Re: [time-nuts] LT3042, etc. Re: HP E1938A schematics.
Hi All capacitors have series inductance as well as resistance. As you go past about 100 KHz or so, this gets to be very noticeable. Exact details will always vary with capacitor type and size. Modern switchers (just like modern logic gates) run ever faster edge rates. The faster the switch, the more high frequency energy in the edge. A switcher running at 2 MHz and doing 90% efficiency will probably need a switching speed faster than 10 ns. This all just keeps pushing the PSRR requirements up and up in frequency. Bob > On Jul 9, 2019, at 1:30 AM, Hal Murray wrote: > > >>> Not only are they low noise, but they have spectacularly good HF >>> rejection across the regulator up to 10s of MHz. > >> In the 5071A, I wanted high bandwidth PSRR and stumbled across a designer's >> manual (HP internal document) for the MMS Modular Measurement System. They >> described a regulator with a common base pass transistor and an op amp, with >> a bandwidth approaching 1 MHz. > > I'm used to thinking that the bypass caps on the board will take care of high > frequencies. > > What's the advantage of a PSRR in the MHz range? Is it as simple as reducing > the number and size of the caps needed? > > In this area, is there a significant difference between analog and digital > sections? > > > -- > These are my opinions. I hate spam. > > > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] LT3042, etc. Re: HP E1938A schematics.
On 7/8/19 10:30 PM, Hal Murray wrote: Not only are they low noise, but they have spectacularly good HF rejection across the regulator up to 10s of MHz. In the 5071A, I wanted high bandwidth PSRR and stumbled across a designer's manual (HP internal document) for the MMS Modular Measurement System. They described a regulator with a common base pass transistor and an op amp, with a bandwidth approaching 1 MHz. I'm used to thinking that the bypass caps on the board will take care of high frequencies. They're actually low pass filters - it's an RC or LC filter, with either an explicit R or L, or just the parasitic R and L from the wiring. When you need, say, 100 dB rejection, you can try and build a couple 60 dB Low pass filters (typically some sort of C L C pi network) in series, or you can use a smaller filter followed by an LDO. The filter approach has voltage drop that varies with the load current. If you're feeding something like a MMIC amplifier (where the supply feeds the drain/collector of a transistor and has essentially Zero PSRR), you want a stable voltage because that sets the gain and P1dB - a series RC/LC filter is a problem. Essentially, the regulator provides a very low source impedance over a wide band What's the advantage of a PSRR in the MHz range? Is it as simple as reducing the number and size of the caps needed? and getting rid of variable voltage drop in the filter. In this area, is there a significant difference between analog and digital sections? I would think so.. in digital circuitry, one typically isn't as concerned about quiet supplies per se, except as it might affect the thresholds, but you are concerned about the source impedance (ground bounce and things like that) For analog circuits, you need really quiet bias supplies, since a lot of amplifiers have terrible inherent PSRR - consider a common emitter circuit - noise on the base bias or collector bias basically goes right to the output, perhaps with gain. The new LDO for negative voltages is very attractive for FET circuits that need negative bias on the gate. We designed and built a small HF receiver for a space application a couple years ago (2017) that has a noise figure <3dB from 5-30 MHz - We wanted to measure the galactic background noise, so the design requirement was "receiver noise < galactic background with 6 meter long dipole". The way we got there from a noisy 12V battery bus, and with a big FPGA drawing watts, and fast ADCS, was CLC filters going off the power supply board with the buck converters, CLC filters going on the downstream board, followed by a LT3042 to the RF circuits. And steel shielding cans. We did fiddle around a bit with the intermediate regulator voltages from the buck converters to make sure we could accommodate the drops in the CLC filters - but that was changing a single resistor or two. BTW, boost converters are a lot noisier than buck converters, and buck/boost designs that tolerate a wide input voltage swing change their noise properties a LOT when transitioning from buck to boost. I don't know if we could have left any of that out - we didn't have time to do extensive experimentation, so there was definitely an air of belt+suspenders+staples+duct tape in the design. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] LT3042, etc. Re: HP E1938A schematics.
In message <20190709053037.2d244406...@ip-64-139-1-69.sjc.megapath.net>, Hal Mu rray writes: >What's the advantage of a PSRR in the MHz range? Is it as simple as reducing >the number and size of the caps needed? The caps have only ever acted as a low-pass filter, to move the noise down in the frequency range of the power-supply regulation. The digital switching noise happens in (pico- and nano-)Coloumbs, not in volts or amps, which means that as the supply voltage decreases, it becomes percentwise larger voltage noise. To deal with that passively you would need bigger caps with better high frequency performance, and lower ESR and more, expensive PCBs (more layers etc.) The trend is therefore to move the corner frequency of the passive low-pass filter higher, that allows you to use ceramic capacitors, instead you must increase the bandwidth of the final power-supply, typically a LDO, and move it physically closer to the load. On high-end kit, it is not uncommon to see a big chip surrounded by a ring of tiny LDO's spaced as little as a centimeter apart along its periphery. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] LT3042, etc. Re: HP E1938A schematics.
>> Not only are they low noise, but they have spectacularly good HF >> rejection across the regulator up to 10s of MHz. > In the 5071A, I wanted high bandwidth PSRR and stumbled across a designer's > manual (HP internal document) for the MMS Modular Measurement System. They > described a regulator with a common base pass transistor and an op amp, with > a bandwidth approaching 1 MHz. I'm used to thinking that the bypass caps on the board will take care of high frequencies. What's the advantage of a PSRR in the MHz range? Is it as simple as reducing the number and size of the caps needed? In this area, is there a significant difference between analog and digital sections? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] LT3042, etc. Re: HP E1938A schematics.
RRR . you are right about drop and noise.. For others not familiar with this behaviour, one thing when looking is PSRR etc on LDOs..take a good look at this value VERSUS dropout / headroom . Most devices are in the toilet when dropout is nigh... In my SDRs I (used) to run 2V around for 1.8V point of load LDO supplies. PSRR 100k went up alot when I bumped the supply rail to 2.2V... On 9/07/2019 10:44 AM, jimlux wrote: On 7/8/19 4:53 PM, Richard (Rick) Karlquist wrote: On 7/8/2019 11:09 AM, jimlux wrote: Not only are they low noise, but they have spectacularly good HF rejection across the regulator up to 10s of MHz. In the 5071A, I wanted high bandwidth PSRR and stumbled across a designer's manual (HP internal document) for the MMS Modular Measurement System. They described a regulator with a common base pass transistor and an op amp, with a bandwidth approaching 1 MHz. Quite state of the art for the time. I copied the design in several 5071A modules. We used it to regulate +5V from the Vicor modules down to ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] LT3042, etc. Re: HP E1938A schematics.
On 7/8/19 4:53 PM, Richard (Rick) Karlquist wrote: On 7/8/2019 11:09 AM, jimlux wrote: Not only are they low noise, but they have spectacularly good HF rejection across the regulator up to 10s of MHz. In the 5071A, I wanted high bandwidth PSRR and stumbled across a designer's manual (HP internal document) for the MMS Modular Measurement System. They described a regulator with a common base pass transistor and an op amp, with a bandwidth approaching 1 MHz. Quite state of the art for the time. I copied the design in several 5071A modules. We used it to regulate +5V from the Vicor modules down to +4.5V working voltage for 100 series ECL or analog. It worked quite well. Now I could replace it with an LT3065. Rick N6RK Barely - I've got bit in the past by coming too close to the LDO minimum voltage. 300mV for those parts, so as long as your +5 had decent regulation, and the ripple didn't drop below 4.8V, and you didn't draw more than 300 mA. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] LT3042, etc. Re: HP E1938A schematics.
On 7/8/2019 11:09 AM, jimlux wrote: Not only are they low noise, but they have spectacularly good HF rejection across the regulator up to 10s of MHz. In the 5071A, I wanted high bandwidth PSRR and stumbled across a designer's manual (HP internal document) for the MMS Modular Measurement System. They described a regulator with a common base pass transistor and an op amp, with a bandwidth approaching 1 MHz. Quite state of the art for the time. I copied the design in several 5071A modules. We used it to regulate +5V from the Vicor modules down to +4.5V working voltage for 100 series ECL or analog. It worked quite well. Now I could replace it with an LT3065. Rick N6RK ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
