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dac architecture

Started by John Larkin January 5, 2019
On Jan 18, 2019, Phil Hobbs wrote
(in article<iYOdnU7D_bbmS9zBnZ2dnUU7-IfNnZ2d@supernews.com>):

> On 1/17/19 10:13 PM, Les Cargill wrote: > > John Larkin wrote: > > > We were talking about TCXOs. One measures temperature and drives a > > > varicap through some nonlinear transfer function to get minumum net > > > TC. > > > > > > We don't want a digital design (ADC, lookup table or polynomial, DAC) > > > because that might add phase noise. I guess you could use a static > > > polynomial with the equivalent of nonvolatile DPOTS as the > > > coefficients. > > > > > > > > > This occurred to me, not as anything practical maybe but as an > > > interesting architecture. > > > > > > <https://www.dropbox.com/s/8ls632mndcxqby8/DAC_TCXO.JPG?raw=1> > > > > > > It's sort of a thermometer-code ADC, but each comparator incrementally > > > adds + or - one increment to the output. > > > > > > As the temperature increases, we jog the output up or down one > > > increment at a time. > > > > > > The sequence of switch settings become a delta-sigma code to make the > > > output. > > > > > > The comparators could be sort of linear, not step outputs, to kind of > > > interpolate a bit. Some flash ADCs did something like that, soft > > > comparators. > > > > > > 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
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
Phil Hobbs wrote...
> >> But did you describe this method in "Building Electro-Optical >> Systems: Making it all Work"? >> >> Joe Gwinn > > No, but it's in an old patent: > <https://electrooptical.net/static/eoi/patents/US06259712__.pdf>.
Do you have any other goodies like that, which you have only published as a patent? -- Thanks, - Win
On Saturday, January 5, 2019 at 5:47:51 PM UTC-5, Phil Hobbs wrote:
> On 1/5/19 2:12 PM, 698839253X6D445TD@nospam.org wrote: > > John Larkin wrote > >> We were talking about TCXOs. One measures temperature and drives a > >> varicap through some nonlinear transfer function to get minumum net > >> TC. > >> > >> We don't want a digital design (ADC, lookup table or polynomial, DAC) > >> because that might add phase noise. I guess you could use a static > >> polynomial with the equivalent of nonvolatile DPOTS as the > >> coefficients. > >> > >> > >> This occurred to me, not as anything practical maybe but as an > >> interesting architecture. > >> > >> https://www.dropbox.com/s/8ls632mndcxqby8/DAC_TCXO.JPG?raw=1 > >> > >> It's sort of a thermometer-code ADC, but each comparator incrementally > >> adds + or - one increment to the output. > >> > >> As the temperature increases, we jog the output up or down one > >> increment at a time. > >> > >> The sequence of switch settings become a delta-sigma code to make the > >> output. > >> > >> The comparators could be sort of linear, not step outputs, to kind of > >> interpolate a bit. Some flash ADCs did something like that, soft > >> comparators. > > > > Yes, > > but you can get more linear varicap effect by using for example 2. > > This paper shows some topologies and their effect: > > https://www.everythingrf.com/uploads/whitepapers/IEEE_BCTM_092010_2.pdf > > > > Then use a linear opamp feedback loop? > > > > I am using something like fig 1b on page 3 for my 25 MHz PLL reference for Eshail2. > > > At a given frequency you can do a good job of linearizing a varactor by > using one inductor in series, resonated just off the high-capacitance > end, and one in parallel, resonated just off the low-capacitance end.
Way to wreck the Q.
> > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC / Hobbs ElectroOptics > Optics, Electro-optics, Photonics, Analog Electronics > Briarcliff Manor NY 10510 > > http://electrooptical.net > http://hobbs-eo.com
On 1/20/19 11:18 AM, bloggs.fredbloggs.fred@gmail.com wrote:
> On Saturday, January 5, 2019 at 5:47:51 PM UTC-5, Phil Hobbs wrote: >> On 1/5/19 2:12 PM, 698839253X6D445TD@nospam.org wrote: >>> John Larkin wrote >>>> We were talking about TCXOs. One measures temperature and drives a >>>> varicap through some nonlinear transfer function to get minumum net >>>> TC. >>>> >>>> We don't want a digital design (ADC, lookup table or polynomial, DAC) >>>> because that might add phase noise. I guess you could use a static >>>> polynomial with the equivalent of nonvolatile DPOTS as the >>>> coefficients. >>>> >>>> >>>> This occurred to me, not as anything practical maybe but as an >>>> interesting architecture. >>>> >>>> https://www.dropbox.com/s/8ls632mndcxqby8/DAC_TCXO.JPG?raw=1 >>>> >>>> It's sort of a thermometer-code ADC, but each comparator incrementally >>>> adds + or - one increment to the output. >>>> >>>> As the temperature increases, we jog the output up or down one >>>> increment at a time. >>>> >>>> The sequence of switch settings become a delta-sigma code to make the >>>> output. >>>> >>>> The comparators could be sort of linear, not step outputs, to kind of >>>> interpolate a bit. Some flash ADCs did something like that, soft >>>> comparators. >>> >>> Yes, >>> but you can get more linear varicap effect by using for example 2. >>> This paper shows some topologies and their effect: >>> https://www.everythingrf.com/uploads/whitepapers/IEEE_BCTM_092010_2.pdf >>> >>> Then use a linear opamp feedback loop? >>> >>> I am using something like fig 1b on page 3 for my 25 MHz PLL reference for Eshail2. >>> >> At a given frequency you can do a good job of linearizing a varactor by >> using one inductor in series, resonated just off the high-capacitance >> end, and one in parallel, resonated just off the low-capacitance end. > > Way to wreck the Q.
How so? Cheers Phil Hobbs