Nice hack, Gizmo!
Very well done! Thank you!
__________
Alex33,
Members of this forum have occasionally reported that this specific oscilloscope line can perform decently up to a bandwidth of 170 MHz. Not to mention the little-known 150 MHz DS1152E-EDU devices, whose existence confirms the previous statement.
In my opinion there must be some sort of QC at the Rigol assembly line, since not every cheap component they use in their oscilloscopes can be pushed beyond its limits and perform well. I am talking about the DS1000E/D front-end 40 MS/s ADCs being overclocked to 100 MS/s (see Dave's
EEVblog #19: Rigol caught with their pants down!). Not that there is any deliberate difference between the AD9288-40 and the AD9288-100 A/D converters Rigol uses, besides their price tag. It is just that the chip manufacturer characterises the faster ones as full bandwidth products and those ones that cannot perform decently at full speed, as lower bandwidth ones; and instead of binning the latter ones they sell them as lower speed components and at a lower price.
By the same principle, the assembled oscilloscopes that cannot perform at full speed (due to the lower performance components they are assembled with) must probably be labeled as lower speed devices (i.e. DS1052 instead of DS1102) which, when pushed to meet the performance of the faster ones, they simply fail to do it right.
By the way, here are all the
DS1000E/D schematics I have drawn when I disassembled my unit, trying to find out the cause of the excessive noise it exhibited right out of the box, and a few comments on the 20 MHz bandwidth limit varicap biasing.
In a few words, I think that the varicap stage is not responsible for the 90 MHz bandwidth knee you have measured.
-George