[hsdd] High-Speed Digital Design Newsletter - - VRM Stability - Part I: Feedback
- From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
- To: <hsdd@xxxxxxxxxxxxx>
- Date: Mon, 10 Sep 2007 10:53:36 -0700
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VRM Stability - Part I: Feedback
HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 10 Issue 03
Precision electronic circuits need good quality power. Digital
electronics usually combine a voltage regulator module (VRM) with
a network of capacitors. The same structure appears in many other
types of electronic products.
No matter what the application, all voltage regulator circuits
involve the principle of feedback. In any circuit feedback must
be carefully controlled because, by its very nature, feedback
invites the risk of self-oscillation.
Before diving into a full-fledged analysis of feedback in a VRM
application, I shall present a simple example. This circuit
example comes from an old Fender Rhodes piano. It has a
particularly simple structure that exhibits beautifully the
concepts of feedback, conditional stability, and several other
interesting problems.
_____
Notes
You may already know that the Fender Rhodes piano introduced in
1969 epitomizes the electronic jazz-fusion sound of the
pre-synthesizer era. Hear it on Chick Corea's album "Light as a
Feather", or Miles Davis' "Bitches Brew". See it in the 1980
film, "The Blues Brothers", where Ray Charles plays "Shake a Tail
Feather" in the music store scene. Electronic keyboard
instruments made today still strive to achieve that unique,
classic "Rhodes" sound.
If you are a Fender Rhodes fan and just can't wait to hear that
sound again, here's a live recording of the 'Rhodes, as played by
Herbie Hancock.
http://www.haroldrhodes.info/history/audio.php
I love music, but digital electronics in my real passion. I'm
looking forward to teaching my first ever Canadian public
seminars in Ottawa September 17-20. Then I'm in Dallas October
1-4.
Attention Intel employees! I will be teaching High-Speed Digital
Design at Intel Corporate in Hillsboro on October 29-30. This
course is exclusively for Intel, and is open to Intel employees
worldwide. Visit www.sigcon.com/Intel2007.
_____
VRM Stability - Part I: Feedback
The instrument on my bench does not sound like Herbie Hancock's
piano.
I am looking at a 1972 Fender Rhodes Mark I electronic "suitcase"
piano. It belongs to my good friend Chris "Breathe" Frue, a
talented jazz multi-instrumentalist. Breathe hovers over the
instrument with a somber tone and long face. He has just pulled
it from his attic after decades of disuse.
Switching on the power, Breathe grimaces as the piano emits a
sound no longer reminiscent of the cool, jazzy, musical marvel of
his youth. It buzzes like a swarm of angry bees circulating up
from the ground in a whirl of vengeance (those who have heard
that sound don't soon forget it). Above the buzzing hoard floats
a high, piercing, teapot-squeal, undulating wildly. None of the
keys work.
"Breathe", I explain, reaching for a screwdriver, "things may not
be as bad as they appear. Self-oscillation requires two things:
an amplifier, and a path for feedback. Any sound at all tells us
the speakers are OK, probably also much of the power system and
parts of the audio amplifier chain. A solution to your problem
might only require identifying the source of feedback and cutting
it off."
Breathe brightens as he begins to realize that I am not going to
merely blow the dust out of the case and start checking
transistors. He has never witnessed a methodology for debugging
electronics.
At my instruction, Breathe opens up the cabinetry and begins
laying the circuits out on my bench.
The Fender Rhodes "suitcase" model comes in two pieces (Figure
1). The keyboard portion rests on the top. It holds the
electro-mechanical apparatus and pre-amplifier circuits. The base
unit below holds the main connection panel, power amplifier and
speakers.
<http://www.sigcon.com/images/news/v10_03_1.gif>
The block diagram comprises five main pieces (Figure 2). First, a
set of electronic pickups, one for each of the 88 keys, wires to
a common jack (J1). That leads to a pre-amplifier embedded within
the keyboard unit (only one of two stereo channels is shown).
<http://www.sigcon.com/images/news/v10_03_2.gif>
The pre-amplifier connects through a shielded cable about 3 feet
long leading from the keyboard unit to jack J2 on the main panel,
located within the base unit. A second shielded cable hidden
entirely within the base unit passes the low-level audio signal
through intermediate jack (J3) to a power amplifier. The power
amplifier returns the amplified signal through a cable (J4) back
to the main panel, from which the signal feeds the speaker (J5).
Power connections to the pre-amplifier and the power amplifier
are not shown.
Breathe has by now completely extracted the electronics from the
cabinetry. All the parts lie exposed on my bench, plugged
together in their original configuration. We power on. The system
replicates its self-oscillation problem. I welcome this
development, as I despise time wasted trying to replicate flaky
or erratic problems.
"Begin by isolating the problem," I suggest. "Unplug pieces of
the system one by one to see what changes."
Breathe disconnects jack J1, making no difference in the problem.
Upon removing J2, the low buzz evaporates. The teapot stays with
us, but changes in pitch, warbling. "That's it," exclaims
Breathe, jotting down notes on our block diagram, "the buzz must
be coming from the pre-amplifier."
"A reasonable guess, but it could also be a problem further
downstream." I point to the power amp. "Oscillation problems
often depend on the loading of each stage. Unplugging J2 changes
the loading at the input of the power amplifier, which could be
the real culprit. For now, we can't necessarily tell what causes
the buzz, only that you found one thing that stops it. I'm more
excited with the fact that the squeal continues. Let's forget the
buzz for now and find out what, in the system to the right of J2,
causes that squeal. "
Inherent to the block diagram, the way I drew it, is a big loop
of cables, going from J3 into the power amp, and from there back
to the main panel at J4. Even a tiny amount of feedback between
these input and output signals could cause the squealing problem.
"It used to work," says Breathe, "twenty years ago. What changed?
Why do we get feedback now that wasn't present in the original
design?"
"Corrosion." I pull out the connector at J5 and the squeal stops.
Of course we stop hearing it, because I just unplugged the
speaker. An oscilloscope probe at J4 confirms, however, that the
squeal has indeed stopped. This step confirms that our feedback
problem involves, somehow, current flowing through the speaker.
My test did not change the voltage coming out of the power
amplifier, only the current.
"Breathe, let's think about where the speaker current flows in
this system." I trace the signal path from the power amp through
its cable to J4 and on to the speaker (red arrows in Figure 3).
Nothing surprises Breathe about that path. Now I draw another
path showing returning signal current coming back from the
speaker to the power amplifier. Here lies the mystery of the
squeal.
<http://www.sigcon.com/images/news/v10_03_3.gif>
The current returning from the speaker (green arrows) passes
along the black wire from the speaker to connector J5. There it
shunts to the main panel chassis. From the main panel chassis the
returning current must complete the last part of its journey,
finding a path somehow from the main panel chassis back to the
power amplifier chassis.
Figure 3 represents the last portion of the current pathway with
fictional impedance Z1. In actual fact, at audio frequencies the
current divides with portions flowing over the ground connections
of cables connected to both J3 and J4. The details aren't
important-what matters is that this last portion of the path has
some finite impedance Z1, so for simplicity I represent it as a
single component.
Current traversing Z1 induces a voltage across it. That voltage,
V[noise], appears between the main panel chassis and the power
amplifier chassis. The magnitude of V[noise] equals the speaker
current, I[speaker], times the magnitude of impedance Z1. Now,
here's the important part of my discussion: the power amplifier
input responds to voltage V[noise].
To see why, short the signal at J2 to its local ground,
transmitting no signal beyond that point. The power amplifier
input stage now receives the difference between (a) its input
pin, connected directly through J2 to the main panel chassis, and
(b) its own chassis, which lies at a distinctly different voltage
than the main panel due to the returning speaker currents flowing
through Z1. The amplifier therefore sees, as its input, a voltage
V[noise] proportional to its own output current. When a system
sees an input signal that varies in proportion to its own output
we call that effect feedback.
A small amount of feedback makes no sensible effect on the power
amplifier circuit, however, corrosion present in the ground legs
of connectors J3 and J5 may increase the overall resistance of
Z1, magnifying the noise voltage V[noise]. If the feedback grows
sufficiently large, it can make the power amplifier
self-oscillate.
With a feedback theory foremost in my thoughts, I suggest to
Breathe that, "Lowering the resistance of the inter-chassis
ground connection might help. That would reduce the inter-chassis
voltage, possibly suppressing self-oscillation." I reconnect the
speaker. The squeal returns as expected. Then I prepare a short
piece of bare wire, #10 AWG. With that wire, I short the main
panel chassis directly to the chassis of the power amplifier.
"See, the squeal stops. Better grounding, less feedback."
Working towards a possible fix, Breathe and I disconnect the
cables at J3 and J4 and also at their opposite ends. We clean all
the connections. The ground shells on the RCA plugs are covered
with a white powder. They appear to be un-plated tin. These get a
very light sanding with #600 grit sandpaper. Everything gets
scrubbed with CRC Electronic Cleaner, applied with Q-tips. We
reconnect, and the system from J2 onwards powers up perfectly
with no squeal. Audio signals applied to J2 come through the
power amplifier beautifully with no distortion. Squeal fixed.
We are only halfway done with our restoration work, and Breathe
has already learned a lot about feedback, amplifier stability,
and the importance of good grounding. I will apply those concepts
to the study of power system stability in the next issue, where
we uncover the genesis of the low-frequency buzz of death-a sound
you'd like to hear only in a horror film.
Best Regards,
Dr. Howard Johnson
_____
Points to Remember (Summary)
Input current flows into the power amplifier through the jack J3.
On its return journey back to the pre-amplifier, the input
current traverses impedance Z1.
The output current from the power amplifier flows through jacks
J4 and J5 to the speaker. On its return journey back to the power
amplifier the output current also traverses impedance Z1.
Impedance Z1 therefore constitutes a common impedance shared by
both the loop of input current and the loop of output current
associated with the power amplifier (Figure 4).
Common impedance Z1 creates crosstalk (feedback) between the
input and output circuits. That feedback occurs in proportion to
the magnitude of Z1.
If impedance Z1 grows over time because of corrosion, then
feedback grows over time also.
Any amplifying system will oscillate if supplied with sufficient
feedback.
<http://www.sigcon.com/images/news/v10_03_4.gif>
_____
Application to Digital Electronics
I encounter grounding problems in high-speed digital circuits all
the time. One usually identifies such problems as "ground
bounce", or using the more modern term, "simultaneously switching
output noise" (SSO). Whatever the name, the concept remains the
same: when the inputs and outputs of any circuit share a common
current path, the impedance of that path must be kept suitably
low.
In general, any single-ended system that transmits power in a
loop or mesh structure requires exceptionally good grounding. The
reference terminals of all the transmitters and receivers in the
system must be connected together. They must be connected by
impedances so small that the voltages present across those
impedances, caused by all the returning signal currents, cannot
sensibly affect the circuit.
Grounding becomes especially complicated in digital circuits. An
audio circuit only needs suitably low resistance ground
connections. A high-speed circuit also needs low inductance
connections.
My final public seminars of 2007 are just weeks away!
I will be teaching both of my classes, High-Speed Digital Design
and Advanced High-Speed Signal Propagation, in Ottawa (Sept.
17-20) and Dallas (Oct. 1-4).
Group discounts are available for all classes.
http://www.sigcon.com/seminars.
Use Promo Code: NL07 for discount
The National Semiconductor "Analog By Design" show hosted by Bob
Pease has invited me back for another taping Oct. 15th -- I'll
let you know when it comes out. In the last issue I did a cool
section on "digital pre-emphasis" circuits:
http://www.national.com/nationaltv/
EDN Magazine may soon post a short film about RoHS. In this film
I interview Joe Fjeldstad, an expert in electronic
interconnections, about the impact of RoHS on the environment,
and on electronic reliability. When I know the URL, I'll pass it
along.
_____
Questions & Comments: All students who attend our High-Speed
Digital <http://www.sigcon.com/seminars.htm> Design seminars
have the opportunity to talk directly with Dr. Johnson about
signal integrity issues.
If you have an idea that would make a good topic for a future
newsletter,
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