Power and data on a single coax cable - RF Cafe Forums

 Post subject: Power and data on a single coax cable

Posted: Sun Feb 21, 2010 5:24 pm 

Hi all,
I am looking to use a single coax cable, say 10m long, for both power and data transmission between 2 units, which are sealed in their own metal boxes with no possibility for external antennas.
The power signal is 100-500kHz, 20-100Vp-p AC wave, close to sine, but with some harmonics on it.
I am hoping to put 2 embedded RF modules into the 2 boxes and connect them to the coax via capacitive or inductive means to protect them from the power signal. This way, I hope, the coax will replace the antenna in between the modules and will shield them from external interference. I need data rates > 56Kbps.

Is this possible and what type of RF module would you suggest?
Thanks.

 Post subject: Re: Power and data on a single coax cable

Posted: Fri Feb 26, 2010 1:03 pm 

DC power on the center conductor is used all the time. Satellite antennas (Earth-based) use that on their horn-mounted amplifiers as a matter of course. There are loads of articles by Ham operators and professional companies on how to do it. Basically, all you need to do is place a high-impedance (at RF) choke in the DC feed line and then recover it at the far end the same way.

...{local}...........coax cable........{remote}
RF --(-)------------------------------------(-)- RF
.......|.....|.........................|....|
.....GND...|.........................|...GND
..........choke...................choke
.............|.........................|
......DC----..........................---- DC

 Post subject: Re: Power and data on a single coax cable

Posted: Sun Feb 28, 2010 6:56 pm 

What is your RF frequency? I would recommend characterizing (if at all possible) the power signal in the RF band - especially since the power is not pure sine and will have harmonics to it - to verify that any leakage (if at all) does not degrade the RF performance.

 Post subject: Re: Power and data on a single coax cable

Posted: Tue Mar 02, 2010 5:27 pm 

Well, yes and no. You can not send a digital baseband signal at 56 kbps on a cable with lots of 100 KHz to 500 KHz noise on it! It just won't work. You would have to lowpass filter the digital signal so heavily that you would have all sorts of intersymbol interference.

Now, it you modulate the 56 kbps signal onto something like a 70 MHz carrier, and demodulate it at the other end...then it is a piece of cake.

Like someone else said, the common way to send power on a coax cable is to convert the power to DC first, then diplexer filtering will be easy.

_________________
Rich
Maguffin Microwave Consulting
www.MaguffinMicrowave.com

 Post subject: Re: Power and data on a single coax cable

Posted: Mon Mar 08, 2010 10:24 pm 

Thanks for your reply.
I really need AC for power on this coax because one of the units can be on a rotating table in a sealed metal chamber and a rotating transformer is the only way.
At the same time the only data path is through the same coax on a high frequency carrier. I hoped someone would mention experience with bluetooth used in a similar configuration. Would be a perfect fit with its data rate.

"Wired" Bluetooth anyone?

 Post subject: Re: Power and data on a single coax cable

Posted: Thu Mar 11, 2010 12:23 pm 

I agree with other replies: this approach is messy. Using some lateral thinking, why not use slip rings to transfer data across a rotating cell?
The CCTV industry uses composite miniature cable for this type of application, whereby data for pan/tilt heads and zoom lens control use a CAT6 screened twisted pair, and the video uses a miniature 75ohm coaxial.
Or you may have other constraints that rule that out.

_________________
At bottom, life is all about
Sucking in and blowing out.

 Post subject: Re: Power and data on a single coax cable

Posted: Sun Jul 25, 2010 6:41 pm 

Interesting problem. In this age, there are any number of little RF modules that will get data from one side to the other.

I wouldn't limit myself to the newer technologies like Blue Tooth and Zigbee. The integration would likely prove more of a hassle than they're worth.

RF solutions has some fairly tame modules as does Linx Technologies and RadioTronix.

You should get far more signal than you need or want on the receiver side, so along with a simple L-C filter, drop in an attenuator on both sides, at least 10dB I'd think. This will help you're electronics survive the occasional spike on the AC side and keep your receiver from being over driven by the transmitter.

If you want to put some more project risk / fun into it (with less production cost), you might try experimenting with Ti's TLC2933A PLL.

- Mike

 Post subject: Re: Power and data on a single coax cable

Posted: Sun Jul 25, 2010 11:27 pm 

Thank you all for your replies.
I am still researching this as an idea, nothing practical is done yet.
I send the same question to several companies that make RF modules, but no one said "It will work with our module".
Seems like this is an "out there" problem and no one has done it yet.
We already use similar configuration with 8MHz carrier, made with discrete components. We can get only 9600baud, it is bulky, hard to tune and a lot of trouble. I am looking for a simple "drop in" solution that is a lot faster and simple to tune.
I will continue checking this post, may be someone will mention similar experience.

Thanks again