Home |
Search |
Today's Posts |
#1
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
At my nav station I have a bus bar with jumpers between the connectors
to provide NMEA data to a variety of equipment which wants to see info such as lat/long. It provided a nice way to connect multiple pieces of equipment to a single NMEA output. The problem seems to be with the data output from my SR161 AIS receiver. I have the AIS data going to single set of connectors on the bus bar which are bridged to the cable which goes to my RayMarine C80 chart plotter. All of the above cables have large ferrite chokes on both ends. When I connect the cable to the SR161 AIS receiver, the VHF radio which is located very near the above equipment receives static on occasion. By this I mean that I hear random periods of static on the radio which disappears only when I turn the squelch up quite high. If I unplug the data cable from the AIS receiver, the static disappears. How can I shield the bus bar so that I can distribute the data and bridge the AIS data such that I don't generate RF which interferes with the VHF? -- Geoff |
#2
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
GeoffSchultz wrote:
At my nav station I have a bus bar with jumpers...snip Connecting multiple equipments to a single RS-232 output can drop or overload the signal levels and affect the quality and performance some or all of the connections. I think that can generally start happening with more than two or maybe three three devices connected. That might be the root cause of your problem here. In a more professional installation, a signal multiplexer would normally be used for what you are doing. Using one of those will usually add some more features too, like allowing more than one talker to share NMEA data with multiple listeners. And some will allow data exchange with proprietary flavors of RS-232 (like Raymarine's SeaTalk) and have Bluetooth and other more modern whizzbang features. If you are crafty, you can make your own: http://www.vimms.org/multiplex/docu.html Or you can buy one of the good ones intended for marine use: http://www.shipmodul.com/en/index.html http://www.nolandengineering.com/ http://brookhouseonline.com/nmeamux_ext.htm Jack -- Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net (also receiving email at jacker at midmaine.com) |
#3
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
Using a bus bar for something like this is generally causing problems
because of the physical layout. A signal wire is supposed to have the return wire very close, in order to radiate in the least possible way. So if the TxD signal from the RS-232 port is going to some bus bar contraption while the ground is going to a different place, you effectively create an magnetic loop antenna, radiating as hell. To prevent radiation, keep signal and return in ONE cable at all atimes. What also helps it to limit the dV/dT of a signal, or in other words: slow down the flanks of the signal by using a low pass filter directly on the Sub-D9 connector of the AIS receiver. Such a filter could consist of a 47 ohm series resistor in the TxD signal line, followed by a 10nF capacitor to the signal ground. This will surpress high frequency signals that could interfere with an SSB or VHF set. Meindert |
#4
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
GeoffSchultz wrote in
oups.com: How can I shield the bus bar so that I can distribute the data and bridge the AIS data such that I don't generate RF which interferes with the VHF? -- Geoff VHF is much easier to fix than HF...... Where is the VHF ANTENNA, in relation to the AIS data lines? Where is the VHF radio? If they are near it, move them, instead of trying to fix the unfixable, namely, trying to shield stupid NMEA open-wire technology- in-the-plastic-boxes. Move the radio/antenna away from the noisy nonsense...which is MUCH easier. The VHF antenna system is a closed system, nicely shielded from the antenna to the radio. One bad thing happens...loose connectors or corroded shields open up this coaxial system to the outside noise intrusion. Check all connections to make sure the SO-239 connectors are tight. If any of the shields can be moved in the connectors, rebuild the connectors so the shields are making an ALL AROUND good connection. Crimped-on coax connectors are fine. So called "solderless" connectors from RatShack are NOT. They come apart too easily. Larry -- While in Mexico, I didn't have to press 1 for Spanish. It just isn't fair. |
#5
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
On Jul 4, 6:57 am, Jack Erbes wrote:
GeoffSchultz wrote: At my nav station I have a bus bar with jumpers...snip Connecting multiple equipments to a single RS-232 output can drop or overload the signal levels and affect the quality and performance some or all of the connections. I think that can generally start happening with more than two or maybe three three devices connected. That might be the root cause of your problem here. In a more professional installation, a signal multiplexer would normally be used for what you are doing. Using one of those will usually add some more features too, like allowing more than one talker to share NMEA data with multiple listeners. And some will allow data exchange with proprietary flavors of RS-232 (like Raymarine's SeaTalk) and have Bluetooth and other more modern whizzbang features. If you are crafty, you can make your own: http://www.vimms.org/multiplex/docu.html Or you can buy one of the good ones intended for marine use: http://www.shipmodul.com/en/index.html http://www.nolandengineering.com/ http://brookhouseonline.com/nmeamux_ext.htm Jack -- Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net (also receiving email at jacker at midmaine.com) Thanks for the pointers to multiplexors. I have 3 devices connected to the 4800 bps NMEA output and I don't seem to have any problems with data reliability or noise. It's when I connect the 38400 AIS data that I get the problem. I don't have time to analyze my options right now as I'm in the process of storing the boat for hurricane season, but when I get home I'll re-think my solution and perhaps get something that bridges to the SeaTalk bus so that I can get full access to that data. -- Geoff |
#6
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
On Jul 4, 7:06 am, "Meindert Sprang"
wrote: Using a bus bar for something like this is generally causing problems because of the physical layout. A signal wire is supposed to have the return wire very close, in order to radiate in the least possible way. So if the TxD signal from the RS-232 port is going to some bus bar contraption while the ground is going to a different place, you effectively create an magnetic loop antenna, radiating as hell. To prevent radiation, keep signal and return in ONE cable at all atimes. What also helps it to limit the dV/dT of a signal, or in other words: slow down the flanks of the signal by using a low pass filter directly on the Sub-D9 connector of the AIS receiver. Such a filter could consist of a 47 ohm series resistor in the TxD signal line, followed by a 10nF capacitor to the signal ground. This will surpress high frequency signals that could interfere with an SSB or VHF set. Meindert I understand. I guess that I should simply solder the two cables together and bypass my patch panel arrangement. Thanks for the suggestions. -- Geoff |
#7
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
On Jul 4, 6:25 pm, Larry wrote:
GeoffSchultz wrote groups.com: How can I shield the bus bar so that I can distribute the data and bridge the AIS data such that I don't generate RF which interferes with the VHF? -- Geoff VHF is much easier to fix than HF...... Where is the VHF ANTENNA, in relation to the AIS data lines? Where is the VHF radio? If they are near it, move them, instead of trying to fix the unfixable, namely, trying to shield stupid NMEA open-wire technology- in-the-plastic-boxes. Move the radio/antenna away from the noisy nonsense...which is MUCH easier. The VHF antenna system is a closed system, nicely shielded from the antenna to the radio. One bad thing happens...loose connectors or corroded shields open up this coaxial system to the outside noise intrusion. Check all connections to make sure the SO-239 connectors are tight. If any of the shields can be moved in the connectors, rebuild the connectors so the shields are making an ALL AROUND good connection. Crimped-on coax connectors are fine. So called "solderless" connectors from RatShack are NOT. They come apart too easily. Larry The VHF antenna is forward, on top of the mast; the VHF radio is at the nav station; the patch panel is mounted behind the VHF radio; the AIS cable run from the patch panel to the C80 which is at the wheel; the two cables are only in the same proximity for a few inches. Moving the VHF really isn't an option. I think that I'll follow up on Meindert's suggestions. -- Geoff |
#8
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
"GeoffSchultz" wrote in message
ups.com... AIS cable run from the patch panel to the C80 which is at the wheel; Ah, I don't know how your wiring is setup, but the C80 had an gavanically isolated NMEA input. To maximize surpression of interference, make sure you have one single cable with a twisted pair between the RS-232 plug of the AIS receiver and the NMEA input of the C80. Do not combine the signal return with the power ground of the AIS receiver or any other device. This ensures the currents in both wires are equal and thus maximum surpression of radiation is achieved. Eventually you could use screened cable, use a twisted pair for the data and return and connect the screen to ground at the C80 side only. The C80 is an IEC945 approved device so it is safe to assume that it's signal ground is clean. Meindert |
#9
posted to rec.boats.electronics
|
|||
|
|||
Supressing RF Interferance from NMEA Junction
GeoffSchultz wrote:
snip Thanks for the pointers to multiplexors. I have 3 devices connected to the 4800 bps NMEA output and I don't seem to have any problems with data reliability or noise. It's when I connect the 38400 AIS data that I get the problem. I don't have time to analyze my options right now as I'm in the process of storing the boat for hurricane season, but when I get home I'll re-think my solution and perhaps get something that bridges to the SeaTalk bus so that I can get full access to that data. I don't have a multiplexer or a boat that needs one but If I did I'd probably give them serious consideration. Being able to mix baud rates and different flavors of RS-232 and NMEA would have to come in handy sooner or later. The AIS thing is new to me, I've not been around it or used it at all yet but it sure sounds like a good thing and and something I'd like to have working as well as possible. I think having to diddle with the old hardware when you add new hardware to boats, is just one of the prices paid for Meindert is too modest to have said it here, but he is the designer and proprietor of the Shipmodul line I linked in my post. He knoweth of what he speaks and posts here regularly. He can always be relied on to straighten out and better explain my layman's mumblings about what is happening on stuff like this. Jack -- Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net (also receiving email at jacker at midmaine.com) |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Cheap navigation system | ASA | |||
A question about NMEA, AIS and Raymarine | Electronics | |||
NMEA: What confusion is that? | Electronics | |||
Connect Raymarine ST60 Multi to laptop | Electronics | |||
GpsMap 276C NMEA Interface Problem | Electronics |