[hpsdr] Demeter progress

[Hollingsworth, Tommy]

 Jeroen:
I think you need to do a little more research into EMI and signal integrity.
The noise/EMI generation is not really driven by the frequency of the
waveform.  It is driven by the rate of rise of the leading edge and trailing
edge alone.  Think dI/dT and dV/dT.  Putting an analog regulator after the
switching regulator does not necessarily do anything to filter out the EMI
generated by primarily the dI/dT in the switch(transistor).  You can be much
more effective with the appropriate use of a pi filter with very carefully
selected ferrites.  The ferrites used in the filter have to be chosen for
the exact frequency or frequency range of the switching regulator.  This in
combination with a very meticulous board layout to minimize the loop size of
the paths carrying the highest dI/dT is needed to keep noise and EMI down
low.  

The use of a linear regulator after a well designed switching supply
certainly does not hurt, but do not think that alone will give you low noise
and or low EMI.  

As to your question about how does load impact the spectrum:  Map the dI/dT
of the design at a nominal level.   Apply the maximum load and map the
dI/dT.  You will find that the spectrum may or may not shift with load)
depends on your topology.  With the higher load the dI/dT amplitude will
rise dramatically and hence radiated and conducted noise will rise the same
way.  As in a radio it is important to watch the frequency of the radiated
signal going into an antenna and the current going into the antenna (both
real and complex).

Your last paragraph is wrong.  Keep reading, it will come to you in time.
Build a simple switching regulator and perform the test you just described.
Keep increasing the load until it self destructs.  It will not become more
stable as load increases until you actually apply a short.  Stability
usually follows shortly after that. :)  To get a real crude idea of radiated
noise or EMI, take the integral of the worst case dI/dT.  This is easily
done graphically.  You need the peak current and the period.  Yes you might
think the spectrum was worse at lower frequencies; but no, the integral was
close to zero since the amplitude was very small.  


Good luck with your effort.
73 Tommy Hollingsworth KV5T


-----Original Message-----
From: hpsdr-bounces at hpsdr.org [mailto:hpsdr-bounces at hpsdr.org] On Behalf Of
Jeroen Bastemeijer
Sent: Thursday, July 12, 2007 7:23 AM
To: HPSDR
Subject: [hpsdr] Demeter progress

***** High Performance Software Defined Radio Discussion List *****

Dear All,

There are a lot of thoughts around about the implementation of Demeter (just
read all the previous E-mails). As the whole HPSDR-project is based on
modules, Demeter should be able to adapt (alomost) to each individual HPSDR
configuration.
A modular power supply means a power supply with certain "options" (like
e.g. a status display or a battery charger) but also with a variable power.
For one configuartion 5V @ 1Amp might be sufficient while another
configuration might need 5V @ 3.5 Amps. Building a power supply with 5V @ 20
Amps will certainly be sufficient for all kind of configurations...... but
I'm sure this is not the solution we are looking for.
To make a power supply scaleable I have been looking around on the internet,
and giving my creative thoughts some room...

There are a few things which would fit in the Demeter concept:
1. Using a PIC for the SMPS controller (you can check the Microchip website
for AN216) This will certainly add the High Performance to Demeter.
2. Build a multiphase controller. Just google for "multiphase SMPS" and you
will find loads of information. This solution will give some extra
flexibility: People who need less power, can ommit a couple of "phases" 
as one or two phases can provide enough power for their solution. People who
need more power can extend the SMPS to four phases. As this technique is
rather new, I hope we don't get any patent-issues when we apply it for the
HPSDR-project.
3. Another nice technique is 'soft-switching'. This will limit the influence
of big switching spikes.

A general question I'm trying to get an answer on is the following: How does
the output noise spectrum (please note: not the level of noise but the way
it is distributed through the spectrum) of  a SMPS depend on the load of the
SMPS?

The thought on which this question is based is the following: Looking at the
spectrum of a 50% square wave you only see odd harmonics. When the
duty-cycle starts to differ from 50% more even harmonics start to appear.
The regulation principle of some types of switchers depends on a PWM
switching signal. This means that they will produce more noise at low loads
(duty cycle < 50%), the lowest noise at half the power rating (duty cycle
50%) and more noise for higher power levels. If this is true, it would be
worth to try to match the power supply as close as possible to the power
which is required for a certain HPSDR-configuration.

That's all folks, for now at least,

73 Jeroen PE1RGE

--
Ing. Jeroen Bastemeijer

Delft University of Technology
Department of Electrical Engineering
Electronic Instrumentation Laboratory
Mekelweg 4, Room 13.090
2628 CD Delft
The Netherlands

Phone: +31.15.27.86542
Fax: +31.15.27.85755
E-mail: J.Bastemeijer at TUDelft.nl
GPS: Lat N52.00002 Lon E4.37157 Alt 46.2m

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