Rigol DHO804 Test and Compare Thread

[Fungus]

I'm still waiting for an explanation of the Siglent's FFT problems shown in Dave's review of the DHO800...

It has been revealed Dave can't drive FFT properly.
https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/msg5195079/#msg5195079

Why are you telling me? Take it up with Dave.

All I know is that Dave got it working on the Rigol, he got it working on the R&S, if he failed on the Siglent then there's a problem with the Siglent. Either in the UI or the implementation.

[Randy222]

Hi Randy,

Measured -3dB point lies on 200Mhz.

Also, if ch1 triggers from ch4, can you still get full 1.25Gsa/s on ch1 ?

Just tested it - samplerate will be then reduced to half although only one channel is active..

Arrrrg.

I assume then if the device does 1/2 on sample rate you will get 2x the record length when capturing for any given memory depth?

Does the 2ch w/ ext have the same issue when using ext as the trigger?

[Martin72]

Why are you telling me? Take it up with Dave.

Dave wasn't the one crowing all over the place that the siglent had a problem with the FFT...
This has now been readjusted three times with three different 1104X-Es, without any problems.
Speaking of problems, I had another hitch with the rigol after a long time, this time a "half".
Happened when I had the FFT function active to recreate the gimmicks, with FM mod and so on.
Suddenly the FFT display disappeared and didn't come back.
The signal itself was strangely discolored and you couldn't resolve it any further, the time base setting was out of order.
Very strange.

[gf]

12.5 MSa/s implies a 0...6.25MHz maximum span, therefore the 10MHz center is out of range.

[Martin72]

Yes, that's right - But that was displayed when the crash occurred, before that it was much more, or rather, I had taken the picture at that point by chance - It was 12.5MSa/s everywhere when the scope "froze".

[Martin72]

I think I've figured it out.
At 5ms/div the FFT disappears, correctly because the sample rate is then too low.
But it comes back when enough sample rate is available - from 2ms.
But earlier it didn't come back and the scope was permanently stuck at 12.5MSsa/s.
So don't play around with it too much...
Otherwise, the FFT function behaves in the same way as its big brother, the 4204.
In other words, hardly anything can be set, no different modes, which is a particular pity that there is no averaging.
And what I already noticed in February with the 4204 is also present here.
Generator with wavecombine function, 10Mhz frequency, ch1 10Vpp, ch2 1mVpp 10.1Mhz frequency, 50Ohm termination.
I have done this with another scope with a "flattop" window and it works perfectly.
With Rigol it looks different when you switch from Hanning to Flattop.

[ebastler]

hardly anything can be set, no different modes, which is a particular pity that there is no averaging.

The absence of averaging is clearly a gap. Makes the FFT output so much cleaner, and makes signals clearly visible that you otherwise have to guess at -- and it should be easy to implement. I think it's catching up with Rigol here that they implemented Averaging as an acquisition mode (only), not as a math function. That's a nice solution when you want to use Averaging on time-domain signals: no overhead and conveniently activated. But it means that averaging is not available post-FFT. A dedicated option would be needed there.

Besides that, I am struggling with the "hardly anything can be set" complaint. Span and Center are set directly in the FFT dialog; sampling rate and record length are set via the time base and memory size in my understanding. Are there critical parameters which can't be adjusted at all? Or is the complaint that the settings are not as clear and direct as one would like? -- Not aiming to defend the Rigol scopes here; I would like to better understand what is missing (or awkward). Thanks!

With Rigol it looks different when you switch from Hanning to Flattop.

Looks like the Flat-top window function is plainly broken on the Rigol. It is just that particular window function which gives the unexpected results, right? Maybe they don't deal with the negative weights properly? Looks like a bug.

[Fungus]

FFT center and span are really easy to set on a touch screen...

[Martin72]

Besides that, I am struggling with the "hardly anything can be set" complaint.

Deepl translation error..
There is nothing to complain about in the menu operation, on the contrary, I think it is one of the most successful menu structures at the moment.

[ebastler]

Besides that, I am struggling with the "hardly anything can be set" complaint.

Deepl translation error..
There is nothing to complain about in the menu operation, on the contrary, I think it is one of the most successful menu structures at the moment.

I didn't understand it as "it is hard to set anything", but as "there are not enough things which can be set". Is there something in the FFT which you would like to adjust but can't?

[iMo]

Working with the DS1054Z is like traveling backwards in time.
I hadn't noticed this before, but now, with the DHO804 in direct comparison, I have to say:
Farewell and thank you for once arousing my interest in a private scope and fulfilling my wish to own one.
But your time is simply up, no one should buy you anymore, given the DHO800.
A small example of how it could not be more drastic.
50mV/div, then stop, then enlarged to 20mv/div.
As I said, time's up.

Hmm, what I see is the 1054Z shows the raw voltage (60 points with the steps), the DHO shows the sinc interpolation (through the 64 points). You cannot judge on the resolution based on that pictures, imho..

[TurboTom]

I guess the engineers at Rigol confused the Flattop window function with the Tukey window which actually has a flat top in the time domain, maybe a language thing... Wouldn't worry too much about this since the most relevant window functions von Hann, Blackman-Harris and rectangular are available and appear to work correctly. A hint for using FFT on DHO scopes: don't use auto sampling depth but rather choose a fairly high number (at least at 1MPts the FFT still utilizes the full memory). Only if very fast FFT update rate is required at reduced available RBW, it makes sense to select less sample memory since Rigol's FFT engine is pretty fast anyway.

True, it's a pity that FFT avaraging isn't available (yet) -- Rigol should have included this instead of this color-grading FFT display nonsense (available on DHO1000 series, don't know if it's included in the DHO800 as well), but otherwise I don't find the FFT U/I too troublesome or unlogical to use. It's possible to get pretty decent results easily. And since sales for the anticipated high-quantity instrument range just started, we may actually see some reasonable firmware-polishing in future. At least I've still got some hope for this... 

[ebastler]

A hint for using FFT on DHO scopes: don't use auto sampling depth but rather choose a fairly high number (at least at 1MPts the FFT still utilizes the full memory). Only if very fast FFT update rate is required at reduced available RBW, it makes sense to select less sample memory since Rigol's FFT engine is pretty fast anyway.

I assume the DHO1000 has the same somewhat haphazard "RBW" indication as the DHO800 -- did you manage to figure out what it means?

I am not concerned with the precise naming (should it be "RBW", "resolution", "frequency interval" or whatever). But can you tie it back to the sampling rate and number of FFT data points in a systematic way? Does it depend on the choice of window function too? And what is the missing unit -- is it always Hz?

Thanks for your insights!

[Martin72]

Is there something in the FFT which you would like to adjust but can't?

Number of points for example, I'm used to being able to manually set the number of memory points (which are also not displayed on the rigol).
And RBW means resolutionbandwith, and the unit Hz is omitted on the rigol, depending on the situation, I didn't pay close attention to when this happens.

[Fungus]

color-grading FFT display nonsense (available on DHO1000 series, don't know if it's included in the DHO800 as well),

It is, and you can do it in multiple windows. 

[ebastler]

It is, and you can do it in multiple windows. 

But it still is just a color banding depending on the vertical position, right? Reminds me of those old video games with a transparent, colored screen overlay, before color monitors became affordable.

[Performa01]

The absence of averaging is clearly a gap. Makes the FFT output so much cleaner, and makes signals clearly visible that you otherwise have to guess at -- and it should be easy to implement. I think it's catching up with Rigol here that they implemented Averaging as an acquisition mode (only), not as a math function. That's a nice solution when you want to use Averaging on time-domain signals: no overhead and conveniently activated. But it means that averaging is not available post-FFT. A dedicated option would be needed there.

Average acquisition mode and/or math function are something completely different than averaging of an FFT trace.

Full featured DSOs have Average as
1.   Acquisition mode, yet with reduced max. memory depth
2.   Math function
3.   Substitute for VBW setting of an SA for the FFT traces.

All three could be used at the same time, yet the value of this would be questionable.

The SDS1104X-E provides 1 and 3 from the list above.

Besides that, I am struggling with the "hardly anything can be set" complaint.

I do not know what can be set in the DHO, maybe Martin will enlighten us. Yet there are other maybe even more important things.

For instance, am I the only one who wonders how the “RBW” can always be “100” (without dimension), regardless of the window function?

From the sparse info on the screen we get that the FFT sample rate is 62.5 MSa/s. We don’t know the number of FFT points, so we can only assume it’s the same as the record length, which is displayed as 625.00 kpts. What a coincidence that the frequency step happens to be exactly 100 Hz in this case. And of course the frequency step is independent of the window function. The RBW on the other hand can be anything from 90 Hz to 380 Hz, hence the info on the FFT is not only misleading but plain wrong. Hardly a professional touch…

Looks like the Flat-top window function is plainly broken on the Rigol. It is just that particular window function which gives the unexpected results, right? Maybe they don't deal with the negative weights properly? Looks like a bug.

Who cares? As long as you can browse the web and play doom on it. You just need to get aware of the real priorities in T&M…

[ebastler]

Average acquisition mode and/or math function are something completely different than averaging of an FFT trace.

Full featured DSOs have Average as
1.   Acquisition mode, yet with reduced max. memory depth
2.   Math function
3.   Substitute for VBW setting of an SA for the FFT traces.

But if a Math function 2. were available which averages over successive sweeps, couldn't it be be applied to the FFT to obtain 3.?

For instance, am I the only one who wonders how the “RBW” can always be “100” (without dimension), regardless of the window function?

From the sparse info on the screen we get that the FFT sample rate is 62.5 MSa/s. We don’t know the number of FFT points, so we can only assume it’s the same as the record length, which is displayed as 625.00 kpts. What a coincidence that the frequency step happens to be exactly 100 Hz in this case. And of course the frequency step is independent of the window function. The RBW on the other hand can be anything from 90 Hz to 380 Hz, hence the info on the FFT is not only misleading but plain wrong. Hardly a professional touch…

As stated in my questions to TurboTom, a few posts above, I have been wondering that too. If the only flaw in the displayed "RBW" is that it is not actually an RBW but a frequency step, I can live with that (although Rigol should fix it, of course, and add a unit while they are at it). But I did see some screenshots or videos earlier where unexpected "RBW" values were displayed, which did not seem to make sense even as a frequency step and assuming a Hz unit. Maybe Tom or Martin can shed some light onto whether and when this happens?

[2N3055]

Besides that, I am struggling with the "hardly anything can be set" complaint.

Deepl translation error..
There is nothing to complain about in the menu operation, on the contrary, I think it is one of the most successful menu structures at the moment.

I didn't understand it as "it is hard to set anything", but as "there are not enough things which can be set". Is there something in the FFT which you would like to adjust but can't?

FFT settings:

Parameter | Factory Settings

Operation | OFF
Source | CH1
X | Span-Center
Unit | dBm/dBV
Center Frequency | 5 MHz
Frequency Range | 10 MHz
Vertical Scale | 20 dB
Offset | 0 dBV
Window Function | Hanning
Color Grade | OFF

Peak Search | OFF
Peak Number | 5
Threshold | 5.5 dBV
Excursion | 1.8 dB
Order | Amp Order


This is not FFT. This is some kind of simplified Spectrum view.
There are no settings for :

1. FFT mode : Normal, Average, Max Hold (at least)
2. Number of points/bins
3. RBW cannot be set. It is displayed in window title (which by the way is not visible if you do multiple windows, negating the benefit) and is some kind of  function of timease. You twiddle timebase left and right until you like how it looks.

Problems with flattop window are not irrelevant. It is used for best amplitude accuracy, when trying to measure peaks amplitude.

Taking into account this simple cheap scope, I will not talk about  markers and all the stuff connected to it.

[ebastler]

Is there something in the FFT which you would like to adjust but can't?

Number of points for example, I'm used to being able to manually set the number of memory points (which are also not displayed on the rigol).

I thought that's set via the acquisition memory setting (up to 1 MPts)? Is the issue that it's not in the FFT dialog, or that the available steps are too coarse? Or does that setting not actually set the number of FFT points at all?

[ebastler]

1. FFT mode : Normal, Average, Max Hold (at least)
2. Number of points/bins
3. RBW cannot be set. It is displayed in window title (which by the way is not visible if you do multiple windows, negating the benefit) and is some kind of  function of timease. You twiddle timebase left and right until you like how it looks.

1. I think we all agree that lacking Average mode is an annoying omission, as discussed separately above. I don't know about the relevance of Max Hold mode, but that's probably due to my lack of knowledge about some applications (regulatory/EMC maybe?).

2. As discussed, it's set via the acquisition memory size in my understanding. That way of doing it may be lacking something; see my questions to Martin just above.

3. Now I am confused. If you choose the window function, record length and sampling rate -- how can you set the RBW independently?

[2N3055]

As stated in my questions to TurboTom, a few posts above, I have been wondering that too. If the only flaw in the displayed "RBW" is that it is not actually an RBW but a frequency step, I can live with that (although Rigol should fix it, of course, and add a unit while they are at it). But I did see some screenshots or videos earlier where unexpected "RBW" values were displayed, which did not seem to make sense even as a frequency step and assuming a Hz unit. Maybe Tom or Martin can shed some light onto whether and when this happens?

Basic problem is that this is a measurement instrument but nobody knows what it shows.

If it measures and shows kilobananas then say that so user knows.

If RBW is RBW in a SA sense then it should be configurable and documented how it does rescaling.
If they use RBW for frequency bin width, that is simply wrong.

Also since real FFT parameters are not shown, and RBW is aparently dimensionless number, how you know it is correct in what it shows. And since you can see how the rest of the scope is tested, at this point I would not trust FFT for any measurements.
Until this is untangled, FFT is simply a graphical visualization, like Fungus nicely shows in his multi window image. You have 3 spectrum windows without basic parameters shown. You know the window function used and resampled sample rate. nothing else.

[2N3055]

1. I think we all agree that lacking Average mode is an annoying omission, as discussed separately above. I don't know about the relevance of Max Hold mode, but that's probably due to my lack of knowledge about some applications (regulatory/EMC maybe?).

2. As discussed, it's set via the acquisition memory size in my understanding. That way of doing it may be lacking something; see my questions to Martin just above.

3. Now I am confused. If you choose the window function, record length and sampling rate -- how can you set the RBW independently?

1. Max hold is as much used as averaging. For instance frequency sweep..
2. Number of bins can be separate from sample rate and acquistion length.
3. I apologize for confusion. I mentioned RBW as it is now only thing they have. If they have RBW then it should be configurable like in a propper SA. In which case by setting RBW, scope changes number of bins to get close and then recalculates FFT bins to RBW bins taking into consideration used window.

[ebastler]

Basic problem is that this is a measurement instrument but nobody knows what it shows.

If it measures and shows kilobananas then say that so user knows.

If RBW is RBW in a SA sense then it should be configurable and documented how it does rescaling.
If they use RBW for frequency bin width, that is simply wrong.

Also since real FFT parameters are not shown, and RBW is aparently dimensionless number, how you know it is correct in what it shows.

Sorry, but I feel this did not add anything to the prior discussion besides negative emotions. Can we please keep separate matters separate, and be specific about them?

(a) What Rigol displays is apparently not RBW but frequency step, so they should fix either the label or the way the calculate it. Already said more than once.
(b) Of course it should always be displayed with a unit. Already said more than once.
(c) Which "real FFT parameters" are you missing? The number of FFT points, as already said more than once; what else?

[ebastler]

2. Number of bins can be separate from sample rate and acquistion length.

How would that be done? Doesn't the time resolution and length of the transformed time series define the spectral resolution and range of the FFT? Unless you do a piecewise FFT of partial segments of the time series, or preprocess the time series by averaging etc., of course.