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

Bob Camp kb8tq at n1k.org
Fri Oct 21 08:13:38 EDT 2016


> On 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.

The EFC on the OCXO is less sensitive than 1 ppm / V. It is a slam dunk 
to say that the whole OCXO is less sensitive than the EFC by a good margin ….


> 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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