[hpsdr] Demeter progress

[J.D. Bakker]

At 16:18 +0200 13-07-2007, Jeroen Bastemeijer wrote:
>Thank you for your E-mail. I do agree with you that dI/dt is a factor in
>the noise generation. However, it is just a part of the noise-generation
>problem and not the whole problem as you put it. In the mail I was not
>talking about the frequency of the waveform but about the waveform
>itself. The duty cycle (the ratio of the high-time and period of a pulse
>signal) strongly determines the frequency spectrum of a pulse signal. By
>doing Fourier analysis on these waveforms one can clearly see the
>influence of a high dI/dt (or dU/dt) AND the influence of the duty-cycle.
>The duty-cycle is parameter which can be influenced, within boundaries
>off course, by adjusting the power-handling-capabillity of the SMPS. An
>overrated SMPS will work with lower duty-cycles compared to a SMPS
>wihich works at almost the top of its power-handling.
>Filtering of the SMPS is necessary, but when we can minimize the amount
>of "noise" generated, the filter can be easier to construct.

In theory, when looking at the Fourier series of square/rect waves ? Sure.

In practice I've never seen a PWM SMPS where there was a significant 
difference in EMI/RFI with a load varying between, say, 15% and 100%. 
The reason that some switchers *will* have higher noise at low loads 
(below the 15% I mentioned) is that they will switch into pulse 
skipping/PFM mode to save power.

As others have mentioned before, the bulk of a switcher's noise is 
dI/dt or switch-node ringing related. Both of those have enough 
energy in the several-MHz range that every trace/loop becomes an 
antenna, and most LF filtering will be ineffective.

For example, take a 50kHz switcher followed by a 25kHz LC filter and 
an LM1117 LDO post regulator. That combination will filter the 
overtones of the switching out so far as to be practically irrelevant 
(>60dB suppression), yet dI/dt and ringing noise will practically 
pass straight through.

>                                              I have an old switching
>power supply based on the well known UC3842 which will be the DUT.

That's a pretty old beastie, not exactly famous for being quiet.

Good luck with your tests,

JDB.
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