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. >> > >
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