Not to be too much of a buzzkill ... however, have a look at the attached images.
10MHz signal generated from DSA815-TG (50ohm output) set to zero-span mode, N-to-BNC cable with 50ohm inline termination to DS2202 (reflashed with modified FW).
Observed waveform shows 74mVpp amplitude.
Bandwidth being -3dB or 0.707.. we find 74mVpp / sqrt(2) = 52.3mVpp (-3dB voltage)
At 250MHz zero-span output, the observed amplitude is 52.4mVpp
Thus, it appears that 250MHz is the actual bandwidth of a DS2000 series scope
I do not like the pulse generator method of finding the bandwidth because it appears that there is much confusion as to how to properly measure the pulse width (not the slope of the curve) and even in doing so, 0.1ns can generate a very significant difference in derived BW. It seems this method is more prone to gross systematic errors. Kinds of like the recent speed of light debacle
Would anyone care to further discuss these findings?
The issue that you have run into here is the loading effect due mainly to the input capacitance of the DSO channel.
As an example, the DS2000 series is specified as having approx 16pf input capacitance.
At say 300Mhz, this is approx 33ohms reactance across the 50ohm external termination.
High end oscilloscopes have a correct 50ohm impedance termination built in.
It is possibble to get around this issue by using a suitable high frequency probe. eg. an active probe.
From the point of view of measuring the DSO channel bandwidth, it is possible to do this with a spectrum analyzer and tracking generator.
The idea is to monitor the signal level directly at the input to the DSO and adjust the Generator output to maintain a constant signal level at the DSO input.
Any drop in signal level seen on the DSO screen is then due to frequency response of the internal circuitry after the input connector.
So this is measuring the DSO frequency response only and ignores the effect of changes in the input loading.
This can be done as follows:-
Connect the Generator output to the DSO input channel using a BNC "T" piece on the front of the DSO.
The Generator output is connected to one side of the "T" and a 20dB 50ohm inline attenuator to the other side of the "T".
Then from the 20dB inline attenuator connect back to the spectrum analyzer input.
The 20dB attenuator acts as a 50ohm termination for the Generator.