Does RF effect BS2?
Tobias
Posts: 95
· I am having a problem with a bs2, I have 3 5v inputs that go through a 1k resistor into the bs2 and a 10k pull down resistor. I have ten of these controllers in different trucks but with these two trucks I am having a problem that when I key the mic on my CB radio the bs2 sees a high on one of the inputs sometimes #1 or
#2 but only on these two trucks I have done some troubleshooting like changing the channel, checking the swr which is good, and turning down the output power on the CB.· The cable that supplies the power to the plastic enclosure where the bs2 is enclosed is shielded. I have experienced it on both the BS2 and the BS2e. What is going on? Your responds is greatly appreciated.
Thanks Toby
#2 but only on these two trucks I have done some troubleshooting like changing the channel, checking the swr which is good, and turning down the output power on the CB.· The cable that supplies the power to the plastic enclosure where the bs2 is enclosed is shielded. I have experienced it on both the BS2 and the BS2e. What is going on? Your responds is greatly appreciated.
Thanks Toby
Comments
Most of these antennas use the vehicle as a ground plane [noparse][[/noparse]part of the magnetic field that generates the transmission signal]. Since some of your CBs do not interfere with use, try to see what is different about their output from the CB and the antenna configuration. That is likely to lead you to a good solution.
Simply put, the CB doesn't load the antenna until you hit the mic button. Then a surge of electromagnetic waves surrounds the vehicle. My guess is the surge creates a transient that is being picked up by the inputs. With FETs it doesn't require much - microamps.
Also, check out if the power output of all the CB units are the same. More power output, more hassle. You appear to be already looking at that. Getting complete RFI shielding can be nearly impossible as all the wires going into the BasicStamp can pick up a signal.
Some primative low pass filtering on the inputs might help. I guess that would be bypass capacitors to trap fast spikes. Opto isolators work as low pass too. Beau recommends them to clean up such automotive spikes.
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
Post Edited (Kramer) : 3/17/2008 11:25:43 AM GMT
thanks Toby
A capacitor probably isn't necessary. Are these inputs floating, or do you have a pull-up or pull-down resistor on them? ... If there is a resistor, then perhaps a lower value might be needed. If there is no resistor I would maybe suggest one depending on how they are used and what they are connected to ... what do these inputs go to, and what does the schematic look like for them?
"Opto isolators work as low pass too. Beau recommends them to clean up such automotive spikes." ....hmm. I try to avoid Opto-isolators unless they are absolutely necessary.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Transmission antennas and the coax lines going to them can be quite involved to study.
A lot of power is output and has to get to exactly the right place or it is reflected back into somewhere. It is just as much an art as it is a science. CBers generally don't concern themselves with it as much at HAMs because the distances and the power outputs are not as huge. If you have an 100 watt transmission station, the antenna really has to be right, or you are just ruining your neighbor' TV shows due to sloppy work. I gets hard to tell you one solution as different kinds and configurations of antennas require different solutions. Just maybe the SWR meter reads okay, but on physical inspection you see the cable or connections are deteriorated. Weather gets in everywhere unless you bury things in silicon caulk. The coax can be creating the problem either from the shielding being improper or because it is not balanced with the antenna and the transmitter properly. And many CBs buy an antenna that they think will just 'self adjust' to whatever they want to do.
It seems that the 15ft coax is needed due to the wave length of the tranmission frequency. And the 12ft coax causes a line imbalance that sets up interference. This web site has the Navy's training manuals for electronic technicials. You can review the basics there and get a solid answer to why. http://www.zianet.com/NMAMARS/downloads/neets.htm
I guess you see why I suggested to stay focused on the antenna system. Having the majority of your Basic Stamps working seemed to indicate redesign of the BasicStamp setup alone wasn't going to make the problem go away.
Forgive me, but I may have made the whole idea of resolving input spikes more complicated. Beau really provided a much better answer at a 5k, 2.5k, or 1k pull down is a far better place to start with fixing the spike on the Stamps input. I didn't think I could be resolved so easily because it was RF.
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
(1) with a 10K pulldown and a 1K pullup, your 3.5v input is only 3.18 volts at the Stamp pin.· This gives very little noise immunity.
(2) 2.5 is a fairly high SWR, and your CB radios may be reducing their output in self-protection.· Perhaps you got a better match with short coax and the transmitter went to full power, thus imcreasing the interference seen by the Stamp.· The shorter coax wouldn't reduce the SWR, but it might result in a better match because it would change the phase of the reflected power.
(3)·you may have grounding problems, which are a·frequent problem·in vehicles.· Have you made sure your negative power connections, as well as the positive ones for the radio and the Stamp, go straight to the battery through reasonably fat wires, say #10, or #12 at the thinnest?· Also the radio feed, especially, should be separate from all other wiring in the vehicle.· It's amazing how voltages can jump around when you don't do that.· The stuff feeding the 3.5v may have its ground a volt or two lower, or higher,·than the stuff (the Stamp) that is being fed.· That could reduce your 3.5 volts by a lot, or conversely place an unintended voltage on the Stamp pins, making them see a 1 when it should be a 0, or vice versa.
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· -- Carl, nn5i@arrl.net
My experience is primaraly with SSB 2-30 Mhz and plastic boats. 3-5 watts is low power compared to the 150-1kw units i see but the theory should hold up.
Thanks, Toby
What is the proper valued resistor to use for the pullup and pulldown circuit?
Thanks, Toby
Can you provide a schematic of your current setup and possibly approximate wire lengths to your inputs as well as power supply wire lengths ?
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.