A Tektronix TDS 684B Oscilloscope Uses CCD Analog Memory

Wow, great timing!

I recently got a TDS684A and also made the same discovery, and wrote half a blog post about it which remains unfinished/unpublished. I don't have much of an EE background (at least, not on this level) so my article was certainly worse.

Relatedly, I dumped the firmware of mine (referencing Tom's notes on a similar model) and started writing an emulator for it: https://github.com/DavidBuchanan314/TDS684A

It boots as far as the vxworks system prompt, and can start booting the Smalltalk "userspace" but crashes at some point during hardware initialization (since my emulation is very incomplete!) - some screenshots/logs: https://bsky.app/profile/retr0.id/post/3ljzzkwiy622d

Edit: heh, I just realised I'm cited in the article, regarding the ADG286D

Nice work. Just a comment: ADC clock is very sensitive to capacitive loading of the probe as it cannot drive that large load, it most likely fails to generate enough signal swing. It's not advisable to probe clock signals like that.

In the 90s Conrad (think Germany's RadioShack) sold a little audio recorder module that I believe was CCD based. It could only store 16 seconds and quality was terrible but audio stored completely in solid state without moving parts was revolutionary back then.

Thinking about it, I might still have the device somewhere in the attic.

> If you ever remove the interconnection PCB, make sure to put it back with the same orientation. It will fit just fine when rotated 180 degrees but the scope won’t work anymore!

I remember pulling a 486 out of its socket in the 1990s and putting it back with the wrong orientation. There was a poc and a bit of smoke. Something on the mainboard had burnt and it wasn't working anymore.

I used smell to locate the fault, a big trace on the PCB, which I soldered back and magic, it all worked again...

I didn't even know such things exist. When I wanted to know the delays of 7400-series logic gates, I built a logic analyzer using Arduino and stroboscopic effects which provided resolution on order of 1 ns. But with CCD it seems that it could be done much easier, and allowed to measure exact voltage and not just logic levels.

Unrelated but related:

I have an entry level standalone oscilloscope that i got but never used. I once looked for tutorials and unpacked it, ready to test, but:

It's covered in that kind of plastic that goes all gooey if left unattended for a long time.

Any hints on how I can clean it up so i can touch it again?

This isn't nearly as technologically radical, but a Tek scope of that era that we had in the lab then had a sequential colour display. A monochrome CRT with a fast-changing electronic colour filter in front of it. Totally sharp (for the era) colour graphics with no subpixels. Until you moved your head and you briefly saw the individual colour frames. The HP stuff was stodgy and boring and predictable by comparison.

The HP logic analyzers back then had a really neat touchscreen interface based on criss-crossing infrared beams in front of the CRT face. The only thing that I've ever used that felt even better than a capacitive touchscreen, though obviously lower resolution.

Not any kind of EE, just a nerd: could this kind of CCD circuitry be used to model a neural network, but in analog-land?

> The input to the ADC is clearly already chopped into discrete samples, with a new sample every 120 ns. We can discern a sine wave in the samples, but there’s a lot of noise on the signal too. Meanwhile the TDS684B CRT shows a nice and clean 1 MHz signal. I haven’t been able to figure out how that’s possible.

Is this maybe using some form of correlated double sampling?

> It looks like the signal doesn’t scan out of the CCD memory in the order it was received, hence the signal discontinuity in the middle.

Or maybe the samples are also interleaved in the low-order bits in some way. This could be because the organization of the CCD isn't symmetric for the input and output paths, perhaps to reduce area or power, since only one path has to be fast. This would make sense because if you implement the CCD using n parallel CCD bucket brigades you only have to put a fast S&H and multiplexer in front of it, then you can drive the CCD brigades at a fraction of the actual sample rate, and the capacitive load of each of those clock phases is much lower as well.

Maybe this CCD/delay-line/bucket-brigade trick can finally inspire someone to make a cheap DIY device that can sample USB3 signals, useful e.g. to check signal integrity. A missing tool in my toolbox.

I mean, I think this would be a very nice project for someone with hardware skills and some time on their hands, and it would be useful too.

Tektronix instrumentation from this era (as well as HP/Agilent, and many of the military-affiliated labs) used pretty magical engineering tricks.

In order to be able to design equipment, the instrumentation generally needs to outperform the equipment, sometimes by a significant margin. If I'm looking at the eye of a digital signal, I need to capture much faster than the signal.

It'd be fun to have a book of tricks from this era. At some point, it will fade into obscurity. Right now, it's a whole different bag of tricks for the state-of-the-art. They feel less... more textbook and less clever.

On the other hand, what's nice is that in 2025, decent equipment is cheap. There's a breakpoint where below around 100MHz, you can't do basic work, and above you can. That's roughly where FM pickup and a lot of oscillations sit. That used to cost a lot, but as technology progressed, we're at a point where a decent home lab can be had for well under a grand.