Baud Rate

A line driver circuit normally includes both a transmitter and a receiver.

If we consider a transmitter (for example at 250 kbaud as used by DMX) the signal, as seen on an oscilloscope, will have sharp rise and fall edges, looking like a "clean" digital waveform. In tyeh frequency domain, the energy of the signal is mainly centred on a frequency at one half of the baud rate of the system.

Without any filtering or signal shaping, this set of square-edged pulses also results in frequency harmonics much higher than the baud rate. The transmit signal would therefore have a wide spectrum when viewed on a spectrum analyser.

This signal travels along the cable. The line driver at the receiver has to decide which bauds were signalled by the sender, but the resulting signal at the receiver is seldom exactly the same as was sent. There are two problems, some of the energy of the sender has failed to reach the receiver, and some other unwanted signal energy (interference/noise) will also be received. A wideband receiver accepts any signal received during a baud period, irrespective of its frequency. This makes it particularly vulnerable to noise/interference.

Limited bandwidth signals

Early communications engineers discovered that limiting the spectrum of the signal at the receiver significantly improved the performance. A band-limited signal (e.g. using a low-pass filter) reduces susceptibility to any unwanted interference/noise outside the filter band. It is therefore wise to use a receiver that filters the signal at frequencies above the baud rate. Slew-rate limiting at the transmitter works by slowing the edges of the RS-485 signal down, reducing the signal's high-frequency components.

The communications engineered also reasoned that transmission of any signal outside the pass-band of the remote receiver was of no value - at best the receiver would filter and remove the signal, and it would have no effect on the output of the receiver. At worst, some of the signal may leak from the cable adding interference to other transmissions. There was no point in sending this signal energy.

The optimum solution was to apply the same shaping of the signal at the sender and the receiver. If both used the same form of filter, then this would result in less interference to other signals and highest probability of correct reception. This filter is implemented as the slew-rate limiting circuit of the line transceiver.

Slew Rate

Slew-rate limiting at the transmitter works by slowing the edges of the the signal down (i.e. reducing the signal's high-frequency components). It also adds a (small) propagation delay through the line driver.

A high baud rate line receiver that accepts a wide bandwidth input signal is more susceptible to interference at higher frequencies picked-up as the signal travels along the cable (e.g. frequencies >> 250 kHz in this example). This reduces the ability to detect the wanted signal, whereas a band-limited signal (e.g., one using a low-pass filter matched to the bandwidth of the signal) reduces susceptibility to any unwanted interference/noise outside the filter band.

An appropriate choice of driver/receiver circuit at the transmitter and receiver hence both reduces radiated emissions and reduces susceptibility to noise and improper termination.

Trace showing the effect of limiting slew rate at a receiver when sending a 125 kHz square wave. Green traces show the input to receiver from the cable with ringing and noise, whereas the purple trace captured at the output is cleaner. Note also the propagation delay (shift on the horizontal axes) through the receiver, and the need to sample at the centre of the baud.

See also:

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2018)