I really wish I’d seen this thread before buying a DSO203.
Everyone is selling these as 72MHz analog bandwidth.
My analysis of the schematic (hardware 2.6 from the Wiki) is that the last Op Amps just before the ADC are single pole lowpass filters with a maximum -3dB frequency of 4.4MHz. That is about right for 36Msps per channel.
Using the built-in signal generator: On a 100MHz BW Tektronix TDS 220, the signal generator looks like a reasonably good square wave all the way up to 8MHz. With the Quad with only Channel A active, at 4MHz it looks like a sine wave with a minor flattening on the tips. At 6MHz, on the Tektronix it looks like a decent square wave, while on the Quad it is a sine wave with lower voltage. At 8MHz, on the Quad the amplitude is much lower.
So with my unscientific tests and past experience, I’d call this 4MHz analog bandwidth. I consider that useable for audio troubleshooting, and for digital signals no faster than 100kHz.