[SI-LIST] Re: SMPS
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: "Russel Hughes" <Russel.Hughes@xxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 10 Aug 2005 10:10:13 -0500
This is good -- thanks for the link -- but it can be difficult to
use a sniffer and get repeatable measurements when making changes=20
or doing tests. It is a great way to find emission peaks and get=20
some idea of where they are coming from. However if it just gets=20
high near your processor you don't know if input clock or SDRAM
strobe lines or ... is the cause.=20
So we make up a test set-up which can be "tied down" even taping
cables to benches so results are somewhat repeatable. Of course=20
we don't know what far field emissions results will turn out to be
but if we knock something down by 6dB or more things usually are=20
better at the test site ...=20
Curt
p.s. Keith's web site address may have been truncated in my first=20
message -- make sure to add lio.htm to the end of the link.=20
Curt McNamara. P.E.=20
Senior Electrical Engineer=20
Logic Product Development=20
411 Washington Ave. N Suite 101=20
Minneapolis, MN 55401=20
Tel 612-436-5178=20
Fax 612-672-0443=20
curtm@xxxxxxxxxxx=20
www.logicpd.com=20
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-----Original Message-----
From: Russel Hughes [mailto:Russel.Hughes@xxxxxxxxxxxxxxxxxxxx]=20
Sent: Wednesday, August 10, 2005 9:57 AM
To: Curt McNamara
Cc: si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: SMPS
Thanks for the links, looks good. We are using a homemade sniffer probe
with an analyser which is proving useful, saves about 10K USD on a
purpose built one.
see: http://www.qsl.net/we6w/projects/rf_probe.txt for construction
details=20
Regards
Russel
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Curt McNamara
Sent: 10 August 2005 15:43
To: Russel Hughes; David Greig; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: SMPS
This is not entirely conclusive since with changing load currents the
power supply can act differently. However, it is an indication to look
elsewhere for now.
If you have a method to observe emissions in your lab -- either by=3D20
directly connecting a spectrum analyzer to a cable or by using=3D20 an
antenna quite close to your product -- you can trace down your source.
One method we use is to probe candidate circuits -- you can use a=3D20
dental explorer to hit pins (watch for ESD!) or even a scope probe which
is not connected to anything. When you hit the "interesting" circuit
either the emissions will go down as you change the impedance of the
circuit or they will shift in frequency.
I can give more ideas but Keith Armstrong has put together a wondrous
collection of troubleshooting tips here:
http://www.emcia.org/Freeinformation/KeithArmstrong/KeithArmstrongPortfo
lio.htm
Curt
Curt McNamara. P.E.=3D20
Senior Electrical Engineer=3D20
Logic Product Development=3D20
411 Washington Ave. N Suite 101=3D20
Minneapolis, MN 55401=3D20
Tel 612-436-5178=3D20
Fax 612-672-0443=3D20
curtm@xxxxxxxxxxx=3D20
www.logicpd.com=3D20
This message (including any attachments) contains confidential
information intended for a specific individual and purpose, and is
protected by law. If you are not the intended recipient, you should
delete this message and are hereby notified that any disclosure,
copying, or distribution of this message, or the taking of any action
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Russel Hughes
Sent: Wednesday, August 10, 2005 9:34 AM
To: David Greig; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: SMPS
Hi,
Yes they are synchronous buck, the datasheet is at:
http://focus.ti.com/docs/prod/folders/print/tps54350.html . I have run
the PSU with resistive loads and we get no emissions at that frequency,
so it looks like it is not the SMPS. Thanks to all that helped, must try
pay you back!.
Best regards
Russel
-----Original Message-----
From: David Greig [mailto:david@xxxxxxxxxxxxxx]
Sent: 09 August 2005 17:18
To: Russel Hughes
Subject: RE: [SI-LIST] Re: SMPS
One other thought, you will see inductor mosfet capacitance ringing if
your in discontinuous mode. I still don't recognize the numbers. Are
they synchronous buck? Another LC to watch for is on the input filter
side, more so with ceramic caps. Very easy to have a few nH and several
uF or nF and uH loops that are not damped. Even if all else damped, the
gate drive currents, di/dt could attribute to your 63Meg on the
reference plane.
Best Regards
=3D3D20
David Greig
______________________________
GigaDyne Ltd
http://www.gigadyne.co.uk
______________________________
-----Original Message-----
From: Russel Hughes [mailto:Russel.Hughes@xxxxxxxxxxxxxxxxxxxx]=3D3D20
Sent: 09 August 2005 15:58
To: David Greig
Subject: RE: [SI-LIST] Re: SMPS
Hi,
Thanks for the hints. The devices are made by Texas Instruments.
Best regards
Russel
-----Original Message-----
From: David Greig [mailto:david@xxxxxxxxxxxxxx]
Sent: 09 August 2005 15:48
To: Russel Hughes
Subject: RE: [SI-LIST] Re: SMPS
If the 63Meg occurs in bursts can you correlate this to the mosfet gate
drive signals? Could be caused by inductance in gate drive loop and
millar capacitance, potentially also cross conduction related to this or
occurring independently. Also, but less likely, SRF of the inductor.
Don't recognise those part numbers, who makes them?
Best Regards
=3D3D20
David Greig
______________________________
GigaDyne Ltd
http://www.gigadyne.co.uk
______________________________
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Russel Hughes
Sent: 09 August 2005 15:27
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: SMPS
Hi,
We are having problems identifying a 63MHz spurious signal. All
other spurious signals have been identified as harmonics of clock
frequencies. We are wondering if it is possible that it is generated via
the SUMPS, is that feasible, it seems to high for an SMPS harmonic?. The
chips are TPS5340 and TPS6421, layout as recommended by the manufacturer
and also low ESR electrolytic caps with parallel 1nF 10nF for
decoupling.=3D3D3D20
Thanks in advance=3D3D3D20
Russel
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