dac architecture

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