Choosing between entry-level 12-bit DSOs

[gf]

please note what you keep talking is a combination of sample rate and Sinc reconstruction. turn off Sinc and your point will become moot, granted signal can be a little weird when spectral content contains element more than half the sampling rate, using "Line" or vector plot. but not as severe when your turn on Sinc.

Umm... How would you describe, say, a 170 MHz sine signal sampled at 312 MSa/s, when shown in dot mode or with linear interpolation? It would look severely misleading in my book. Sinc interpolation is not the issue here.

170 MHz folds back to 142 MHz in the first Nyquist zone. So it looks like you had sampled a 142 MHz sine wave.

[Mechatrommer]

Umm... How would you describe, say, a 170 MHz sine signal sampled at 312 MSa/s, when shown in dot mode or with linear interpolation? It would look severely misleading in my book. Sinc interpolation is not the issue here.

and you keep showing the worst case scenario to prove your point when user really fucked up trying to probe >170MHz using 4CH cluelessly. in reality we will avoid such test condition. we will turn on only 2 or 1 channel in operation. even if we have to use all 4CH, when in doubt, we can switch to 1CH to probe the suspicious high frequency signal. granted its not foolproof, thats why you have to gain knowledge and theories as you go, otherwise if you keep doing your setup example you mentioned again and again and made mistake again and again, this job is actually not for you, ymmv.

170 MHz folds back to 142 MHz in the first Nyquist zone. So it looks like you had sampled a 142 MHz sine wave.

if you really try to probe 170MHz fundamentals at 312 MSa/s really you have something wrong within you, not the scope, scope is just a tool. usually spectral content in excess of Nyquist limit are harmonics whose magnitudes are much smaller hence aliasing will not affect much the fundamentals shape, unless your circuit is oscillating unexpectedly. ymmv.

analogy is manual transmission car vs automatic transmission car. manual is not foolproof, automatic transmission is, but for some people like us, we still prefer manual and know what we are doing, and also know what we should not do. because we need performance not available in automatic cars. ymmv.

[ebastler]

and you keep showing the worst case scenario to prove your point when user really fucked up trying to probe >170MHz using 4CH cluelessly. in reality we will avoid such test condition. we will turn on only 2 or 1 channel in operation. even if we have to use all 4CH, when in doubt, we can switch to 1CH to probe the suspicious high frequency signal. granted its not foolproof, thats why you have to gain knowledge and theories as you go, otherwise if you keep doing your setup example you mentioned again and again and made mistake again and again, this job is actually not for you, ymmv.

Not sure where the agitation comes from. Feel free to choose frequencies you consider more realistic. My point was just that aliasing remains aliasing, also in dot mode or with linear interpolation.

[Mechatrommer]

Not sure where the agitation comes from. Feel free to choose frequencies you consider more realistic. My point was just that aliasing remains aliasing, also in dot mode or with linear interpolation.

sorry sometime i got mixed up with 2n**55 posts, but i read his argument all too often. so its somewhat similar to your post... Sinc reconstruction CAN exaggerate aliasing more than Line plot, that was my point. we discussed Gibbs effect that will not be seen on Line plot, esp on fast rising edge square signal. but my reply still stands, if a user wants to probe fundamentals in violating of nyquist limit, then the user got problem, or he/she may not know enough, not the scope... ymmv. otoh talking about properly designed BW limited scope, it will not be able to see ill behaved oscillation in circuit, sometime aliasing can give enough suspicion something happened, ymmv.

[shapirus]

But facts are facts and DHO900 is hardware that is a shame of Rigol and should not have left prototype phase. It is a 250MHz scope (claimed) that in worst case samples at 156MSps/s which is FINRSI type of specmanship.
And since many people hack their DHO800 to 900 that defect is applicable to 800 too.

When does it switch to 156Msa/s?
(other than zooming out horizontally to >= 500 us/div)

[2N3055]

p

But facts are facts and DHO900 is hardware that is a shame of Rigol and should not have left prototype phase. It is a 250MHz scope (claimed) that in worst case samples at 156MSps/s which is FINRSI type of specmanship.
And since many people hack their DHO800 to 900 that defect is applicable to 800 too.

When does it switch to 156Msa/s?
(other than zooming out horizontally to >= 500 us/div)

900 switches when all 4 ch on and MSO

[Mechatrommer]

But facts are facts and DHO900 is hardware that is a shame of Rigol and should not have left prototype phase. It is a 250MHz scope (claimed) that in worst case samples at 156MSps/s which is FINRSI type of specmanship.
And since many people hack their DHO800 to 900 that defect is applicable to 800 too.

When does it switch to 156Msa/s?
(other than zooming out horizontally to >= 500 us/div)

when you turn on all 4CH and 16bit LA and expect you can probe 250MHz with the setup

[BillyO]

Question for those that know.

With the DHO900 (non 'S' model) can you still do automated Bode plots with an external Sig. Gen.?

[Fungus]

I've mentioned before that I think Rigol should drop to 125Mhz when more than 2 channels are active.

(Probably a very difficult hack...)

[ebastler]

I've mentioned before that I think Rigol should drop to 125Mhz when more than 2 channels are active.
(Probably a very difficult hack...)

Might be tricky as a retrofit hack, but it should be straightforward for Rigol. The front-end amplifier has software-controlled filters anyway...

They could even fall back to 70 MHz in 4-channel MSO mode. 

[Mechatrommer]

With the DHO900 (non 'S' model) can you still do automated Bode plots with an external Sig. Gen.?

offically? no. but unofficially, other than hacking it to S model in vendor.bin, you can make PC SW to control both and plot in PC, its possible.

[shapirus]

900 switches when all 4 ch on and MSO

I see. Yeah that's kinda expected.
A hacked 800 doesn't do it, it stays on 312.5.

It however starts to fail well below Nyquist (which is 156.25 MHz at this sampling rate): the displayed signal begins to have lower frequency noise at about 100 Mhz and it becomes next to useless at ~130 MHz.

Frequency counter works only up to 142 MHz.

Here's a 30s frequency sweep from 70 MHz to 170 MHz. Nevermind the intermittent glitches and some jitter, they may be a result of the signal gen that I'm using (TinySA Ultra). A 50 Ohm feed-through terminator was used on the scope's BNC input.

Memory length was deliberately set to a maximum to get a minimal waveform update rate to get rid of the display persistence effects, which are not desirable in this case.

(the 156.25 MHz point is at ~00:25)

[Fungus]

It however starts to fail well below Nyquist (which is 156.25 MHz at this sampling rate): the displayed signal begins to have lower frequency noise at about 100 Mhz and it becomes next to useless at ~130 MHz.

sin(x)/x will start to go wrong at sample rate / 2.5

That video shows exactly what the math says it should.

[shapirus]

sin(x)/x will start to go wrong at sample rate / 2.5

That video shows exactly what the math says it should.

Yeah, and it actually works fine up to 500 MHz at 1.25 GSa/s (save for the greatly reduced amplitude because of the input filter).

[ebastler]

900 switches when all 4 ch on and MSO

I see. Yeah that's kinda expected.
A hacked 800 doesn't do it, it stays on 312.5.

No, it's not really expected. The DHO900 is the only scope I'm aware of which behaves this way.

In typical DSOs, the analog sampling rate is limited by the ADC. (Which is shared across two or four channels, hence sampling rate per channel drops as you activate more analog channels.) But the digital channels are read separately -- they do not need the ADC, and hence don't eat into the ADC bandwidth available for analog sampling.

In contrast, the DHO900 series seems to be limited by the bandwidth of the FPGA/DRAM interface. It uses the same ADC as the larger DHO1000 series, yet runs it at only 1.25 instead of 2 GHz. And when you want to read digital data in parallel, that eats into the available sampling rate.

A hacked DHO800 will behave the same -- as soon as you actually connect and enable a digital probe. Mechatrommer has done that and can comment.