Re: placing addiional caps across existing caps to reduce noise

John_H wrote:
"rickman" <gnuarm@xxxxxxxxx> wrote in message
news:1157032693.254923.135250@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On a separate note, I can't believe some of the things we do here. Our
digital circuits are part of RF equipment so we are typically very
concerned with even low levels of noise in the RF region. To make sure
our boards are quiet we have an RF person review the design and board
layout. I was assisting on a design for a simple MCU board with an
attached GPS receiver. The RF guy was very concerned about various
noise sources that had burned him in the past and did a lot of what I
thought was overkill in the power distribution. I just found out that
he had the 6 layer stackup done with two ground planes and no power
plane! I suggested to the layout guy that it would be ok to flood fill
the signal layers with power plane and he said they are doing that, but
connecting to ground instead of power!!! So there is no effective
bypassing on this board above a couple hundred MHz and the freq of the
receiver is around 1.5 GHz. Do you think we will see any interference?

The situation might not be so bad. When an RF engineer is interested in
quiet power, there are filters between any noisy digital power planes and
the power for the RF section, effectively eliminating any bypass gains
achieved from the beautifully bypassed (but still RF-sensitivity
compromising) power planes.

Yes, he added an LDO between the switcher and the digital power because
the digital power goes to the GPS module. This is a bit pointless
because an LDO is only effective up to a few 10s of kHz. Then there is
a ferrite bead which is not very effective until you get to high MHz.
So that leaves a huge hole from about 100 kHz to maybe 100 MHz. Also
the ferrite bead is only an impedance, not a cure. Noise can still
couple to the load if it is not a low impedance.

On the other hand a good power plane will decouple noise at the source
and prevent it from reaching the ferrite filter. So good power planes
are *always* a good thing for reducing noise.


There is a large variety of RF caps specifically used for in-line power
decoupling that are effective *only* at the high frequencies partly because
that's the only frequency of interest in the RF device. An oscillator at
1.5 GHz cares little about what's happening at 200 MHz if that 200 MHz noise
has been filtered out before hitting the effective 1.5 GHz bypassing.

How do you make a capacitor less effective at low frequencies???
Wouldn't that be an inductor?


The truely effective board level decoupling can result in much better
mixed-signal performance where discretes are connected directly to the
shared power plane. True RF still seems like a much different animal to me
where grounds really are king (with properly filtered and cascaded power
distribution is regal as well).

I don't know about the royal lineage. I do know that digital circuits
are kept quiet with good power decoupling including low impedance
across the spectrum. In this case the only way to filter 1.5 GHz in
the power spectrum is with plane to plane capacitance.

.

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