Reverse engineering FNIRSI-5012H

[ataradov]

The trigger code now works for 100% of samples at 125 MSPS. So i don't think this would be an issue.

[cliffyk]

The trigger code now works for 100% of samples at 125 MSPS. So i don't think this would be an issue.

Alex, let me first commend you on your knowledge and diligence!

I have wondered though, has anyone done a simple Nyquist "flat-line" test of the 5012H sample rate?  I would--however I do no longer have a 500 MHz signal source. As I am certain you are aware, a simple test of feeding a DSO a signal of a frequency equal to the DSO's sampling rate will result in a flat line trace. 499 MHz would alias to 1 MHz as would 501 MHz.

If a 500 MHz input doesn't cause a flat line display it is not sampling at 500 MSps.

[ataradov]

Its clock is derived from a cheap crystal oscillator. Measuring any sort of timing performance is a futile effort. We know "nominal" sample rate because we feed it to the ADC and we can measure it. There is no need for roundabout methods. Also, its front-end will not let 500 MHz through. There is 200 MHz buffer op-amp.

The 125 MSPS is achieved by interleaving 2 channels, so matching and timing between the odd and event samples in this mode is also very questionable. 

Remember, this will never be a real measurement equipment.

[ataradov]

Here is a schematic of the so called front-end of this scope. None of the capacitor values are known, obviously.

Transistors are part of the optocouplers.

The paired transistors are the same optocouplers, but they are driven at the same time (LEDs on the other side are in series). If anybody knows what is going on with those paired transistors, please chime in.

[lordium]

Seems like they just short the 1.3M to get easier ranges to work with (sure it's bjts, and not mosfets?).   x 0.9, 0.3, 0.1 ,0.05, 0.025, 0.01 (close enough)

[BravoV]

The paired transistors are the same optocouplers, but they are driven at the same time (LEDs on the other side are in series). If anybody knows what is going on with those paired transistors, please chime in.

Assuming those are CPC1002, I think its optomos relay / mosfet switch.

CMIIW 

[ataradov]

CPC1002 shorts the AC/DC capacitor. The range switches are PC817 optocouplers.

And those pairs are controlled separately for different ranges.

Each transistor or pair of transistors represent a separate volts/div setting.

[exile]

I think it has to do with Emitter-collector voltage quite low. (zenerdoid effect)
And I think it starts to affect before 6volt as the data sheet says. By coupling 2 in each direction it can withstand higher voltage.

That's what I think, but I can be wrong.

[magic]

Yeah, given that they are in antiseries I would guess it's something to do with reverse breakdown.

And FYI, the opamp is configured for unity gain with low series resistance, so per the DS it's supposed to have massive gain peaking at 300MHz and -3dB point at 450MHz. Not sure why they did that.

[splin]

And FYI, the opamp is configured for unity gain with low series resistance, so per the DS it's supposed to have massive gain peaking at 300MHz and -3dB point at 450MHz. Not sure why they did that.

That graph is for .1Vpp output. For a full scale, 1Vpp ADC input, slew rate distortion will start at about 35MHz according to the graph on page 8. However, unusually, the graph also implies that the slew rate is a function of the output signal level - approx 65V/us @ 5Vpp rising to 110V/us at 1Vpp and nearer 300V/us at 100MHz which is closer to the datasheet 300 to 360V/us values. (The latter are quoted for a gain of 2 but the gain wouldn't normally affect slew rate significantly).

So perhaps slew rate won't actually be a limit until 150MHz or above for signals < .9Vpp. Does the graph suggest an unusual compensation scheme in the amp or is the graph suspect?

[magic]

Probably the plot is right, and all plots are for 5V supply and gain of 2 so you don't even know what will happen at unity gain at 3.3V. But it's unlikely to be anything good indeed, this chip is simply too slow and quirky for the claimed BW. (Assuming the ADC really is 1Vpp full scale, I didn't follow the thread).

I only remarked about the possibility of getting any sort of 500MHz signal in there at all

[OwO]

ADC might be the MXT2088: https://item.taobao.com/item.htm?spm=a230r.1.14.19.2eea1df5ep0STN&id=590948654001&ns=1&abbucket=11#detail

I just searched for "AD9288" + the Kanji for "substitute" on taobao and that turned up. More digging found this page which lists more ADCs: http://www.mxtronics.com/n107/n808/n809/n815/index.html

Unfortunately I couldn't find most of those on taobao, especially the 1GSPS+ ones. Maybe they are all sold under the ADI/TI part numbers instead.

[ataradov]

Thanks for this information. Interestingly, if you search AliExpress for "MXT2088", it will come up with a bunch of listings for "AD9288" that otherwise do not mention MXT2088 at all. So it looks like there some hidden connection/keywords in their system.

It is also good. Because while it is obvious  that all those AD9288 are fakes, they are quite a bit cheaper than the original. And for applications like this scope it really makes no difference.

[mg3100]

that all those AD9288 are fakes

At least they seem to function  .

Last month I ordered  a couple of P-Channel power mosfets and even though they looked genuine and had all the markings in the right place
I couldn't get them to function. In the end they turned out to be N-Channel mosfets  ... and of course no reply from the seller.

[ataradov]

If there is one item you should never buy from China, it would be power mosfets. It seems like they are the most faked item ever.

[mg3100]

I hope LCSC sends me genuine gd32f407vet6 

[ataradov]

I've implemented the basic version of the triggering setup. Basically a single trigger. I start the ADC DMA and run the trigger detection code. After the trigger is detected, I wait for ~64K more samples and stop the capture.

The issue is that on continuous AC signals the trigger is detected immediately, so there is no pre-trigger history. So it looks like I just need to collect 64K samples without running the trigger detection code, and then start  it. I guess it is fine, since this is really only an issues for periodic signals, and they will hopefully continue running.

A bigger issue is that current trigger detection code actually just detects that sample is above the threshold, not the moment of crossing. So if trigger detection code starts when the signal is already higher than the trigger, it marks the first sample as the trigger point. It is not a big deal, but will cause some loss of performance.

[dmendesf]

Trigger needs to have a state. Initial state is "non-triggered". For each sample in the buffer you do the trigger logic (for a rising edge trigger, logic is "if last sample is below trigger value and actual sample is equal or bigger than sample value, then state is triggered").

[ataradov]

Well, yes, I know how triggers are working in general. In this case we are extracting the last bit of performance with SIMD instructions, so it is not that trivial. Looking at the last sample every cycle is not an option.

So it looks like I would have to look for the opposite condition and then for the real trigger condition. It should not add a lot of instructions. I'm a bit distracted at the moment to think hard about it. I'll figure it out tomorrow.

[EL-TRONIC]

I bought FNIRSI-5012H and I got this today...
Please see images, is there any way to change language?
Maybe new firmware? Where to download?
Thanks in advance...

[ataradov]

Please see images, is there any way to change language?
Maybe new firmware? Where to download?

There are no images attached.  Also, there is no way to download or upgrade the firmware.

[EL-TRONIC]

[EL-TRONIC]

Is there any settings to change language?
Thanks

[ataradov]

I don't see any settings to change the language. So it looks like you are stuck with Chinese. Thankfully there are not that many menus you actually need to use past the initial setup.

[EL-TRONIC]

Thanks Alex