If you want to see anything other than the fundamental sine wave component of the 100 MHz signal, then the oscilloscope bandwidth needs to be much higher to cover the harmonics. Otherwise a 100 MHz oscilloscope viewing a 100 MHz signal will only display the 100 MHz fundamental sine wave component, and possibly with a 3dB amplitude error.
FTFY.
It need be noted label BW can be substantially different from actual -3dB BW as is the case with the 100 MHz rated SDS2104X Plus which the 1st we received I tested with 3 yes 3 signal sources to convince myself the ~185 MHz result was actually real !
Some oscilloscopes are weird. Even old high bandwidth instruments often break the Gaussian response -3dB rule, and the 0.35 rule does not apply to them either. For other instruments, the specified bandwidth was more of a guideline than a rule, and the instrument was *at least* that fast. Modern examples of weirdness include the Rigol 1000Z series where full power bandwidth varies with the V/div setting.
Supporting that result is they are also supplied with 200 MHz probes.
Tektronix specified their oscilloscopes for -3dB bandwidth *at the probe tip*, so a 200 MHz Tektronix oscilloscope with a 200 MHz Tektronix probe yielded 200 MHz. At least for them, the probe bandwidth specification represents the highest frequency which the probe will reproduce faithfully and not the -3dB bandwidth.
As pointed out by tggzzz, a bandwidth limited signal can be reconstructed with a sampling rate greater than twice the bandwidth no matter where in the frequency spectrum it is, within the bandwidth of the sampler.
In theory? Yes.
In practical terms? Not so much. The reconstruction filter would become very unwieldy as you approach Nyquist.
In a subsampling bandpass application, the input signal bandwidth is tightly constrained by a high selectivity bandpass filter, so the difficulty of reconstruction is considerably relaxed. Ideally aliasing products will be close to or below the noise floor.
ie. I've sat a potentiometer and manually dialed a frequency where I no longer observe the AM modulation effect mentioned earlier. The frequency I ended up with was right there in the 2.5x ballpark. Maybe it could have been 2.4x but it's definitely not as low as 2.2x.
2.5x may be a "non-specific/vague figure" but it works in practice.
I find 2.5x times to be optimistic when sin(x)/x reconstruction is required. The effect you mention can be modeled as the result of non-linear mixing between a pure sine wave and the sampling clock, with aliasing of some of the mixing products. The reconstruction then has multiple solutions and each pass shows one of those solutions. When a fast edge is applied, the same thing happens even when the reconstruction filter correctly filters the higher frequency aliasing products because the mixing products are still there close to the aliasing frequency. I find the result annoying in the extreme.
The solution is to use a faster sample rate. Sometimes there is no substitute for high bandwidth and fast sampling rate.
This scope is old enough that it doesn't have sin x/x interpolation. As switchabl mentioned the older Agilent scopes typically use linear interpolation, and it isn't at all useless. If your choices are dot mode and linear interpolation, linear interpolation is just fine. When your signal is oversampled, you can't tell the difference anyway. This scope has rated max sample rate of 4 Gsa/s, and analog bandwidth of 500 MHz, so it's 4 times oversampled at rated bandwidth, and much more than that for lower bandwidth signals.
For Tektronix you have to go back to 1990 and the 2232 series of DSOs for the last of their DSOs which did not support sin(x)/x reconstruction, and those are what I consider to be the first "modern" DSOs, at least from Tektronix. Their 2430 series first made in 1986 had sin (x)/x reconstruction and I think all of their instruments after that, other than the 2232 series, did.
Honestly I have never missed sin(x)/x reconstruction on the 2230 and 2232. Usually equivalent time sampling removes the need, and cases where single shot acquisitions need to be made with full bandwidth signals are rare.