FL2K replace MT34TL/AS11D buck chip with LDO

Matthew Quirke matthew.quirke at gmail.com
Fri Feb 8 18:53:43 UTC 2019


Hi 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
>


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