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

Scott Stobbe scott.j.stobbe at gmail.com
Sat Oct 22 16:15:00 EDT 2016


I think we are coming up to the noise floor here. We have identified a
potential signal A from a 7912 (likely similar to a 7812, no guarentee).
The remaining question is if you apply signal A to the power pin of a
thunderbolt what is signal B added to the output. We know where we want
signal B to be, below the noise. Knowing the PSSR for each of the power
inputs would be one way to answer this question.

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

> Hi
>
> There is an *enormous* difference between regulation and it’s impact on
> stability (which is what this drifted off into) and PSRR and it’s impact on
> phase noise (which got lost at the tread moved on). PSRR does indeed
> matter, but a fairly simple linear regulator (or pair of cheap ones or
> coil plus
> pair) can take care of that.
>
> The EFC reference in the TBolt comes from the TBolt board and not from the
> OCXO.
> It is not in any way connected to the -12 V supply line.
>
> Bob
>
>
>
> > On Oct 22, 2016, at 12:41 PM, Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:;>> wrote:
> >
> > Bob, this is good data and insights thanks for taking the time to share.
> > Ultimately the question Nick is hoping to answer is what is the point of
> > diminishing returns for voltage regulation. I think there are plenty of
> > folks on this list that have shared data suggesting a switching regulator
> > which meets these specifications:
> >
> > Table 3-2 Power Specifications (Board-only)
> > +12v +/-10% max current 750 mA
> > +5v +/-5% max current 400 mA
> > -12v +/-10% max current 10 mA
> > Ripple
> > +5v 50 mV peak-to-peak, 15 mVrms
> > +/-12v 75 mV peak-to-peak, 20 mVrms
> >
> > definitely has a degrading impact on the 10MHz phase noise.
> >
> > I would happily agree that PSRR of an opamp buffering the EFC line is not
> > going to have a significant impact on performance with a jellybean LDO.
> >
> > Do you know if the +- 5V reference is derived on board or is it from an
> EFC
> > reference provided by the ocxo?
> >
> > On Sat, Oct 22, 2016 at 11:31 AM, Bob Camp <kb8tq at n1k.org <javascript:;>>
> wrote:
> >
> >> Hi
> >>
> >> The -12 V line is not the reference for the EFC. It *is* one supply into
> >> the op amp circuit
> >> that drives the EFC. Since the EFC is +/- 5V, there must be both a
> >> positive and
> >> a negative supply into the driver circuit for full output swing. The -12
> >> can be anything between about
> >> -13 and -7 without significantly impacting the function of the device.
> In
> >> fact, people
> >> with EFC voltages that are slightly positive have accidentally proven
> that
> >> the
> >> TBolt will work (lock up and function correctly) with zero volts on the
> >> -12V line.
> >>
> >> Bob
> >>
> >>> On Oct 22, 2016, at 10:24 AM, Scott Stobbe <scott.j.stobbe at gmail.com
> <javascript:;>>
> >> wrote:
> >>>
> >>> 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 <javascript:;>>
> 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:;>
> >>>> <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:;>
> <javascript:;>>
> >>>> wrote:
> >>>>>
> >>>>>> Hi
> >>>>>>
> >>>>>>> On Oct 21, 2016, at 7:44 PM, Scott Stobbe <
> scott.j.stobbe at gmail.com <javascript:;>
> >>>> <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:;>
> >>>>>> <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:;>
> >>>>>> <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:;>
> <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:;>
> >>>>>> <javascript:;>> wrote:
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>>> On Aug 30, 2016, at 8:48 PM, Cube Central <
> >> cubecentral at gmail.com <javascript:;>
> >>>> <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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> <javascript:;>
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> >>>>>>>>>> and follow the instructions there.
> >>>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>
> >>>>>>> <7912_1PLC.png><7912_AllanDeviation.png><7912_
> >>>>>> TempCo.png>_______________________________________________
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