Discerning FL2K devices with LDOs versus switching regulators

Mac A. Cody maccody at att.net
Sat May 12 03:34:56 UTC 2018


Marcus,

Thank you for your prompt, detailed, informative, and (even)
entertaining reply.  Many of the things you mentioned I am
already aware of, but thisinformation 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




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