dac architecture

Why? Works perfectly, R2R is as old as the world:

Got nice video from it (on output of Xilinx FPGA), driving straight into an emitter follower and then a 75 Ohm cable,

Assuming you use 0.1% Rs, is CMOS R_out the problem?

NT

Now it makes sense, thanks!

Best regards, Piotr

Doesn't have to be super duper linear, though +-10% is lots tight enough, because all it does is change the loop gain a bit. You can do that well with an inductor in series and one in parallel.

Cheers

Phil Hobbs

Just use a regular DAC and an LPF and set the knee above a frequency that matters. The output will mostly be zero, anyway.

It's temperature so it's already heavily integrated. You just don't want too much process gain.

You can't usefully lowpass filter 1/f noise. It's really a different regime, especially when you care about LF phase noise.

Some years ago I was building stabilized lasers for geophysical applications (a downhole interferometric gravimeter). The basic idea is that you can measure the density of rock by measuring gravity at the surface (where the rock is pulling down) and then at depth, where some of the rock is now pulling up. It was also intended for reservoir management, where we'd leave one sensor at the bottom of the well and correlate its data with that at the surface. (There are important gravity variations due to barometric pressure, even.)

The laser had to have an Allan variance below 10**-10 at 100000 seconds (about a day), so I locked a communications-type DFB laser to an air-spaced etalon made from optically-contacted Zerodur, which was itself temperature-controlled. (Optical contacting makes a hermetic seal, which gets rid of the drift due to air density.)

The locking technique is one I invented almost 30 years ago: you sit halfway up an interference fringe, subtract the photocurrents from the transmitted and reflected beams, and servo at the null. That gets rid of the AM noise contribution. You have to attenuate the reflected beam a bit, because it's stronger than the transmitted beam due to cavity losses. As long as you're super paranoid about fringes due to unwanted surface reflections, it's a very very stable locking mechanism, and doesn't require super-high finesse etalons like Pound-Drever-Hall.

Interestingly it turns out that if you adjust the attenuation so that

dR/d omega + dT/d omega = 0

at the same frequency where

R-T = 0

the out-of-band frequency noise decouples from the total amplitude measurement as well, so theoretically you can do intracavity measurements at the shot noise. (The loop suppresses the in-band noise.)

Filtering was not a useful concept.

Cheers

Phil Hobbs

Did you ever write that up?

Nah, I haven't published a paper since I left IBM. The technology worked great, but the company went down the tubes (so to speak) when the founder and main technical guy went off on the most spectacular midlife crisis in my acquaintance--apparently he deserted his wife and five children, then skipped off to China and shacked up with a 22-year-old rich girl in Shanghai or someplace.

(February 22nd is the 10th anniversary of my consulting business--we're going to throw a party.)

Cheers

Phil Hobbs

Wait, what? Is that a thing in TCXO's ? I meant the lowpass really as just more integration - I do know you have to let these things get to equilibrium before you can trust them.

Sounds like it then.

That's crazy. Seems like it would also be a fine seismograph...

I would rather think not :) I thought we were talking about a temperature controller.

No, but it's in an old patent: .

I'll probably put it in the third edition--it's a great trick.

Cheers

Phil Hobbs

Well, it depends what you're interested in. There's a patent on total-metal detection in process water, which was really fun:

and of course the rest are at

Cheers

Phil Hobbs

On Jan 18, 2019, Phil Hobbs wrote (in article):

You mentioned in a response to the above that you had not published anything since your IBM tenure.

But did you describe this method in "Building Electro-Optical Systems: Making it all Work"?

Joe Gwinn

Do you have any other goodies like that, which you have only published as a patent?

Way to wreck the Q.

How so?

Cheers

Phil Hobbs