Sv: [sdiy] Tube signal conditioner ?

[cfmd at swipnet.se]

> Hello friends,

Hello Michael!

> i am on the quest for some kind of Schmitt-Trigger or
> Comparator that allows me to clean up a possibly noisy
> signal to an exact 0/1 level.
> 
> Purpose of this is for my tube DAC. At this moment, i am using
> tubes to amplify the TTL output from my digityl circuitry to drive
> a R/2R ladder. But, TTL seems not to have very exact signal
> lexels (H is anywhere between 3.9 and 4.4 V, L between 0.4 and
> 0.9 V) between outputs of different chips.

Ah, classical digital meets analog problem.

> So, i want to make some kind of "hard switching", by means of a tube.
> How can i achieve this ? OK, i have the schematics of that old philbrick
> tube opamp, and could use it as a comparator, but i feel there must be
> a more simple solution for that.
> 
> Anybody any ideas ?

Well, what I would recommend is that you "wash" your digital signals into a good power domain. You can do that by pretty simple means.

If you take a 74HCT gate, it will perfectly well understand the TTL levels, but
it will give you MOSFET wiring to Vcc and Vss. So, what you want to do is to set up a quite ground and power field. Preferably you have separate stabilisation (using say a uA723 or similar regulator) to feed only the conversion chip(s).
Wire the power-feed according to the rules with propper bypass caps, low inductance caps, low inductance paths etc. You can also intentionally lowpass filter the input TTL signals to get cleaner "shots". The power-line (Vcc->Vss) should be quite like a grave even when you fiddle with all the bits like a crazy horse on a wild streak. Then, this will help to bring your noise levels down.

If you want to do it "propper" you should actually reclock everything in a register.

These are just a few ideas from the top of my head.

The trick of bringing the signal into a clean power-setup I've used before with good result.

Cheers,
Magnus

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