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Matthew Quirke matthew.quirke at gmail.comHi Marcus, thanks for your input, Yes my bad? I was referring to these ‘spurs’ which seem to be modulation products AND harmonics of the switching frequency of the buck converter ( @ 1.1Mhz ) - As Steve sugged in his video - here @ [22min.22s] for clarification. https://www.youtube.com/watch?v=VRvLVjLQSaw&t=1366s The LDO approach appeals to me to get rid of the most noise and I have a few suitable LDO’s sitting on my desk looking very bored. ;) On Sat, 9 Feb 2019 at 01:02, Müller, Marcus (CEL) <mueller at kit.edu> wrote: > I'd like to expand on that: the fl2k is just a DAC without > reconstruction filtering. Like every DAC without a filter, harmonics > mathematically must exist. That's not a bug or something due to > imperfect design – that's just the theory behind DAC; especially: has > nothing to do with the power supply :) > > In essence, you can think of a (perfect¹) DAC as something that > produces a series of extremely sharp impulses of the correct amplitude > at sampling rate, with zero in between. So, let's imagine these > amplitudes would form a nice cosine of say 20 MHz if you "connected the > dots" – but you don't do that connecting electrically just yet. > > * * > * | | * > +--*---+---|---|---+---*---+---|---|---+---*-- … > * * | | * > * * > ^ ^ > |____________ T=1/(20MHz) _____________| > > When you think of the spectrum of that, I'd say we both agree that > there's power at 20 MHz, so obviously the PSD must have a peak at 20 > MHz. But hey, if this was *actually* a sine of 20 MHz, then there would > be "smooth" connections between the signal impulses (the "*" in my > ASCII drawing). There are not; so, there must also be a signal > component which "suppresses" the 20 MHz sine in between. Whilst there's > fine math theory which lets us derive this directly, I simply imagine > there being 1-0-1-0… wave that I multiply with the "pure" sine to > arrive there – first one of twice the sampling rate, to "zero out" the > middle between amplitude instants, then another one with higher > multiples of the sampling rate and so on. > > As someone dealing with HF, you probably know what happens when I > multiply one sine with a wave of a different frequency: You mix one > tone by the other. And that's exactly what you'll see at the output of > *every* DAC: the spectrum you produced in the baseband, and a > repetition every sampling rate distance. These repetitions are > fittingly called /images/, and you often (and you, especially, in this > case) suppress them with a simple low-pass filter. The effect of that > /reconstruction/ filter is that it actually smooths out these impulses > – it "connects the dots"! > > Good thing is that the sampling rate of the fl2k is plenty high enough > to cover your HF band at once – you don't need to rely on this imaging > to get to any portion of that band. In case you haven't seen that, the > fact that imaging exists is even useful: By selecting an image that is > far above the baseband, you can, with a DAC that has ~160 MHz sampling > rate, even generate signals in the 900 MHz GSM band, for example, > without any mixer. > > So, what you need is a filter with a cutoff frequency above HF and > below half your sampling rate. > > By the way, power supply effects *can* lead to harmonics, but usually > in amplifiers: when your power supply is insufficient for your > amplifier, then that amplifier can't reach high amplitudes as fast as > it should – that's a nonlinearity. Nonlinearities lead to mixing, i.e. > harmonics. > > My suspicion is that you meant "spurs", not "harmonics", caused by the > buck converter. You're referring to > http://people.osmocom.org/steve-m/fl2k_slides/osmo-fl2k.html#(17) ,right? > Well, in fact, these seem to be harmonics, but my best guess it's > they're harmonics of the boost switching clock, mixed with the signal > of interest, not of your signal itself. We typically refer to tones > that aren't harmonics of the signal of interest but are generated > within a device as spurs. Anyway, that's just semantics, in the end. > You want to get rid of them; first, make sure you actually need to do > that - they are present in 1 MHz steps, as it seems on that slide > above, so chances are that if you choose your rates cleverly, you can > avoid seeing them in-band at all. > > Before replacing any complete buck converter, I'd try to uppen its > output smoothening: find the large output caps after the inductors, and > parallel/on top solder another 10µF; follow the positive voltage trace > and solder a good 10nF onto where that connects to the fl2k; connect a > very solid wire to ground to the other end. Maybe that's already > sufficient at surpressing things. > > Best regards, > Marcus > > ----------------------------------------------------------------------- > ¹ "real" DACs often work slightly different, e.g. do zero-order hold or > such, but that doesn't actually matter for the principle of images – > what matters is that the "difference" between the "pure" smooth signal > and the DAC's output has frequency components at multiples of the > sampling rate. > > On Fri, 2019-02-08 at 07:59 +0100, Sylvain Munaut wrote: > > Hi, > > > > > Has anyone tried replacing the MT34TL/AS11D buck converter/supply > > > chip with a LDO regulator as Steve Markgraf suggested - my goal is to > > > remove the harmonics, for HF tx. > > > > Remove the harmonics ? The fl2k will always have harmonics, that's > > just the way it is. Only way to get rid of theses is to post-filter. > > What a clean supply does is remove the spurs. > > > > Cheers, > > > > Sylvain > -- *Matthew Quirke* New Zealand contact me: Matthew.quirke at gmail.com mobile:+64 022 185 77 22 Skype-me: matthew_quirke This message may be protected by a GnuPGP digital signature. For secure email communication please use my public PGPkey (matthew.quirke http://keyserver.ubuntu.com) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.osmocom.org/pipermail/osmocom-sdr/attachments/20190209/928aea38/attachment.htm>