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

[Scott Stobbe]

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>
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> 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> wrote:
>> >
>> >
>> >> On Aug 30, 2016, at 8:48 PM, Cube Central <cubecentral at gmail.com>
>> 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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>
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