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29 May 2013

An Overview of Banner Specs

"Banner specs" is a term that oscilloscope makers use often. If you've ever met with one of the vendors' salespeople, you're likely to have heard it. But what are banner specs and what do they mean to you?

Example of oscilloscope banner specs
Here is an example of
banner specs, in this case
for Teledyne LeCroy's
HDO 4000 series

Basically, banner specs are the key specifications that the manufacturers hope will catch your eye when you're looking for a new oscilloscope (see the figure). Some will disparage "banner specs" as purely a marketing catchphrase, and yes, it can be, but these specifications are much more than that.

Taken as a group, an oscilloscope's banner specs will give you a good high-level indication of what kind of instrument you're looking at. Depending on your application requirements, an oscilloscope's banner specs will let you know whether a given instrument is potentially the right one for you.

So what are banner specifications? Essentially, we're talking about four key parameters:
  • Bandwidth: The bandwidth of an oscilloscope indicates the point at which the measured amplitude on an amplitude/frequency chart has decreased by -3dB (or 70.7%) of the original value in relation to its level at a lower reference frequency. Basically, an oscilloscope's bandwidth is the frequency range over which it makes reliable measurements. An old rule of thumb, the Five Times rule, is that the bandwidth of an oscilloscope should be five times the signal frequency being measured. Following this rule will result in measurements with errors due to frequency limitations to within +/-2%. 
  • Sampling rate: An oscilloscope's sample rate is the number of times per second the instrument's digitizer samples the waveform for conversion to digital values. Or, we can say it's the number of samples per second the analog-to-digital converter (ADC) can store in memory. A higher sampling rate will capture more waveform detail, meaning that it's less likely that critical information will not be captured. Don't forget to look at minimum sample rates as well as maximums; the former is important when observing slowly changing signals over long time periods.
  • Acquisition memory: The maximum amount of memory space the oscilloscope has in which to store acquired waveforms. Sometimes this is expressed in terms of record length, or how many sampled and digitized waveform points the oscilloscope can acquire for a single waveform record. One can set an oscilloscope to sample at the maximum rate all the time, but doing so will be impractical at longer timebase periods. That's why most oscilloscope makers build their instruments so that the sampling rate is automatically adjusted to match the timebase in order to get the most out of the available memory.
  • Resolution: The ADC’s resolution determines the fidelity with which it digitizes and reproduces signals on-screen. The oscilloscope parameter most improved by ADCs with greater resolution is the instrument’s vertical precision. ADC resolution has been a forgotten parameter until quite recently, because virtually all of the instruments on the market were built with 8-bit ADCs in the acquisition system. Of late, however, instruments with 12-bit ADCs have appeared, resulting in a huge jump in vertical precision.