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A High-Performance Open Source Oscilloscope: development log & future ideas

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asmi:

--- Quote from: tom66 on November 17, 2020, 03:34:47 pm ---asmi, could you link to that Cypress solution?  I will give it a look, but it does (as nctnico says) seem like added complexity for little or no benefit.
--- End quote ---
https://www.cypress.com/products/ez-pd-barrel-connector-replacement-bcr
I would recommend to buy this eval kit: https://www.cypress.com/documentation/development-kitsboards/cy4533-ez-pd-bcr-evaluation-kit It's very cheap ($25), and allows you to evaluate all features of the chip.
But I find it's hilarious that you already declared it to be complex and provide no benefit without even seeing it :palm:


--- Quote from: tom66 on November 17, 2020, 03:34:47 pm ---In my fairly modern home with a mix of Android and iOS devices I have one USB-C cable and zero USB-C power supplies.  My laptop (a few years old, not ultrabook format) still uses a barrel jack connector.  Girlfriend's laptop is the same and only 1 year old.  I've no doubt that people have power supplies with Type C,  but barrel-jack connectors are more common and assuming this device will ship with a power adapter, it won't be too expensive to source a 36W/48W 12V AC-adapter whereas a USB Type-C adapter will almost certainly cost more.
--- End quote ---
Take a look at Amazon - you can buy 45 W USB-C power supply for like $15-20.
Barrel jacks are good exactly until you connect the wrong one and cause some fireworks.


--- Quote from: tom66 on November 17, 2020, 03:34:47 pm ---And there will be that not-insignificant group of people who wonder "why does it not work with -cheap 5W smartphone charger-?"  When you have to qualify it with things like, only use >45W or more rated adapter, then the search-space of usable adapters drops considerably.

--- End quote ---
I kind of suspect that idiots are not exactly the target audience for DIY oscilloscope project :-DD
But again, nothing stops you from having both options if you really want that stone-age barrel jack. It's trivial to implement.

tom66:
I didn't regard Cypress solution as too complex.    I'd not seen it until you literally just linked it!  I just have an aversion to USB for power supplies because it's not ideal in many cases and for *this product* it is a complex solution with little obvious benefit.  As nctnico says, what does it add?  It needs to add a good thing to be worth the complexity.

There's a big TVS on the input that clamps at ~17V on the present board.  If you put reverse polarity or too many volts in it either blows the fuse or crowbars the external supply.  A barrel jack is about the most rugged DC connector you can get for the size whereas USB-C port could get contaminated with dust/dirt or have connection pads damaged - after all it is a 20 pin connector.  I've had plenty of headaches with USB connectors before failing in odd, usually somewhat intermittent ways: both the Lightning connector on my old iPhone and the USB Micro connector on my old Samsung S5 failed in intermittent fashion and required replacement.  So personally I see the barrel jack as better from an engineering environment perspective where you have more dust and contaminants than typical. 

And you have a point about the target-market being technical but then you will also have non-technical people that might want to use such an instrument e.g. education or hobbyists.  That USB-C supply is twice the retail price of a comparable Stontronics PSU with a barrel jack output, and it's not clear what it offers over the barrel jack for most users.

Don't get me wrong, nothing is set in stone yet,  it might be the best solution for Mk2 of the product.  I will of course listen to feedback in that regard.

dougg:

--- Quote from: nctnico on November 17, 2020, 03:31:46 pm ---Still isn't adding USB-C adding on more complexity to an already complex project? I recall Dave2 having quite a bit of difficulties implementing USB-C power for Dave's new power supply.

--- End quote ---

There are lots of 1 chip solutions. Have a look at tindie.com for examples with schematics in most cases. If a USB-C power adapter (or battery, the RP-PB201 is a cheaper + more powerful than the Mophie 3XL that I mentioned previously) doesn't have enough power for the 'scope (you always get at least 5 Volts (Vsafe) at 1.5 Amps) then flash a red LED.

Dave seemed to be intimidated by the 659 page USB PD spec (plus the 373 page USB Type C spec). Dave "spat the dummy" for theatrical effect; either that, or he should get out in the real world more often! For example: write a device driver for product A to talk to product B via transport C (e.g. USB, Ethernet, BT) using OS/environment "D". That may involve thousands of pages across multiple specs. You don't read them like a novel, you use them like a dictionary. And when product A fails to talk in some situation to product B, you contact support for product A (say) and point out that it doesn't comply with the spec they claim to implement with a reference to chapter and verse (of the relevant spec).

I proposed two USB-C ports to replace the barrel connector _and_ the USB Type A receptacle. So either one could be power in, while the other functionally replaced the USB Type A host. In that latter role USB-C is more flexible as it can play either the role of (data) host or device. So your PC connection could be via USB-C where the PC is "host" and the 'scope is the device. OTOH you could connect a USB memory key and the 'scope would play the host role (and source a bit of power).

Whoever suggested connecting a USB-C power adapter to a USB dongle might find that hard to do if the USB-C power adapter has a captive cable. [They would need a USB-C F-F dongle.] If the power adapter didn't have a captive cable (just a USB-C female receptacle) then the connection can be made but nothing bad would happen (i.e. no magic smoke), the dongle would be powered at 5 Volts but the dongle would find no USB host at the other end of the cable to talk to. Maybe the dongle would flash a LED suggesting something was wrong. When you use symmetrical cables then many more stupid combinations are possible (so devices and their users need to be a bit smarter) but you need less (a lot less) cable variants. That is a big win for not much pain .

dougg:

--- Quote from: tom66 on November 17, 2020, 04:24:09 pm ---There's a big TVS on the input that clamps at ~17V on the present board.  If you put reverse polarity or too many volts in it either blows the fuse or crowbars the external supply.  A barrel jack is about the most rugged DC connector you can get for the size whereas USB-C port could get contaminated with dust/dirt or have connection pads damaged - after all it is a 20 pin connector.  I've had plenty of headaches with USB connectors before failing in odd, usually somewhat intermittent ways: both the Lightning connector on my old iPhone and the USB Micro connector on my old Samsung S5 failed in intermittent fashion and required replacement.  So personally I see the barrel jack as better from an engineering environment perspective where you have more dust and contaminants than typical. 

And you have a point about the target-market being technical but then you will also have non-technical people that might want to use such an instrument e.g. education or hobbyists.  That USB-C supply is twice the retail price of a comparable Stontronics PSU with a barrel jack output, and it's not clear what it offers over the barrel jack for most users.

--- End quote ---

I'm proposing an infinitely cheaper PSU supplied with your 'scope :-) That is, no PSU at all. Sounds like you need 15 Volts in and while that is a common USB-C PSU voltage, the El cheapo ones only supply 5 Volts, and sometimes 9 Volts as well. If you need 15 Watts or less than you could boost a RPi 4 PSU (and they are around $US10). All USB-C PSU schematics that I have seen (that can supply > 5 Volts) have a pass MOSFET that gets switched off in a fault condition. For power only USB-C the 24/22 pin connector can come down to as few as 6 active pins (see https://www.cuidevices.com/blog/an-introduction-to-power-only-usb-type-c-connectors ).

nctnico:

--- Quote from: tom66 on November 17, 2020, 11:21:36 am ---
--- Quote from: nctnico on November 17, 2020, 10:56:11 am ---Just wondering... has any work been done on an analog front-end? I have done some work on this in the past; I can dig it up if there is interest. Looking at the Analog devices DSO fronted parts it seems that these make life a lot easier.

--- End quote ---

I've got a concept and LTSpice simulation of the attenuator and pre-amp side, but nothing has been tested for real or laid out.  It would be useful to have an experienced analog engineer look at this - I know enough to be dangerous but that's about it.

--- End quote ---
I have attached a design I created based on earlier circuits. IIRC it is intended to offer a 100MHz bandwidth and should survive being connected to mains (note the date!).
Left to right, top to bottom:
- Input section with attenuators. Note sure whether the capacitance towards the probe is constant.
- Frequency compensation using varicaps. This works but requires an (digitally) adjustable voltage of up to 50V and I'm not sure how well a calibration holds over time. Using trim capacitors might be a better idea for a first version.
- over voltage protection
- high impedance buffer
- anti-aliasing filter. Looking at it I'm not sure whether a 7th order filter is a good idea due to phase shifts.
- single ended to differential amplifier and analog offset
- gain control block and ADC.

Nowadays I'd skip the external gain control and use the internal gain control of the HMCAD1511/20 devices. It could be a nice Christmas project to see how it behaves.

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