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

Nick Sayer nsayer at kfu.com
Sat Oct 22 16:45:06 EDT 2016 

At this point, I have three prototype boards that are functional as far as I can tell. Unfortunately I don’t have a better reference which would allow me to make comparisons.

If someone with a better reference could make a proper comparison, I’d be happy to send them a prototype.



> On Oct 22, 2016, at 1:15 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
> 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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto:kb8tq at n1k.org> <javascript:;>
>> <javascript:;>>
>>>>>> wrote:
>>>>>>> 
>>>>>>>> Hi
>>>>>>>> 
>>>>>>>>> On Oct 21, 2016, at 7:44 PM, Scott Stobbe <
>> scott.j.stobbe at gmail.com <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto:nsayer at kfu.com>
>> <javascript:;>
>>>>>> <javascript:;>
>>>>>>>> <javascript:;>> wrote:
>>>>>>>>>>>>> 
>>>>>>>>>>>>> 
>>>>>>>>>>>>>> On Aug 30, 2016, at 8:48 PM, Cube Central <
>>>> cubecentral at gmail.com <mailto: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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>>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>> <7912_1PLC.png><7912_AllanDeviation.png><7912_
>>>>>>>> TempCo.png>_______________________________________________
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