<div dir="ltr"><div class="gmail_default" style="font-size:small">
<p style="margin-bottom:0cm;line-height:100%">Hi Marcus, thanks
for your input,
</p>
<p style="margin-bottom:0cm;line-height:100%">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 ) </p><p style="margin-bottom:0cm;line-height:100%"><br></p><p style="margin-bottom:0cm;line-height:100%">- As Steve
sugged in his video - here @ [22min.22s] for clarification.<br></p>
<p style="margin-bottom:0cm;line-height:100%"><a href="https://www.youtube.com/watch?v=VRvLVjLQSaw&t=1366s">https://www.youtube.com/watch?v=VRvLVjLQSaw&t=1366s</a></p>
<p style="margin-bottom:0cm;line-height:100%"><br>
</p>
<p style="margin-bottom:0cm;line-height:100%">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. ;)</p></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, 9 Feb 2019 at 01:02, Müller, Marcus (CEL) <<a href="mailto:mueller@kit.edu">mueller@kit.edu</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">I'd like to expand on that: the fl2k is just a DAC without<br>
reconstruction filtering. Like every DAC without a filter, harmonics<br>
mathematically must exist. That's not a bug or something due to<br>
imperfect design – that's just the theory behind DAC; especially: has<br>
nothing to do with the power supply :)<br>
<br>
In essence, you can think of a (perfect¹) DAC as something that<br>
produces a series of extremely sharp impulses of the correct amplitude<br>
at sampling rate, with zero in between. So, let's imagine these<br>
amplitudes would form a nice cosine of say 20 MHz if you "connected the<br>
dots" – but you don't do that connecting electrically just yet.<br>
<br>
* *<br>
* | | *<br>
+--*---+---|---|---+---*---+---|---|---+---*-- …<br>
* * | | *<br>
* *<br>
^ ^<br>
|____________ T=1/(20MHz) _____________|<br>
<br>
When you think of the spectrum of that, I'd say we both agree that<br>
there's power at 20 MHz, so obviously the PSD must have a peak at 20<br>
MHz. But hey, if this was *actually* a sine of 20 MHz, then there would<br>
be "smooth" connections between the signal impulses (the "*" in my<br>
ASCII drawing). There are not; so, there must also be a signal<br>
component which "suppresses" the 20 MHz sine in between. Whilst there's<br>
fine math theory which lets us derive this directly, I simply imagine<br>
there being 1-0-1-0… wave that I multiply with the "pure" sine to<br>
arrive there – first one of twice the sampling rate, to "zero out" the<br>
middle between amplitude instants, then another one with higher<br>
multiples of the sampling rate and so on. <br>
<br>
As someone dealing with HF, you probably know what happens when I<br>
multiply one sine with a wave of a different frequency: You mix one<br>
tone by the other. And that's exactly what you'll see at the output of<br>
*every* DAC: the spectrum you produced in the baseband, and a<br>
repetition every sampling rate distance. These repetitions are<br>
fittingly called /images/, and you often (and you, especially, in this<br>
case) suppress them with a simple low-pass filter. The effect of that<br>
/reconstruction/ filter is that it actually smooths out these impulses<br>
– it "connects the dots"!<br>
<br>
Good thing is that the sampling rate of the fl2k is plenty high enough<br>
to cover your HF band at once – you don't need to rely on this imaging<br>
to get to any portion of that band. In case you haven't seen that, the<br>
fact that imaging exists is even useful: By selecting an image that is<br>
far above the baseband, you can, with a DAC that has ~160 MHz sampling<br>
rate, even generate signals in the 900 MHz GSM band, for example,<br>
without any mixer.<br>
<br>
So, what you need is a filter with a cutoff frequency above HF and<br>
below half your sampling rate.<br>
<br>
By the way, power supply effects *can* lead to harmonics, but usually<br>
in amplifiers: when your power supply is insufficient for your<br>
amplifier, then that amplifier can't reach high amplitudes as fast as<br>
it should – that's a nonlinearity. Nonlinearities lead to mixing, i.e.<br>
harmonics.<br>
<br>
My suspicion is that you meant "spurs", not "harmonics", caused by the<br>
buck converter. You're referring to<br>
<a href="http://people.osmocom.org/steve-m/fl2k_slides/osmo-fl2k.html#(17)" rel="noreferrer" target="_blank">http://people.osmocom.org/steve-m/fl2k_slides/osmo-fl2k.html#(17)</a> ,right? <br>
Well, in fact, these seem to be harmonics, but my best guess it's<br>
they're harmonics of the boost switching clock, mixed with the signal<br>
of interest, not of your signal itself. We typically refer to tones<br>
that aren't harmonics of the signal of interest but are generated<br>
within a device as spurs. Anyway, that's just semantics, in the end.<br>
You want to get rid of them; first, make sure you actually need to do<br>
that - they are present in 1 MHz steps, as it seems on that slide<br>
above, so chances are that if you choose your rates cleverly, you can<br>
avoid seeing them in-band at all.<br>
<br>
Before replacing any complete buck converter, I'd try to uppen its<br>
output smoothening: find the large output caps after the inductors, and<br>
parallel/on top solder another 10µF; follow the positive voltage trace<br>
and solder a good 10nF onto where that connects to the fl2k; connect a<br>
very solid wire to ground to the other end. Maybe that's already<br>
sufficient at surpressing things. <br>
<br>
Best regards,<br>
Marcus<br>
<br>
-----------------------------------------------------------------------<br>
¹ "real" DACs often work slightly different, e.g. do zero-order hold or<br>
such, but that doesn't actually matter for the principle of images –<br>
what matters is that the "difference" between the "pure" smooth signal<br>
and the DAC's output has frequency components at multiples of the<br>
sampling rate.<br>
<br>
On Fri, 2019-02-08 at 07:59 +0100, Sylvain Munaut wrote:<br>
> Hi,<br>
> <br>
> > Has anyone tried replacing the MT34TL/AS11D buck converter/supply<br>
> > chip with a LDO regulator as Steve Markgraf suggested - my goal is to<br>
> > remove the harmonics, for HF tx.<br>
> <br>
> Remove the harmonics ? The fl2k will always have harmonics, that's<br>
> just the way it is. Only way to get rid of theses is to post-filter.<br>
> What a clean supply does is remove the spurs.<br>
> <br>
> Cheers,<br>
> <br>
> Sylvain<br>
</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div dir="ltr"><div style="text-align:left"><p style="margin-bottom:0cm"></p><div><b style="color:rgb(0,69,134)"><font face="arial, helvetica, sans-serif">Matthew Quirke</font></b></div><font face="arial, helvetica, sans-serif" style="text-align:center"><div style="text-align:left"><span style="letter-spacing:0.2px">New Zealand</span><br></div></font><p style="text-align:center"></p><p style="margin-bottom:0cm"><font face="arial, helvetica, sans-serif">contact me:<br><a href="mailto:Matthew.quirke@gmail.com" target="_blank"><font color="#004586"><font>Matthew.quirke@gmail.com</font></font></a></font></p><p style="margin-bottom:0cm"><font face="arial, helvetica, sans-serif"><font color="#000000"><font>mobile:+64 022 185 77 22<br></font></font>Skype-me: matthew_quirke</font></p><p style="margin-bottom:0cm"></p><p style="margin-bottom:0cm"><span style="font-family:arial,helvetica,sans-serif">This message may be protected by a GnuPGP digital signature. </span><span style="font-family:arial,helvetica,sans-serif">For secure email communication please use my public PGPkey <br></span><font color="#000000" style="font-family:arial,helvetica,sans-serif"><font>(matthew.quirke </font></font><a href="http://keyserver.ubuntu.com/" style="font-family:arial,helvetica,sans-serif" target="_blank"><font color="#004586"><font>http://keyserver.ubuntu.com</font></font></a><font color="#000000" style="font-family:arial,helvetica,sans-serif"><font>)<br></font></font></p><p style="margin-bottom:0cm"></p><p></p></div></div></div></div></div></div>