Electronics > Beginners
RF question about coax
SoftwareSamurai:
I've got a project where I need to send a 2.3 MHz analog signal to an LED via some cable from the main board. I'd like to know if I can use a micro-coax cable for the connection. Let me explain:
My current circuit generates the signal and feeds it to an op amp with a transistor to drive the LED. The LED anode connects to Vcc (+5V). The LED cathode connects to the collector of the NPN transistor. The base is driven by the op amp through a 47 ohm resistor. The emitter goes to a 10 ohm resistor, which then goes to ground. The op amp's negative input is fed back from the emitter via a 1K2 resistor.
With everything on one board, this all works great. But now I need to move the LED to a separate board about one meter away and connect it to the main board via a cable of some sorts.
I know that if I just use a twisted pair, it's going to radiate like an antenna. I don't want broadcast the EM, just the light! (Probably wouldn't pass FCC testing either, which it must do eventually.)
I was thinking about using a micro-coax (0.81mm dia) cable. My theory is that I can put Vcc on the shield (which would connect to the LED's anode) and drive the current through the stinger (which would connect to the LED's cathode).
Q. Is this a stupid idea?
Q. Would using a micro-coax in this way isolate the signal, or would it still radiate like an antenna?
Q. Should I, instead, use two micro-coax cables, feeding Vcc through the stinger of one, and the drive current through the stinger of the other, grounding the shield for both, in order to prevent radiating the signal like an antenna?
Any help would be appreciated!
IanB:
I don't know anything about RF design, but you didn't mention shielded twisted pair? Network cables use this quite successfully and they surely operate at high frequencies. Are you overlooking a simple solution?
amspire:
You can use twisted pair or coax.
All your computer networking cable is just twisted pair.
If you think about it, the two wires are in proximity, and each is transmitting exactly the opposite RF, so they add up to zero or close enough.
Gigabit Ethenet cable uses unshielded twisted pairs transmitting a 125MHz carrier.
ADSL2+ sends up to 1.1MHz frequencies over kilometers over a twisted pair.
Now at only 2MHz, you will have no problem using either over a distance of a meter, as long as the output can drive the capacitance of the cable.
If you use either the coax or twisted pair as a properly terminated transmission line, you will have no capacitance problems (ie if you use a 50 ohm coax, make sure the impedance for AC terminating the cable is 50ohms) but this may not suit your current circuit.
Unterminated, if you are feeding the cable with non-sine wave signals, you can get some ringing in the cable around about 100MHz, so maybe a ferrite bead or two to soak up any HF problems.
Richard
vk6zgo:
What are you actually doing?
Are you modulating the LED with the 2.3MHz?
Or is the LED just to indicate presence of RF?
If it the former,yes you can use coax or twisted pair.
If it is the latter,just detect the presence of RF at the near end of the cable & use any cable to drive the LED,as it will only be DC.
'VK6ZGO
SoftwareSamurai:
--- Quote from: IanB on October 07, 2011, 02:27:22 am ---I don't know anything about RF design, but you didn't mention shielded twisted pair? Network cables use this quite successfully and they surely operate at high frequencies. Are you overlooking a simple solution?
--- End quote ---
I did think about shielded TP, and I might end up doing just that. The techy engineer in me loves to explore all possibilities.
(Besides, how often do you get to play around with micro-coax cables? They're so tiny and cool!)
Thanks IanB.
--- Quote from: amspire on October 07, 2011, 03:33:07 am ---If you think about it, the two wires are in proximity, and each is transmitting exactly the opposite RF, so they add up to zero or close enough.
--- End quote ---
Yes, that's what I was thinking too. Perhaps I'm just being overly-cautious, but this device will need FCC approval eventually, so I really want to make sure whatever wire/connection I use doesn't get flagged as a EM-radiating antenna.
--- Quote from: amspire on October 07, 2011, 03:33:07 am ---Now at only 2MHz, you will have no problem using either over a distance of a meter, as long as the output can drive the capacitance of the cable.
If you use either the coax or twisted pair as a properly terminated transmission line, you will have no capacitance problems (ie if you use a 50 ohm coax, make sure the impedance for AC terminating the cable is 50ohms) but this may not suit your current circuit.
--- End quote ---
Right. I hadn't planned on putting anything else besides the LED at the end of the cable. I'll have to think about impedance matching. :/
--- Quote from: amspire on October 07, 2011, 03:33:07 am ---Unterminated, if you are feeding the cable with non-sine wave signals, you can get some ringing in the cable around about 100MHz, so maybe a ferrite bead or two to soak up any HF problems.
--- End quote ---
Oh, I should have mentioned that the signal is an FM sine wave. But still, ferrite bead(s) might be a good idea. I'll have to R&D that.
Thanks Richard.
--- Quote from: vk6zgo on October 07, 2011, 03:44:17 am ---Are you modulating the LED with the 2.3MHz?
...
If it the former,yes you can use coax or twisted pair.
--- End quote ---
I'm sending an FM sine wave via light.
Thanks VK6ZGO.
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