My radio chassis is grounded. However, if the negative DC power cable is long, the manufacturer recommends a shorter ground direct to the boat common bus. My audio cable connects the chassis' ground between the radio and Pactor II modem. There's no reason to run an additional ground to the modem. I didn't worry about ground on the laptop (at this time). The radio manufacturer also claims that the primary cause of RFI is most likely due to poor grounding on the antenna tuner. I don't think I need to go there yet, since the radio transmit seems to be working well.

My getting high-voltage shocks across the laptop mouse-pad seems to be resolved at this time. Thanks for all advice above and sorry it took me so long to sort out.

Don
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