I agree that -200 dB sounds like hogwash. Just exactly how would someone
verify such a requirement, assuming for the moment that is was real?
But back to Ralph's question. Our power supply team usually uses a tool such a
Saber to model the PDN from the output of the power supply up to the RF
devices. If this model properly accounts for the distribution inductances and
capacitances, and given the output noise/ripple from the power supply, they can
estimate the noise/ripple that would show up at the supply pins to the RF
devices.
For very sensitive applications, it is not unusual to have several linear
regulators in series to achieve the noise reduction needed from the output of
the power supply to the RF chain.
Steve
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Ralph Wilson
Sent: Friday, August 28, 2015 10:29 AM
To: Mikhail Matusov; si-list@xxxxxxxxxxxxx
Subject: EXT :[SI-LIST] Re: Looking for ideas......
Mikhail,
You are likely correct. I'm not an RF engineer, so that's not my bailiwick.
Never the less, I'd like to analyze the PDN to quantify the distribution of
power noise to the RF circuits, so my problem remains.
Thanks!
Ralph
On 8/28/2015 9:23 AM, Mikhail Matusov wrote:
Ralph,
Spurs at -200 dB sound like complete nonsense. This is roughly thermal
noise of a 50 Ohm resistor in 10 Hz band at -273 C. And I haven't seen
an RF application where spurs at -120 dBc would be considered a
problem either.
/Mikhail
-----Original Message----- From: Ralph Wilson
Sent: Friday, August 28, 2015 9:40 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Looking for ideas......
All,
Some of my customers (in this case RF design engineers) are reporting
problems found in their designs where noise on a power net that feeds
the input to a voltage regulator (LDO, switching, or other) is
conducted (but reduced in
magnitude)
to the output of the regulator. The output of the regulator supplies
power (usually through subsequent L-C filtering) to an RF component.
The noise from the upstream power feed reaches the RF component and
shows up as spurious noise on the RF signal. We're talking noise in
the -120dB to -200dB range, so these are very sensitive RF nets. I
would like to be able, in the simulation world, to see if I can come
up with a quantitative analysis of the noise conducted through this
power distribution network - in the most general sense of the term,
an "S21"
from the source power net to the client power pin(s).
In looking at some LDO data sheets, they spec a "ripple rejection rate"
(usually
around -40dB) specifically for this. I'm thinking I can model the LDO
(for AC purposes) as a simply "Pi" circuit equivalent to the ripple
rejection ratio of the LDO - at least as an initial approach.
One problem I am having is the tools (in this case Hyperlynx
SI/PI/Linesim)
don't support this kind of analysis. The power nets are typically
routed as area fills, hence the trace modeler does not recognize them.
Hyp PI sees the power nets, but doesn't do S-parameter-like analysis
(the Hyp AC wizard does do Z-parameter analysis - I've been trying to
think of how to bastardize this to get some meaningful results).
In theory I could go to 3D simulations, but this is not practical for
what I'm trying to do. If I knew where the problem was on a specific
design, I could extract the necessary pieces and probably measure what
I already know. However, I'm trying to look at a fully routed design
and analyze the whole PDN looking for weaknesses in the layout /
routing - without any a-priori knowledge about whether a problem
exists or where one might exist.
Right now I'm sort of stuck - I'm not coming up with any practical
means of doing this analysis.
Looking for ideas.......anyone attempted this before - with success?
Which tool(s)?
Thanks!
Ralph Wilson
Alcatel-Lucent
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