Hi > On Oct 21, 2016, at 12:20 AM, Scott Stobbe <scott.j.sto...@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 …. Bob > > 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> > 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> 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> wrote: >>>> >>>> >>>>> On Aug 30, 2016, at 8:48 PM, Cube Central <cubecent...@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. >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to https://www.febo.com/cgi-bin/m >>> ailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> >> > <7912_0p1_10Hz_60dB_preamp.png>_______________________________________________ > 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. _______________________________________________ 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.