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 <kb...@n1k.org> wrote: > Hi > > > On Oct 21, 2016, at 7:44 PM, Scott Stobbe <scott.j.sto...@gmail.com > <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.sto...@gmail.com > <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.sto...@gmail.com > <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@febo.com <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 <nsa...@kfu.com > <javascript:;>> wrote: > >>>>> > >>>>> > >>>>>> On Aug 30, 2016, at 8:48 PM, Cube Central <cubecent...@gmail.com > <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. > >>>> _______________________________________________ > >>>> time-nuts mailing list -- time-nuts@febo.com <javascript:;> > >>>> To unsubscribe, go to https://www.febo.com/cgi-bin/m > >>>> ailman/listinfo/time-nuts > >>>> and follow the instructions there. > >>>> > >>> > >>> > >> > > <7912_1PLC.png><7912_AllanDeviation.png><7912_ > TempCo.png>_______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com <javascript:;> > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com <javascript:;> > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.