# Analogue and digital signals

1. Jun 21, 2004

### Cheman

From physics sheet - "all signals (digital a reduction in signal power) as they travel through a system and need amplification at certain intervals." Why is this the case?

2. Jun 21, 2004

### chroot

Staff Emeritus
Wires have non-zero resistance.

- Warren

3. Jun 26, 2004

### Cheman

How do you mean? What effect that have - current should still be the same everywhere surely?

4. Jun 26, 2004

### Kurdt

Staff Emeritus
Resistance dissipates the energy originally provided in the form of heat therefore we need to periodically add more energy to the signal.

5. Jun 26, 2004

### chroot

Staff Emeritus
When people say 'signal' they usually mean 'voltage signal' -- i.e. the potential present on a wire. If you apply a battery (referenced to ground) to one end of a very long wire, and measure the potential on the other, you'll see that the potential decreases along the wire. Ideal wires have zero resistance, and thus the potential is the same everywhere along their length -- but real wires have small, but non-zero resistances. Thus, voltages at the far end of a wire get smaller as the wire length gets longer. There are a couple of solutions: use amplifiers, or use current signals instead. Current is the same everywhere in a wire, even though voltage is not. If you pump 10 mA of current into a wire at one end, you can rest assured 10 mA must be coming out the other end.

- Warren

6. Jun 26, 2004

### Integral

Staff Emeritus
Chroot,
Would not the POTENTIAL of the wire be constant, as long as there IS NO CURRENT. This would mean that the potential measurement would VARY with instrument used. If you measured the voltage with respect to ground of your long wire (any wire for that matter!) would get a different result if you used a cheap analog meter or a high impedance Digital Voltmeter. If you could measure the potential of the wire with no current draw you would measure source voltage at any point on the wire.