Hi,
This is my first post so please excuse me if I am not expressing everything clearly!
I just got a Rigol DS1202Z-E scope, 200 MHz, 2 Channels. Since I happen to have an EVAL-AD9912
https://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/eval-ad9912.html#eb-overview DDS board with me, I decided to test out the scope bandwidth with the DDS board. This DDS evaluation board is 1GSPS, with a 400 MHz 7th-order elliptical low-pass reconstruction filter at its output. Therefore I expect the DDS output to have a flat magnitude below its bandwidth limit.
I did the test by connecting the DDS filtered output (SMA on PCB board) with an SMA-BNC adaptor and a 1-foot BNC cable directly to the scope. I only turned the Ch1 on, therefore I am getting 1 GSPS on the scope.
I measured the DDS output on the scope. I get 1.33 Vpp with its frequency set in between 10 MHz and 50 MHz. Then at 100 MHz I get about 1.17 Vpp (88% of the low-frequency value). At around 160 MHz I get 0.92 Vpp (69% of the low-frequency value). At 200 MHz I get only 0.68 Vpp (51% of the low-frequency value).
So, based on this I would claim the scope bandwidth is about 160 MHz, as the 3 dB point would mean a 70.7% (oops, made a mistake at first) amplitude measured on the scope. I also tried measuring the DDS output with the Rigol PVP2350 probe on its 10x option and using the ground spring. The results are the same, thus I think my SMA-BNC adapter solution is good.
So here comes the question: I wonder if my understanding above for the scope bandwidth is correct. Also, does the scope meet its 200 MHz spec in this case, or is something wrong?
I guess there is a slight possibility that my DDS board output amplitude is frequency-dependent. However, I don't have a higher bandwidth scope at home to verify that. I don't have other high-frequency function generators for testing the scope. I don't have a convenient fast-rising edge of something either, so I can't test out the rising time of the scope myself right now. If you have suggestions on how I can build a simple circuit with a fast-enough rising edge, I would greatly appreciate that and be happy to try it out
.