rsl09@kuhub.cc.ukans.edu (04/12/90)
I have an application that requires sending low level signal (in mv range) down a long cable (about 50 meters). The signal is bandlimited to about 13 KHz. To avoid potential interference, I thought it would better to digitize the signal and send digital signal, rather than analog, down the cable. I was wondering if there is a product on the market that can digitize an analog signal, transmit down a long cable, and receive the digital signal at the other end of the cable. Any help or suggestion will be greatly appreciated. Vahid Hesany Graduate Research Assistant University of Kansas
grege@gold.GVG.TEK.COM (Gregory Ebert) (04/12/90)
In article <22845.2623bae8@kuhub.cc.ukans.edu> rsl09@kuhub.cc.ukans.edu writes: > >I have an application that requires sending low level signal (in mv range) >down a long cable (about 50 meters). The signal is bandlimited to about >13 KHz. To avoid potential interference, I thought it would better >to digitize the signal and send digital signal, rather than >analog, down the cable. > >I was wondering if there is a product on the market that can >digitize an analog signal, transmit down a long cable, and >receive the digital signal at the other end of the cable. > >Any help or suggestion will be greatly appreciated. > Why not use shielded cable ? Your frequency is so low that you wont even need to terminate it for a 50m run. The only potential problem would be a *slight* attenuation due to lossy media, but I'm pretty sure that the loss of, say, RG58 or RG59 at 13Khz is zippo for 50m. You will, of course, want to avoid ground loops. Make sure that the coaxial shield is the ONLY ground path between the transmitter and the receiver - Never, never, NEVER connect your power supply ground to 'earth' in more than one location. I had ground loops in my homebrew system; it caused SEVERE RFI interference for about 1/2 block. Took out the ground loops, and there was NO detectable interference.
mikem@col.hp.com (Mike McTigue) (04/13/90)
This may not be what you want but you could consider using a balanced coax system simular to the cables low impedence microphones use. A differential driver and reciever at the sending and recieving ends would eliminate interference easily to the BW you indicated.
whit@milton.acs.washington.edu (John Whitmore) (04/13/90)
In article <22845.2623bae8@kuhub.cc.ukans.edu> rsl09@kuhub.cc.ukans.edu writes: > >I have an application that requires sending low level signal (in mv range) >down a long cable (about 50 meters). The signal is bandlimited to about >13 KHz. To avoid potential interference, I thought it would better >to digitize the signal ... You do not mention the impedance of your signal source; this is VITALLY important to determine what, if any, interference you are protecting against. Digitization is certainly not appropriate (unless you have a large budget, or noisy wiring that you wish to use rather than making a new wire path). A simpler method: amplify the signal at one end, with a high-output-impedance amplifier (i.e. make a current source), and detect at the receive end with an appropriate resistor. There is a popular industrial standard, the "4-20mA transmitter", using this trick. It usually works well with low frequency signals. Or amplify to get a higher voltage signal, which you attenuate at the receive end. Or use a transformer at one end (or both) to break up the "ground loop". It may suffice to use coaxial cable, or you may wish to use twisted pair (the second wire becomes the "sensitive ground" connection for the remote box), or (in extreme cases) you may use two coaxial cables, one for the signal, one for the ground reference. The mere presence of digital circuitry in a box practically guarantees that that box cannot maintain low noise for a small signal, if only due to modulation of the power supply with the digital clocking noise. Avoiding switching circuitry and shielding the cable can easily keep your 13 mV signal intact. I am known for my brilliance, John Whitmore by those who do not know me well.
sukenick@sci.ccny.cuny.edu (SYG) (04/14/90)
Tri-axial connections and cable are supposed to be best for preventing noise.
One lead is for the signal, the other wire is for the ground, and the shield
doesnt carry current and is connected at one end only to either ground or a low
impedance potential close to the potential on the signal wire ("guard"). If you
really want to get fancy, there is dual shielded cable in which you can use the
inner shield as a guard and the outer as a ground shield.
Now the question I have is: Is there a perfered method of connecting
the shield (ie: connect at signal out end or at signal in?)