Sample and Hold

[ravaner]

Hello. I'm not sure that this is the convenient place for my question but ... I'm looking for old schematics of sample and hold or analog to digital converter based on electron vacuum tubes. Thanks by advance for your help.

[Sch3mat1c]

Hmmm... sampler maybe, but I doubt ADC was ever done vacuum state. Simply no need for it, it's faster to plot on an oscilloscope or X-Y plotter and make a photograph of it. They didn't have the widely available data processing, let alone DSP, we have today.

When did Tektronix release their first sampling plugin? I'm guessing it was already transistor...

As for inventing one from scratch, tube S&H would probably be best made with a diode gate (as far as switching signals, Tek's used this since the beginning, when they created the "dual trace" oscilloscope). Triodes don't make as good switches as JFETs, and certainly aren't bidirectional. You still have to contend with leakage, which is astronomical (low uA) compared to SS (<pA easily). That will fundamentally limit the time a signal remains valid for a given ratio of sampling vs. hold time (i.e., longer sampling time = more time to charge a bigger capacitor = more time the voltage remains within tolerance).

As for making an ADC, you could build a comparator + SAR with not too many tubes. Tubes are fine at storing logic states, if not particularly fast. The comparator will be plenty noisy and drifty, when you get to the lower bits (~10mV), so even with compensation, you'll have a limit there. Fortunately you can amplify the signal to accommodate a more tube style voltage (100V instead of 5V?), but that still only gains you a few bits accuracy. One bit sigma delta would be easy, and not too bad on accuracy due to the integrator.

And of course the complement, a tube DAC, would be kind of silly. You're better off stacking calibrated xxx meg resistors from the logic lines to the output, using voltage division to brute force the conversion. With 200V+ to burn, this isn't a big deal. You could still switch currents with diode gates, but you need to generate accurate currents, too.

Tim

[ravaner]

Thanks for reply. I know all that, but it's just for my own culture, and if I get schematics they will be the base for an experiment I want to realize with an electrometer tube CK5886 for a switched integrator. I've tested with special IC's and homemade circuits using JFet and MosFet but problem is speed (always staying in an amateur environment where it's not easy to get high range components).

[bill_a]

Rav.
Huh? Just about anything that's available to pros is available to you. As a pro, I get most of my protoype parts from Digikey, Mouser or Newark, all of which will sell to anybody. So I'm not sure that's a good excuse.
Bill

[Sch3mat1c]

5886, "electrometer pentode". Well I'll be, 10^-15 -- that's 1fA nominal grid current! Haha, and at really low voltages too, like 4.5Vg2, cool. Microamps plate current, of course.

It's my understanding that FET op-amps that good have only recently hit the market. But they have much higher transconductance (or in the case of op-amps, voltage gain). In fact, the insulation quality of even a regular 2N7000 is pretty impressive, just don't get fingerprints on it.

You will certainly not get any speed improvement using tubes.

Tim

[EC8010]

Originally Posted by Sch3mat1c

Hmmm... sampler maybe, but I doubt ADC was ever done vacuum state.

Amazingly, it was, and it was audio. This month's AES journal has a fascinating article about the SIGSALY system that allowed Churchill and Roosevelt to talk securely during WWII. The digital encryption techniques were startlingly modern. Unsurprisingly, the terminal equipment was rather big (forty bays of equipment).

[jrevillug]

Mike's Electric Stuff has a bit about Vacuum state ADC:

Glass Analogue to Digital converter

It is well worth exploring the site - lots of interestnig and fun stuff to do with, erm, electricity. I want a Mercury Arc Rectifier for my MIG welder...

Thanks for the pointer to the AES article - I'll have to try and find that one.

[Soonerorlater]

Excellent site, particularly if you're looking for a new avatar

[tomchr]

Originally Posted by jrevillug

Mike's Electric Stuff has a bit about Vacuum state ADC:

Glass Analogue to Digital converter

I went to the EIPBN ("3-beams") conference some 5-6 years ago. Those guys deal with just about everything on a miniature scale that has photon/ion/electron beams in it. Someone was presenting a MEMS-based ADC based on exactly that principle. Pretty neat stuff.

~Tom

[Miles Prower]

Originally Posted by Sch3mat1c

Hmmm... sampler maybe, but I doubt ADC was ever done vacuum state.

Actually, hollow state ADCs did exist. In 1955, Epsco, Inc. introduced its Datrac B-611 hollow state ADC which offered 11 bit resolution at a 44KHz conversion rate, which is still pretty impressive. The whole unit weighed in at some 150 lbs, at a price of $8500. The performance is still impressive, but definitely pricey. ADCs were used in the space program, and for the first radar signature analysis techniques.

Hollow state ADCs were based on a VT derived from electrostatic deflection CRTs. In the place of a screen, you had two plates: one solid, and another one with perforations in front of it. The pattern of perforations determined how the analog signal was digitally encoded. Horizontal scanning determined the conversion rate, while vertical deflection would put the electron beam at the proper place to digitally convert the corresponding voltage level.

By 1959, discreet BJTs were being used to implement ADCs with the corresponding reduction in size, weight, and cost.