Pretty nice photos. I like it too, to disable blanking and to see the painting beam. I did some tests with the MCP4922, too, but it was too slow for me. Mabe you've found a better way (code) driving it. If you don't mind, I'm intersted in a code snipped and which lib you have used. I ended up in a AD5344, which works pretty well for me on the teensy40/41). So maybe I will give the 4922 a second try. And I'm curious about your powersupply approach. Especial the +-5V and noise. I'm using a switched one, which is too noisy. Last, I'm in the same problem, getting a sharp image an tweaking with focus, astig etc. So, if you get good results with the emitter follower, please share your experience. Cheers Jörg
On Monday, November 20, 2023 at 9:37:35 PM UTC+1 GDR GDR wrote: > wow! while the technical details are all "greek" to me, the images look > amazing. I love the "artifacts" in them. I dont know how you took these, > but it looks awesome! I like it as it is! not sure how this looks like in > "motion" but looks great in the stills... can you post some more? cheers > > On Sunday, November 19, 2023 at 4:25:50 PM UTC-6 Anders Mikkelsen wrote: > >> I recently started playing with scope clocks again, after seeing David's >> circle graphics clock the better part of two decades ago and having had it >> in the back of my mind since. This was prompted by Teensy 4.1 becoming >> available again on the market, and the official port of the SCTV codebase >> for this platform. I got it up and running, and explored some aspects >> around the design, I thought I'd share some info with the group in case >> anybody else can benefit from it. >> >> Firstly, off-the-shelf flyback transformers for low power auxiliary >> supplies work very well in Baxandall oscillators to make the required >> voltages at minimal cost and complexity, without using any custom parts. I >> used a Wurth 750310787, but many others are available. A buck converter can >> be wrapped around the circuit to get good line and load regulation, and the >> resulting BOM cost is less than 5 bucks. An off-the-shelf gate drive >> transformer can be used to drive the filament, providing the required >> isolation and power handling for a few bucks more, making a very simple and >> cheap design. >> >> Secondly, the LT DAC in the design can be replaced by a much more >> affordable (and easy to solder) MCP4922, without taking a hit on the DAC >> update rate. A comparison of the two parts would suggest that the 20 MHz >> maximum SPI rate of this part would significantly impact performance, but >> in practice it ends up working at around 500 KSPS as well. The issue with >> the original part is that it relies on 24 bit SPI transfers, which are >> again implemented as an 8 bit and a 16 bit transfer, and the overhead from >> reconfiguring the SPI peripheral and waiting for the receive buffer to fill >> is significant. The MCP allows native 16 bit operation of the peripheral, >> and use of the transmit FIFO to make the transfers without any intervention >> from the firmware once a set of XY coordinates are passed. I also >> experimented with porting the code to run on an RP2040, which works well so >> far. I haven't assembled the RTC part yet, but everything else seems to >> work with minimal changes to the code. The biggest effort was figuring out >> how to do efficient SPI transfers, which required some raw register access, >> and clock tree reconfiguration for synchronous operation of the SPI >> peripheral (which is not the case with the Arduino default clock frequency >> for this board). >> >> Thirdly, I did struggle a lot with getting a sharp image without having a >> lot of interaction between focus, brightness and astigmatism. Part of it >> came from feeding the cathode from a high impedance point in the divider, >> but a major factor turned out to be the source impedance of the anode >> drive. Most circuits I've seen drive the anode from a potentiometer fed >> from the deflection design. Most CRTs seem to draw an anode current that's >> significantly higher than the screen current, often by an order of >> magnitude, and this current causes the anode voltage to drift around as the >> brightness is adjusted. I initially used the high value of 1 Mohm for the >> pot, which exaggerated this issue a bit as well. In electrostatic CRTs, the >> final anode together with the first set of deflection plates forms a >> cylindrical lens, that affects both image geometry and beam focus. I looked >> at some oscilloscope schematics, and found some that use an emitter >> follower to buffer the output of the astigmatism pot to deal with this, and >> I'm just waiting for some parts in order to try this out in my design. >> >> I tried it with a range of old CRTs, and most of them struggle with >> getting a sharp dot near the edges of the screen. I suspect this is partly >> due to my sagging voltages, but I'll see if the improvements address it. >> Does anybody know what to expect in terms of corner spot size for typical >> non-PDA electrostatic CRTs in the range of 30 - 100 mm screen diameter? >> This is at 1200 V, and with enough beam current to make the scope clock >> image readable in a room with normal indoor lighting. >> >> Some pictures attached in order to compensate for all the words. >> >> AM >> > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/1d47ee9a-6973-4cb8-91cb-b6b828142b4dn%40googlegroups.com.
