What is the difference between analog and digital BW?
Analog Bandwidth (BW)
BW is the amount our signal will be attenuated by the oscilloscope's front end section. This includes from the input connector, preamplifier, track and hold circuitry, and ADC and memory components - specified at the -3 dB point - as a function of input frequency.
BW ratings are at the input to the amplifier and that your probes might also attenuate your signals. If you are looking at signals > 50 MHz, I suggest an active FET probe. For 1 GHz and higher, active differential probes work the best.
Remember that -3 dB is down in amplitude by almost 30%. You probably don't want a 30% error in your amplitude measurements. Consider the highest BW you need to measure with best accuracy.
Caution!Many oscilloscopes have BW ratings that reflect their best performance but only in certain voltage ranges. If you need accuracy at certain peak-to-peak values, check the scope's accuracy (with probe) at those same full scales.
Also note that the BW of the TRIGGER circuit may NOT be as high as the rated front end BW of the scope.
Digital Bandwidth (BW)
Some scope manufacturers apply DSP to extend the BW. The technique is difficult to apply without group delay artifacts and isn't suggested without a comprenhensive understanding of the possible impacts.
The sample rate impacts the scope's ability to capture the peak values in waveforms.
As a general rule, you want at least 5 sample points on the highest frequency cycle you expect to digitize. Single shot capture then suggests a 40 GS/s scope can look at waveforms up to about 8 GHz.
Be cautious of short record length scopes that can only sample at maximum rates for short periods of time. Scopes with less than 1 Mpt of acquisition memory operate on just a few timebase settings at their maximum sample rate.
Application Timebase Useable BW Useable BW
32k Mem 8M Memory
=================================================================
Ethernet 10baseT 10 us/div 2.5 MHz 500 MHz
PLL Dynamic Resp. 100 us/div 250 kHz 500 MHz
CDMA-Qualcom 200 us/div 125 kHz 250 MHz
Video and Imaging 2 ms/div 12.5 kHz 25 MHz
Switcher Power 100 ms/div 250 Hz 625 kHz
Ideally, the sample rate value should be displayed on screen all the time. Be certain you can sample fast enough in real time (single shot) mode on all channels so you can record without aliasing.
Another point of confusion. Sample rates specified for repetitive vs. single shot acquisitions. Real-time refers to single shot. RIS refers to random interleaved sampling. RIS is sometimes called ET or equivalent time sample mode and can only be used with repetitive waveforms. Also referred to as "random repetitive sampling".
Many oscilloscopes have analog BW specifications far greater than their single-shot sample rate's Nyquist (.5 sample rate) frequency. This is so that when repetitive waveforms are viewed, the maximum BW signals can be seen.
Some oscilloscopes reduce sample rates by the number of channels activated. This could cause aliasing by changing the relationship of how fast the oscilloscope is sampling vs. the BW of the signals you are digitizing.