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.
The MT34TL/AS11D(xx) datasheet is available online so it seems very do- able - just considering all the pitfalls - issues/ obvious things I missed ;)
Any input/advice is much appreciated.
-Matt
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
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
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@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.
* * * | | *+--*---+---|---|---+---*---+---|---|---+---*-- …
* | | * * *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 -
Matthew Quirke
-
Müller, Marcus (CEL) -
Sylvain Munaut