With the conventional circuit, you have two saturated transistor switches, each of which needs to turn off to blank the anode. Each may take a few microseconds, and the second stage doesn't start its time delay until the first stage turn off is completely finished. With most designs the transistors are over-driven, which makes the delays worse. Switch-on time of the next stage is not delayed, so you can easily get more than 10 microseconds overlap with the next anode.
The cascode stage has essentially zero switching time, and with a bit of attention to base current and resistor values you can cut the switching time of the PNP to a minimum, so even if there is a slight overlap it's likely to be less than the ionization time of the next tube. I don't claim that this circuit will always eliminate ghosting entirely, but I do assert that I've made half a dozen multiplexed clocks using a similar circuit to this (I use a 2ms digit period), switching the anodes simultaneously with the digit cathodes, and I've never had any ghosting. My example 1mA current to turn on Q2 is ridiculously high, by the way. Experiment with the R2 emitter resistor - it should work with 100k. But as you increase R2, make R1 an equal value, otherwise Q2 won't turn on. On Thursday, February 26, 2015 at 5:46:52 AM UTC-8, Terry S wrote: > > Pete, that's a nice application for the cascode circuit... Help me > understand how it eliminates concerns about dead time and ghosting. > > Terry > > > On Thursday, February 26, 2015 at 3:05:55 AM UTC-6, petehand wrote: > >> >> <https://lh5.googleusercontent.com/-_PML27wwc9w/VO7hHsuYRwI/AAAAAAAAATA/LVcwCOp5mWw/s1600/IN17.jpg> >> I did mean to change R22 to 10k, but I can suggest an even better way. >> This is a circuit I've used to multiplex IN17s, with no dead period and no >> ghosting. It looks terrifyingly unsafe. Let me explain. >> >> When the processor pin is high, Q1 base-emitter voltage is 0 and the >> transistor is cut off. The port pin sees no high voltage. When the port pin >> goes low the transistor turns on as a constant current source, the current >> set by (5 - 0.6)V/R2 or about 1mA. This drops 170V across Q1 and 10V across >> R1, which turns on Q2. Q1 is operating in linear mode, not saturated, so it >> switches in nanoseconds. Resistor R1 is necessary to help Q2 to switch off >> rapidly. >> >> This configuration of Q1 with the implied transistor inside the MPU is >> called a cascode <http://en.wikipedia.org/wiki/Cascode>. >> >> >> >> >> On Wednesday, February 25, 2015 at 3:45:43 AM UTC-8, joenixie wrote: >>> >>> Hmmm... interesting observation Pete, are you talking about changing R21 >>> or R22 to 10K? I chose 100K because I have them in my NixieNeon clock. >>> >>> -joe >>> >>> -- 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 [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/cafbdbfc-8371-42f9-9c65-ad2b38172f66%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
