Hi > On Oct 21, 2016, at 7:44 PM, Scott Stobbe <[email protected]> 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 <[email protected]> > 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 <[email protected]> >> 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 < >>> [email protected]> 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 <[email protected]> wrote: >>>>> >>>>> >>>>>> On Aug 30, 2016, at 8:48 PM, Cube Central <[email protected]> >>>> 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 -- [email protected] >>>> 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 -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
