Puzzling RFI Question

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bob550

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I have my 996xt connected to my computer on a full-time basis. I use a null modem cable from the rear serial port connected to a serial-to-USB adapter cable. For the past couple of months, I have been experiencing interference on a particular VHF frequency for my local PD, 156.090. That interference is present on the 996xt, my Icom IC-R5, and my old Uniden BC-890, where the squelch remains open on that frequency as a result (no CTCSS). In attempting to locate the source through trial and error, I found that the serial-to-USB adapter is the source of the interference. When I disconnect the adapter from the computer, the interference stops. When I disable the adapter in Device Manager, the interference stops. So, has anyone else ever experienced this before? Can anyone suggest any solutions to this?
 

JamesO

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Wrap the serial to USB adapter with a few sheets of foil and see what happens.

You may need to connect a ground wire from the foil to the computer or the radio as well.
 

N0UDG

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I have my 996xt connected to my computer on a full-time basis. I use a null modem cable from the rear serial port connected to a serial-to-USB adapter cable. For the past couple of months, I have been experiencing interference on a particular VHF frequency for my local PD, 156.090. That interference is present on the 996xt, my Icom IC-R5, and my old Uniden BC-890, where the squelch remains open on that frequency as a result (no CTCSS). In attempting to locate the source through trial and error, I found that the serial-to-USB adapter is the source of the interference. When I disconnect the adapter from the computer, the interference stops. When I disable the adapter in Device Manager, the interference stops. So, has anyone else ever experienced this before? Can anyone suggest any solutions to this?
Do you also have this problem if you use the front connector?

Are you using a Uniden cable or a third party cable?
 

bob550

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Do you also have this problem if you use the front connector?

Are you using a Uniden cable or a third party cable?
Since I use the rear serial port, I made the scanner/computer connection with a null modem cable to the serial/USB adapter. I did not use the supplied Uniden cable for long after I got the 996xt last November, so I couldn't say if that would have caused interference. However, I tried disconnecting the null modem cable from the serial/USB adapter leaving only the adapter plugged in to the computer, and still had the interference. That tells me that only the adapter is responsible for the RFI. I also tried using other USB ports for the adapter, with no change.
 

JamesO

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Did you try the tin foil trick??

Faraday cage and/or toriodal clamp on filters do wonders!

The other option may be use an external antenna if the RFI is entering the front end of the receiver..
 

UPMan

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Likely will need to try a toroid on the lead coming from the adapter to the scanner. Might help w/RF coming in through the cable itself (not radiated from the body of the adapter).
 

nr2d

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Likely will need to try a toroid on the lead coming from the adapter to the scanner. Might help w/RF coming in through the cable itself (not radiated from the body of the adapter).
Radio Shack makes a great toroid for RFI issues.

Snap-Together Toroid Choke Core-2-Pack - RadioShack.com

I've used these to eliminate RFI to my digital products from my HF ham transmitter. They may work for your problem also. Here is another RF choke from Radio shack:

Snap Choke Core - RadioShack.com

On the bad side though I had a problem I was working for NASA and the only way we could get rid of the RFI was to disconnect the USB device.
 

bob550

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Did you try the tin foil trick??

Faraday cage and/or toriodal clamp on filters do wonders!

The other option may be use an external antenna if the RFI is entering the front end of the receiver..
I'll try the foil trick tonight when I get home tonight. I also tried ferrite chokes from RadioShack. But, I can only get one loop due to the short length of the adapter cable. I didn't notice any reduction in the interfering signal. I may also try using a USB extension cable between the adapter and the computer to gauge the effect. I can also lower the baud rate via Device Mangler and see if that makes a difference.

I am using an internal antenna for the BC890 as I only use that to monitor local frequencies. However, I use a combination of an external and internal for the 996xt. The internal is used to boost 800 MHz frequencies, and is a mobile mount sitting on a metal file cabinet in the shack room. Be aware also, that this interfering signal is evident one flight up and at the other end of my 40 foot house, although to a lesser degree.
 

UPMan

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You should be able to determine whether the RFI is radiated (where foil will work) or conducted (where foil won't work but a toroid might) by simply disconnecting the USB from the scanner. If the RFI goes away, then it is conducted and will be very difficult to eliminate. You might have to try a different adapter (using different chipset, too) which would hopefully throw RFI at different frequencies.
 

bob550

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Did your system ever work without generating the RFI?
I really can't say for sure. When I first noticed the interference, I had already begun using the serial/USB adapter. It may have done this from day one, but until now, I never suspected that was the cause. I know you're thinking I may have a bad unit. I may, but it does work flawlessly for data transfer in all other respects.
 

bob550

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You should be able to determine whether the RFI is radiated (where foil will work) or conducted (where foil won't work but a toroid might) by simply disconnecting the USB from the scanner. If the RFI goes away, then it is conducted and will be very difficult to eliminate. You might have to try a different adapter (using different chipset, too) which would hopefully throw RFI at different frequencies.
I believe the RFI to be radiated. It does not cease if I either disconnect from the scanner, or disconnect the interconnecting null modem cable from the adapter itself. The RFI is there if only the adapter is connected to the computer.
 

UPMan

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If it is radiated, then I'd try using a USB extension then relocating the scanner to be physically farther from the USB adapter. RFI like this is typically very low power, and the signal strength will fall off a the square to the distance from the adapter, so might just mean a couple of feet.
 

bob550

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If it is radiated, then I'd try using a USB extension then relocating the scanner to be physically farther from the USB adapter. RFI like this is typically very low power, and the signal strength will fall off a the square to the distance from the adapter, so might just mean a couple of feet.
The interfering signal seems to have a range of about 20 feet. I would be unable to move the scanner that far away.

I have covered the RS-232 end of the serial/USB adapter with foil, wrapped as much of that cable as possible around a ferrite choke, added a short USB extension cable, and added another choke to that cable. The RFI may have been reduced in strength as a result, but it's still present.

Also, I should mention that the RFI tends to wander about +/- 25 kHz back and forth! So, sometimes it's interfering with 156.090, and sometimes it isn't.
 

bob550

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Well, I finally took the only step I'd not already taken: picked up a new serial/USB adapter cable and .... no more interference! I suppose I simply had a bad unit, or the RFI was just a byproduct of that brand/model. Thanks to all for your input!
 
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