[time-nuts] Anybody want a Thunderbolt power supply?

[Scott Stobbe]

Interesting, I would have thought that the +12V input would be extremely
well regulated since its shared with the oven heater, I*R drops are going
to show up every where, if your looking for uV levels of stability.
Just a connector has milliohms of contact resistance, let alone routing and
cables...

On Friday, 21 October 2016, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> > On Oct 21, 2016, at 7:44 PM, Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:;>> wrote:
> >
> > A little more data on the 7912.
> >
> > The first plot shows the tempCo of the 7912 measured with ambient
> > temperature swings "7912_TempCo.png". Which is -150 ppm/degC.
> >
> > The second plot is off a 7912 logged for an hour or so, "7912_1PLC.png",
> > nothing too interesting here. However the environmental temperature swing
> > of about 1 degC/hour is pretty conservative for a DUT sitting in free
> air.
> >
> > Finally, an allan devation plot looking at the normalized stability of a
> > 7912 regulator "7912_AllanDeviation.png". Interestingly here, is, how
> quick
> > a 15 mK/min temperature swing shoots above the 1/f floor, it's a matter
> of
> > seconds.
> >
> > Now if your PSRR is 1 ppb/V or better, then all of this is comfortably
> > below the intrinsic noise of a thunderbolt.
>
> The main point is that the internal tempco of the TBolt it’s self is much
> larger than
> the issues surrounding the power supply pins. The +12 is the only one that
> is
> sensitive enough to voltage (change in frequency vs voltage) to contribute
> to any
> significant way to the overall stability.
>
> Bob
>
> >
> > On Fri, Oct 21, 2016 at 12:20 AM, Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:;>>
> > wrote:
> >
> >> Nick had mention that the -12V rail on the thunderbolt has the poorest
> >> PSRR with respect to frequency output, so I first took a look at the
> >> venerable 7912.
> >>
> >> The first data-set was taken with a -13.5 VDC input. Attached is the 0.1
> >> Hz to 10 Hz noise of an essentially quiescently loaded 7912, only a 10k
> >> resistor was added as load for preliminary evaluation. With a 60 dB
> preamp
> >> the scale of the scope plot is 20 uV/div. The 0.1Hz to 10Hz band noise
> is
> >> 15 uVrms, which is about 1.3 ppm rms of the DC mean.
> >>
> >> In allan deviation terms, a quiescently loaded 7912 has a spot noise of
> 7
> >> uV/rtHz at 1 Hz (on the 1/f slope), normalized that's 580 ppb/rtHz.
> >> Equivalently speaking, the flicker noise floor of an allan deviation
> plot
> >> would be sqrt(2*ln(2)) that figure to be 6.8E-7.
> >>
> >> Assuming a thunderbolt should be achieving 1/f floor of around 1E-12, it
> >> would need a PSRR of at least 1 ppm/V. I'm sure someone has gone to the
> >> trouble of actually measuring it.
> >>
> >> So from a 0.1 Hz to 10 Hz noise standpoint, the 7912 isn't terrible
> >> with 1.3 ppm rms noise, considering an LM399 is about 0.1 ppm rms, only
> one
> >> order of magnitude off.
> >>
> >> The bad side of a 7912 is in long-term stability and tempCo, the sample
> I
> >> tested had at least a 150 ppm/degC tempCo, which is going to put a
> serious
> >> lump/bump in the 10s tau to gps crossover point on an allan deviation
> plot.
> >>
> >>
> >>
> >> On Tue, Oct 18, 2016 at 3:05 PM, Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:;>>
> >> wrote:
> >>
> >>> I'm sure I have some 7805s lying around, maybe a 7812/7912. I'm
> >>> interested to see the 1/f noise of a classic regulator, what load
> current
> >>> do you expect? I can bias a 7805 for the same load and measure the 0.1
> to
> >>> 10 Hz noise.
> >>>
> >>> Also if you have a digital scope without a very good builtin FFT,
> octave
> >>> would be one solution.
> >>>
> >>> On Tue, Oct 18, 2016 at 10:46 AM, Nick Sayer via time-nuts <
> >>> time-nuts at febo.com <javascript:;>> wrote:
> >>>
> >>>> Just an update. I’ve built the second prototype board (I skipped over
> >>>> the first design), and it’s powering my tbolt right now.
> >>>>
> >>>> The design calls for 15v in (though it would also work with 13.8v).
> The
> >>>> +12 output comes from a D2PAK 7812. For +5, there is an AP1509 buck
> >>>> converter to make around 6.5 volts, then a DPAK 7805. For -12, there
> is an
> >>>> MC34063 configured as an inverter to make around -13.75 volts and
> then a
> >>>> DPAK 7912.
> >>>>
> >>>> Steady-state, the system appears to be working just fine. The AP1509’s
> >>>> inductor and the D2PAK 7812 are just warm to the touch.
> >>>>
> >>>> I checked for noise and ripple on the outputs and it’s somewhere
> around
> >>>> ±2 mV or so generally. From what I can see on the scope, there’s no
> ripple
> >>>> - it’s all high frequency noise. I am not absolutely certain that the
> noise
> >>>> measurement represents real noise or the limits of my measuring
> ability.
> >>>> I’m just using the scope probes the scope came with, and 2 mV/div is
> its
> >>>> lowest range.
> >>>>
> >>>> I haven’t compared the noise with the ex laptop supply that I was
> using
> >>>> before, but I’d have to believe it’s cleaner. I don’t really have a
> way to
> >>>> check the oscillator’s before and after ADEV. My only other reference
> is an
> >>>> FE5680A, and I think the thunderbolt’s going to be far better at
> lower tau
> >>>> (where this all matters).
> >>>>
> >>>> I know also that ±2 mV is still one and perhaps two orders of
> magnitude
> >>>> higher than some have called for. But before I attempt to reduce the
> noise
> >>>> further, I’d like to know that there are real gains to be had. Would
> >>>> someone with a Thunderbolt and better output noise measuring
> wherewithal be
> >>>> willing to take a prototype and compare it with something that does
> have µV
> >>>> levels of noise and ripple so I can get an idea of what there is to
> gain?
> >>>> If you like, you can make such comparisons public - no secrets here.
> >>>>
> >>>>> On Aug 30, 2016, at 10:37 PM, Nick Sayer <nsayer at kfu.com
> <javascript:;>> wrote:
> >>>>>
> >>>>>
> >>>>>> On Aug 30, 2016, at 8:48 PM, Cube Central <cubecentral at gmail.com
> <javascript:;>>
> >>>> wrote:
> >>>>>>
> >>>>>> I would be interested, I think.  Planning ahead for if the one I
> have
> >>>> for my Thunderbolt fails, I guess.  Are there different models or
> would a
> >>>> photo of the input ports on mine be useful?
> >>>>>
> >>>>> Actually, what I had in mind is to just put a SIP4 header on the
> board
> >>>> for the output and people could wire the “last mile” themselves. The
> input
> >>>> is a 2.1mm barrel connector. You use whatever 15W 12VDC wall wart is
> handy
> >>>> and plug it right in.
> >>>>>
> >>>>> What it really amounts to is that you get +12 volts directly from the
> >>>> input, then there’s a buck converter to drop the +12 down to +5 and an
> >>>> inverter to generate -12 from the +12. Those 3 voltages, plus a
> ground go
> >>>> to the SIP4.
> >>>>>
> >>>>> So it’s just two switching power supplies to turn a +12 volt only
> >>>> supply into the three-way that the Thunderbolt wants.
> >>>>>
> >>>>> It’d be good for around 1500 mA @ 5V and around 50 mA @ -12 (the +12
> >>>> spec is whatever is left from the source supply’s power spec) - more
> than
> >>>> enough for a Thunderbolt. Probably enough for a hard disk or a
> smallish PC.
> >>>> _______________________________________________
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> >>>> and follow the instructions there.
> >>>>
> >>>
> >>>
> >>
> > <7912_1PLC.png><7912_AllanDeviation.png><7912_
> TempCo.png>_______________________________________________
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