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

Bob Camp kb8tq at n1k.org
Fri Oct 21 20:20:32 EDT 2016


> On Oct 21, 2016, at 7:44 PM, Scott Stobbe <scott.j.stobbe at gmail.com> 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. 


> On Fri, Oct 21, 2016 at 12:20 AM, Scott Stobbe <scott.j.stobbe at gmail.com>
> 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>
>> 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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> <7912_1PLC.png><7912_AllanDeviation.png><7912_TempCo.png>_______________________________________________
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