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Mac A. Cody maccody at att.netMarcus, Thank you for your prompt, detailed, informative, and (even) entertaining reply. Many of the things you mentioned I am already aware of, but this information is definitely valuable as a reminder and for those unaware of all of the issues. You put in far more time into your reply than I would have ever hoped for and it is greatly appreciated! I'm looking at using the FL2K for amateur radio HF band applications. I want to employ a sampling rate of 120 Msps and a low-pass filter with a roll-off just above 30 MHz. I'm considering using an Odroid XU4 as an embedded processor, as it has significant processing capability and it has two USB 3.0 hosts. Of course, a suitably enhanced RTL-SDR device would be used for the SDR receiver component. Alternatively, an SDRPlay RSP1A could be used. Regards, Mac On 05/10/2018 03:48 AM, Müller, Marcus (CEL) wrote: > Hi Mac, > > so first of all: any spur is only a problem if it ends up in your > signal. Since we're clearly talking about devices that you can't use > for operation with an antenna withou very much filtering: check whether > you actually get a problem first. To be completely honest, the whole > LDO vs. SMPS discussion often bares technical background, as you'll > find SMPS in high-end radio receiver devices just as well. It's all > about /designing/ your thing to be low noise, not about the "use an LDO > instead of a switcher". > > Now, Steve has offered nice figures about the spurs there, so these > might actually be linked to the switcher. However, the method seems to > be to first measure, then link cause to it; not the other way around. > I'd argue that the device with the many spurs that, actually do look > like one rectangular wave modulated another rectangular wave, was > simply badly designed, probably with underdimensioned means of > eliminating cross-talk between the two switchers (no idea how they > relate). What confuses me is that these spurs roughly fall into a 3 MHz > grid – and that's usually a bit on the high end for switching > frequencies. > > Another device with a switcher *might* be nicely filtered and work > perfectly well. I agree, adding a switching regulater definitely adds a > source of noise, but please don't assume that cheaply designed LDO > systems are superior in signal quality¹; there's modern switch mode > supplies that actually use spread-spectrum methods to spread out the > energy they leak onto many frequencies², and others that you can > synchronize e.g. to sampling clocks so that noise at least aligns and > can be filtered out more easily. > > The point I'm trying to make is that if these spurs are a problem to > you (and I can heartily to figure on slide 17 worrying you), then > you'll want to have spur measurements at different sampling rates at > exactly your USB bus – in the end, the noise of a SMPS very much > depends on how hard it is at work, and a stable input supply and high > output current might be nicer than a dropping input and a current draw > so small that forces the SMPS into discontinuous current mode. > > Regarding spotting: > ------------------- > > Switch mode supplies generally can be found by looking for (large-ish) > inductors close to (large-ish) diodes, typically close to either a > converter IC or in higher-current applications close to a (large-ish) > discrete transistor. Do an image search for "SMD power inductor", and > you'll see how these tend to look like. > > > Regarding remedies: > ------------------- > > Filters, filters, filters³! > You need to select the right Nyquist zone, anyway. So, pick a sampling > rate range that works out for that; shifting your signal in digital > domain so that it ends up where you want it after being shifted by the > sample clock N times allows you to have some leeway there. Then, use > whatever remaining degrees of freedom you have to pick a rate that is > at a supply spur – and filter that out. Whether this is an option at > all of course depends on the RF bandwidth you need. > > Replacing the power supply on-board: > I'm willing to say "it's possible", but I'd also say "at a time > investment higher than simply buying a handful of candidates and simply > sticking with one that works". > Supplies typically have to be electrically well-coupled to the ground > and supply lines, so if you externalize these, you'd replace the > original output stage of the on-board SMPS with larger capacitors, but > these typically have worse RF interference suppression properties, so > you'd add smaller capacitors, but now you have a system with capacitors > of different sizes and internal resistances and inherently some > inductive characteristics of whatever connects the external supply to > these connectors – you can certainly simply build that, and it's not > that unlikely it'd work, especially if you overdimension everything a > bit, but I wouldn't know how to predictably make a "first trial works" > device. > Note that switch mode ICs for these voltages and currents aren't > necessarily solder-friendly⁴ . Rule of thumb: The smaller the package, > the higher the switching frequency⁵ – and as noted above, 3 MHz would > be at the higher end of the spectrum of switching frequencies⁶, but > that's likely because higher switching frequency also makes the > necessary inductance smaller, and hence, the inductor cheaper. > > What I would do > =============== > > Compare a handful of dongles. Because: > a) They're cheap, and time is sparse, > b) can't be that bad to have spare ones lying around, for operation > away from the spurs, or to give to friends who want to try that, or to > honestly resell as tested to work with osmo-fl2k but replaced with a > lower-noise one, > c) to verify hypotheses on how to fix things, without risking to fry > one of the "good ones", and > c) if you can figure out how to improving the best one, maybe it > becomes easy to improve the others, too. Maybe it's easier to observe > an improvement in the ones that are bad. > > Go and measure. That means that I'd both add appropriate output filters > for both the Nyquist zone I want, and measure after that (e.g. using an > RTL dongle, whose spurs I at least know), as well as trying to figure > out where exactly the spurs come from – are they really on the signal > lines, or are they radiated into my measurement by the shield conductor > of the VGA port? When I probe around with an oscilloscope, on which > lines do I see exactly these frequencies I observed? > > Then, improve and adapt. If things are actually radiated by the board, > proper shielding might be the simplest method to improve the situation. > Else, go for easy things like soldering another (better, as in lower > ESR, higher capacity?) capacitor onto the decoupling capacitors or > output smoothing caps on-board⁷ first. > > Best regards, > Marcus > > ======================================================================= > > ¹ You can underdampen these LDOs, just as well, or underdimension them: > linear supplies tend to be cheaper than SMPS for small loads, so the > fact that some manufacturers use SMPSes might point out that you'd need > a relatively beefy and fast LDO and thus expensive LDO to reliably > supply the current needed, and there's plenty that you can mess up when > you're designing an LDO system at the edge of cost efficiency > > ² Though that doesn't sound too desirable here > > ³ Imagine Ballmer going "developers!" on you here. > > ⁴ In highly integrated electronics, ICs with 6 pads in a package of > total size ~ 1 mm × 1.3 mm would be typical if you just need a small > step-down from 1.8 V to 1.2 V efficiently. > > ⁵ Because the higher the frequency, the less charge transfered per > cycle, the lower the switched current, the smaller the switching > transistor. > > ⁶ Please don't really infer that this means you get a chip scale > package – these VGA dongles were built with cost, not size, as primary > target, as you can see from the sparsely populated simple PCBs; you > don't use a high-end phone-building assembly line to build 5 € VGA > dongles, so you don't use <0.05 mm tolerance in placement parts. > > ⁷ Maybe don't take it this far: > https://twitter.com/LaF0rge/status/892872883164336128 > > On Wed, 2018-05-09 at 22:44 -0500, Mac A. Cody wrote: >> Greetings, >> >> In Steve Markgraf's slide presentation >> (http://people.osmocom.org/steve-m/fl2k_slides/osmo-fl2k.html), >> do slides 16 and 17 imply that some FL2K devices have LDO regulators >> while other >> using switching regulators? Obviously, the FL2K devices that have >> LDO >> regulators >> are preferred, due to fewer spurious RF emissions. How can one >> determine which >> FL2K devices have LDOs? Can an FL2K device be reworked to use LDO >> regulators? >> >> Thanks, >> >> Mac