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

[Scott Stobbe]

It all depends what the -12V rail is for, some have said it directly
references the EFC dac. I would hope an ocxo would have a better tuning
gain on its efc pin than supply pin but maybe that's not always true.

On Saturday, 22 October 2016, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> The +12 goes straight to the OCXO. All OCXO’s have a voltage sensitivity.
> That sensitivity is much higher than voltage sensitivity is much higher
> than
> what you see on the other two supply pins.
>
> Bob
>
> > On Oct 21, 2016, at 11:06 PM, Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:;>> wrote:
> >
> > 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 <javascript:;>>
> wrote:
> >
> >> Hi
> >>
> >>> On Oct 21, 2016, at 7:44 PM, Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:;>
> >> <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:;>
> >> <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:;>
> >> <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:;> <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:;>
> >> <javascript:;>> wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>>> On Aug 30, 2016, at 8:48 PM, Cube Central <cubecentral at gmail.com
> <javascript:;>
> >> <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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