The length of a transmission line is limiting the highest possible data rate on that line.
Why are faster signals more likely to become corrupted on long transmission lines than on shorter ones, with focus on frequency-BW of the signal, raise time, etc… ?
Would appreciate any explanations.
Best Answer
SPI uses clock and data. From the sending (master) end, clock and data fly down their respective cables in sync (but delayed by the cable) and they reach the slave end and hey presto, the slave clocks in the data and does what it has to do but, what if it has to send back some data such as a value of something.
OK, it transmits its data synchronized to the local clock it is receiving and doesn't worry any more BUT, that local clock it receives is delayed by the cable and, the data the slave sends back to the master is further delayed by the cable and what happens at the master (when receiving data) is a mess unless the data rate is slow or the cable is short.
The main problem is that data sent from a slave is "timed" to the clock edges at the slave. The data received by the master is clocked into the master by the local clock at the master - the slave clock and master clock are not aligned due to the cable delay.
The OP has changed the question so here's some extra things about cable: -
Longer cables attenuate more - think about powering a motor from a battery - it works fine up close on short leads but if you make the leads longer the terminal voltage seen on the motor gets smaller and smaller as cable length increases. Copper is not zero-ohms.
It gets worse as frequency rises due to a phenomena called skin effect. skin effect reduces the conductivity of a copper wire by forcing currents to only be present in the skin of the conductor. This means smaller cross sectional area for the current hence higher resistance hence greater losses.
Dielectric loss in cable is proportional to frequency - basically energy is stolen from the signal to heat-up the insulating material between the two wires that form the transmission line or cable. This is what wiki says: -
If losses are proportional to frequency then the likelihood of data corruption is also proportional to faster signals. Above a certain point there is another mechanism when cable (such as coax) starts to acts like a waveguide. Again wiki has the word: -