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/ba34ca67-b81a-412b-b89d-b0f67a7204b4n%40googlegroups.com.
