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| Sniffing the Rigol's internal I2C bus |
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| GlassFET:
Someone asked a while back in this long thread if anyone had tested the bandwidths of unlocked DS2000A scopes to verify that the 300 MHz goal had indeed been achieved. I don't know if anyone responded with results, but here are the results of several BW tests we conducted. Five of us have unlocked these scopes, four starting with the DS2702A model and one with a DS2102A model. All five of us chose the full "NS8H" 300-MHz-plus-all-options upgrade. Tests were run using an RF signal generator connected to a short (~1M) 50 ohm cable, with the 50 ohm input impedance selected on the scopes. The -3dB BW results are: Unit 1: 354 MHz Unit 2: 408 MHz Unit 3: 367 MHz Unit 4: 420 MHz Unit 5: 480 MHz (?) Notes: Units 1, 2, 3 were tested with a Wavetek 2510A synthesized signal generator set to 1.0 Vrms (+13 dBm) at 10MHz. The frequency was increased until the scopes read 0.707 Vrms (+10 dBm). Each channel was separately measured one, at a time, and the averages of the two BW results are presented here. Unit 4 was tested similarly with an HP8656A signal generator, except the generator was set to 0.5 Vrms (+7 dBm) at 50 MHz. The set up for Unit 5 is unknown and it might be considered an "outlier". We didn't observe that BW varied whether two channels were selected or just one, but we could see some aliasing noise in the waveform when two channels were selected, because the 2 Gsa/s maximum sample rate has to be interleaved between channels in the two channel mode, which is typical for DSOs. Conclusion: All units tested well exceeded the 300 MHz spec of the DS2302A. Thanks to all here who made this possible! |
| marmad:
--- Quote from: GlassFET on March 09, 2014, 06:34:39 pm ---Conclusion: All units tested well exceeded the 300 MHz spec of the DS2302A. --- End quote --- As mentioned already many times in this thread (and elsewhere in the forum), the fact that you get more bandwidth, in and of itself, is not necessarily a good thing. To accurately measure the signal without sampling alias errors, your DSO must have sufficient sample rate. For a Gaussian-response oscilloscope like the Rigol, 4-6 times the bandwidth is typical. The original 200MHz DS2000 has a dual channel sampling rate of 5 times the bandwidth - which fit the BW filter curve quite well. Running a DS2000 at 300MHz with both channels on means a sample rate of only 3.3 times the bandwidth; an inadequate amount for the Gaussian filter - and will likely lead to alias errors in certain circumstances. So, IMO, the 300MHz is reliable when only using a single channel - but not both. |
| electronics man:
What I don't realy understand is why is the bandwidth only limited in softweare, so is rigol actually selling an upgrade. |
| GlassFET:
--- Quote from: marmad on March 09, 2014, 07:05:17 pm --- --- Quote from: GlassFET on March 09, 2014, 06:34:39 pm ---Conclusion: All units tested well exceeded the 300 MHz spec of the DS2302A. --- End quote --- ...So, IMO, the 300MHz is reliable when only using a single channel - but not both. --- End quote --- That was my intended point about aliasing. I concur that only one channel should be used at close to max BW to avoid aliasing. Nothing shameful here; Tek and Agilent face the same physical realities. |
| Mark_O:
--- Quote from: GlassFET on March 09, 2014, 07:19:43 pm ---That was my intended point about aliasing. I concur that only one channel should be used at close to max BW to avoid aliasing. Nothing shameful here; Tek and Agilent face the same physical realities. --- End quote --- That's true, however what is one supposed to do when you have two channels enabled? You can't just say (as you did) 'don't use both at close to max BW', because you have no reasonable control over the max BW at that point. I.e., there may be spurious harmonic content you have no knowledge of. As marmad pointed out, "The original 200MHz DS2000 has a dual channel sampling rate of 5 times the bandwidth - which fit the BW filter curve quite well." i.e., the sample rate was well-matched to the BW (or vice-versa). So to avoid aliasing with your modded 300 MHZ (or 400!) scope, you must engage a 100 MHz BW limiting filter... which may not be as desirable as the original 200 MHz would have been. It's really a choice between having 200 MHz at 1 and 2 channels; or 300 MHz at 1 channel, with 100 MHz at 2. It's a point worth noting, and remembering. |
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