Why do higher frequencies mean higher data rates, and why do we even need frequencies to begin with?

Question

This is regarding wired connections. I have heard that higher frequencies mean higher data rates since there are more cycles per second you can fit more data in per second. That makes sense but I don't understand why we need them in the first place. Couldn't we have a data scheme that just relies on the presence of voltage being a 1 and the absence being a 0. Just imagine a very long wire and a battery. When I touch the wire to complete the circut it's a 1 when the circut shows no voltage it's a 0. Now imagine I alternate this very very very fast. Other binary schemes can be:

"Low frequency pulse" = 0 "high frequency pulse" = 1, no sound = wait or next bit (imagine pressing 1 vs 9 on a touch tone phone as your coding scheme)

+Voltage = 1, - Voltage = 0, No voltage = no bits yet (To send 1001 you would touch positive, unapply voltage and switch wires, unapply voltage and touch them again, unapply voltage and switch back to the original position).  Once again imagine a machine doing this very very fast.

Answer 1

I don't know what do you mean by "presence/absence" of voltage, but in general, what you describe is exactly how it works:

• High voltage on the data line is (usually) defined as logical 1
• Low voltage is logical 0

The "frequency of the circuit" is just the maximal number of times the transition between voltage levels can happen in a second.

Please note that if you send a long string of logical 1's, there is no need for these transitions at all - you can keep the data line constantly at high voltage. However, the question arises "how many 1's were sent during some period of time"? How do you count them if the data line is at constant voltage? For the purpose of resolving the above ambiguity, there is usually a "control" signal which determines the "pace" of the circuitry - the Clock. On each rising/falling edge of the clock signal, the voltage is sampled from the data bus and its value is checked for determining whether it is logical 1 or 0.

In the case of a circuit where Clock is present, the frequency of the circuit is just the frequency of the clock signal.

There are schemes when the circuit is not synchronized with Clock, but these schemes are much more complex and they are (usually) employed in special application which require very high bit rates.

Answer 2

I hear of circuts using a "carrier frequency" and modulating it and it seems as if the speed is dependent on this frequency (for example ethernet is several hundred MHz). What I mean is why can't one skip all the modulation business and have a circut where they simply flip the voltage high\low super super fast and achieve speeds of 1 THz (= 1 Tb/sec)

The quote above is a comment from the OP but I think it gets closer to the misunderstanding than what he puts in the question.

Pure data transmissions (1s and 0s) don't make efficient use of bandwidth. That's not a problem - the simplicity of transmitting 1 and 0 makes reception of those bits very easy. Between 5V and 0V (5V logic signalling) noise and glitches can come long and make the signal somewhat "different" to the original 0V and 5V but providing there is still a distinction to be made by the receiver, data can be faithfully recovered.

The trouble with normal data is that it can't share a line with another normal data system. The two lots of 1s and 0s end up being additive and sometimes you'll get 0V, sometimes you'll get 5V and sometimes you'll get 10V i.e. rubbish.

A carrier wave, when (say 1kbps) data is applied to it has a bandwidth that is a few thousand Hertz centred on a frequency that might be 20MHz. Below (say) 19.97MHz and above 20.03MHz the side frequencies can be largely removed and not transmitted AND importantly the "modulated" carrier can be received and turned back into the original data.

You could have another data system whose carrier is at 19.9MHz - if modulated with different data (still 1kbps for this example), the useful bandwidth it might occupy is from 19.87MHz to 19.93MHz. The receiver tuned into this carrier won't be interfered with by the transmission at 20MHz.

You could repeat this system and have a multitude of exclusive data systems all sharing the same wire (different carriers of course) and all the data streams are perfectly recoverable by their own receivers.

This is why modulation schemes of one sort or another are used. This one is called frequency division multiplexing. It can use a wire or radio.

It doesn't stop there - you can have time division multiplexing - this type of system allocates a time slot for each data stream. Let's say there are ten data streams each at 1kbps. If the data was sent ten times as fast it only needs to occupy 10% of the usable "capacity" of the cable. Ten systems, each with their own time slot can share one cable.

There are a few other schemes as well but this is beyond the scope of the question.

Answer 3

• Transmitting data by completing a switch wouldn't be as effective as you'd think.

  1. You could have a ground loop

     • The ground of the transmiter circuit and the ground of the receiver circuit could be at different potentials, especially true over longer distances in cables. This is generally more true for transmission lines, so you may not have this issue with a PCB.

  2. Transients from turning ON and OFF

     • Turning on anything is similar to introducing a new current source to circuit or sub-circuit. This could mean spikes in your data-stream which may effect your ICs or other PCB components.

  3. Data isn't transmitted as ON/OFF, but as HI/LO

     • HI is above a certain threshold and LO is below that threshold. A signal that is only turning a switch ON and OFF makes it more difficult to differentiate

  4. There is less disambiguation in a data stream instead of single bit pulses.

     • Consider "1001". How would a receiver view this word? "101" is just as easy to interpret as "1001". But how would you know for sure it's "1001"? Maybe by pre-defining a predetermined signal length, i.e. frequency?

• Using low-frequency and high-frequency signals to denote 0 and 1, respectively, is some sort of frequency modulation method. This would be modulating a carrier wave, similar to your standard FM radio transmission. This method is called frequency shift keying.

• The third method you described is more similar to RS-232 data transmission scheme. It actually uses +3 to + 15V as logical 1 and -3 to -15V as logical 0. Once again, you want to avoid a 0 level so that you have a clear, definable signals.

Answer 4

The reason we use frequencies when looking at data communications is that we typically have to send those communications across a physical medium. Most of our mediums have passive components to them, things that are well modeled using resistors, capacitors and inductors. For example, between any two wires in a cable, there's some capacitance because you have two metal wires separated by an insulator. You also have some inductance because they are wires running in parallel.

At low signaling speeds, you can just get away with a simple mapping from digital bits to voltages. However, as you start ramping things up, you find that the message gets harder to read. It starts to get distorted and you start misinterpreting 1s as 0s. This happens because those passive components have a memory of sorts. Capacitors get charged and discharged.

Now we typically solve this by thinking in the frequency domain. Why? Well, it turns out that the math lets us get rid of those complex derivatives and integrals that appear in the time-based behavior of inductors and capacitors. The math of Fourier transforms and Laplace transforms turn those differential equations in the time domain into nice easy to understand multiplications and divisions in the frequency domain. We can say things like "I have a 60Hz signal, how much is that signal diminished by the wire? (which is a different answer than if you had a 100MHz signal)"

Once we start thinking in the frequency domain, we can look at putting our signal into regions where our wire has predictable behavior. One major aspect of this is that we tend to see behaviors that occur on a log scale. While there may be major differences between the behavior at 60Hz and at 100Hz, there is typically must less of a difference between 1,000,060Hz and 1,000,100Hz. So if we can depend on our signals being in a higher frequency band, they tend to get treated more "flat" by the wire. This makes decoding much easier. This is why we see carrier frequencies. Often we wish to send a low frequency signal using the physical properites of the system at a higher frequency. Frequency modulation is a tool that lets us do that.