What's the Difference Between Data Rate and Bandwidth?
Data rate and bandwidth are sometimes used interchangeably, thanks largely to advertising firms and the media, who turned an important technical term from analog circuit design into a buzzword. The word “bandwidth” is now misused to the point where it has unintentionally taken on a somewhat related meaning from ADC design. In PCB design and circuit design, bandwidth sometimes has a clear distinction that has nothing to do with data rate, and sometimes it refers to some quality of the signal and its interaction with a receiver.
With the difference between data rate and bandwidth being murky, how does it relate back to your PCB design? We’ll look at this deeper in this article so that we can see how to define signal integrity metrics for ultra-high speed channels. These same ideas around signal integrity metrics are reflected in the recent USB 4.0 standard and will become more important in newer high speed signaling standards.
Difference Between Data Rate and Bandwidth
Data rate is exactly what it sounds like: the number of bits transmitted through a channel or by a component per unit of time. Data rate may also be written in baud rate (e.g., the number of signal units per second), which allows us to differentiate between binary and multilevel signalling schemes. This is pretty simple; for a 2-level signal (e.g., NRZ), the baud rate is equal to the bit rate. For 4-level signals (e.g., PAM4), the baud rate is half the bit rate as two bits are transmitted per unit interval (UI).
Bandwidth is generally used by electronics designers of all stripes to refer to one or more of the following:
- -3 dB point. If you’re designing a filter, this is usually used to denote the frequency where the filter’s transfer function (magnitude) drops by 3 dB.
- Frequency range where a component can receive/transmit. I’ve normally seen this used by other researchers working on integration or system design, where there is a need to match a new component/system to receive/transmit within a specific frequency range.
- Signal’s frequency content. A broadband signal may have its frequency content spread across a large range of frequencies, and bandwidth defines the size of this spectrum.
- A channel’s data rate capacity. This definition arises because the data rate (really the baud rate) and frequency content are related, but it is normally used to describe fiber or wireless links rather than board-level interconnects.
The last of two points are more important for the digital designer as this is where the relationship between bandwidth and data rate needs to be made clear for PCB designers. For analog signals, we don’t care about bandwidth unless we are using modulation with a carrier signal (e.g., Ethernet), or we’re working with pulses (such as in lidar) or chirped waveforms (such as FMCW radar). The bandwidth for an analog signal is quite small and can be seen directly on a spectrum analyzer trace. You can generally define the bandwidth as the range of frequencies that is cut off by the noise floor in your oscilloscope trace. The situation isn’t so simple for digital frequencies.
Bandwidth and Digital Signals
Here, when I refer to bandwidth, I’m referring to the frequency content that makes up a digital signal. The bandwidth of a digital signal is not so clear cut because, mathematically, it spans out to infinite frequency. Therefore, in order to establish some useful definition of bandwidth for use in transmission line design for very high speed links, we need to set some relevant upper limit on the bandwidth of a digital signal.
For a 2-level signal (e.g., NRZ), here are some common definitions of bandwidth:
- 5th harmonic. This is a common, but arbitrary cutoff point for digital signal bandwidths. I say this is arbitrary because you could also use any other odd frequency greater than the 5th harmonic. This definition says the bandwidth is 2.5 times the data rate.
- Knee frequency. This particular frequency is normally approximated as 0.35/trise. In other words, it says the bandwidth is generally not related to the data rate, although a higher binary data rate will have a shorter rise time.
- Nyquist frequency. Assuming a receiver only samples a binary digital signal at a rate that is equal to the data rate, then the Nyquist frequency would be equal to half the data rate. This is another common bandwidth metric for binary digital signals.
Here we have two metrics that link bandwidth to data rate: 5th harmonic and Nyquist frequency. Among these, the Nyquist frequency has the greatest generalizability to multilevel bitstreams.
For a multi-level signal, like a bitstream with pulse-amplitude modulation (PAM), the Nyquist frequency is the best definition for bandwidth as it is most generalizable to any number of signal levels. Here, the bandwidth (equal to the Nyquist frequency) can be defined as:
where N is the number of signal levels per UI and D is the bit rate. This nicely fits within Nyquist’s criterion as defined for an ADC, where the sampling rate matches the data rate. The takeaway is: just because we say a channel’s bandwidth is X GHz, it doesn’t mean the data rate is limited to 2X GHz; the signalling standard matters too. In fact, the bandwidth in terms of its frequency content should be defined on a case-by-case basis; there is no single equation.
When I’m looking at the relevant bandwidth for a particular bitstream, I always choose the larger of the knee frequency or Nyquist frequency for 2-level signals. For multilevel signals, I stick with the Nyquist frequency as the relevant measure of bandwidth. When looking at S-parameters or a channel’s transfer function, you can focus on frequencies at and below the bandwidth as the receiver limits the relevant bandwidth of the channel. You only need to worry about losses and impedance matching at frequencies up to the bandwidth.
Once you understand the difference between data rate and bandwidth, you can use the PCB design and layout tools in Altium Designer® to create compliant interconnects. You’ll have a complete set of routing and layout features for high speed impedance controlled designs.
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