RF and Microwave Basics Impact PCB Design
It is a given that printed circuit board designs are utilizing higher frequencies to meet
performance demands. As data rates increase, the resulting bandwidth requirements are
driving the upper limit of signal frequency to 1 GHz and beyond. And while this is a far
shot from millimeter wave technology (30 GHz), it is indeed RF and low-end microwave.
RF requires a design engineering approach that addresses the associated stronger
electromagnetic field effects which naturally occur at these higher frequencies. These
fields can induce signals in adjacent signal lines, or PCB traces, creating undesirable
crosstalk (interference and overall noise), undermining system performance. Return loss
(signal reflected back into the incident oncoming signal) is primarily caused by
impedance mismatch and has much the same impact of added noise and interference to
the primary signal.
There are two effects of high return loss, both of which are bad news. First, signal
reflection back towards the source adds noise to the system, making it more difficult for
the receiver to distinguish noise from the signal. Second, any reflected signal is
fundamentally a degradation of the signal itself since the “meaning” or shape of the
inbound signal can be altered. While a digital system can be far more forgiving since it is
only attempting to recognize a one or a zero (on or off), the use of harmonics for faster
pulse rise times involves weaker signals at higher frequency. And, while we can
implement forward error correction technology to fix some of these effects, the result is
system degradation as capacity gets consumed in redundant transmissions. A much
better answer is to understand and engineer the RF effects to help, not hurt, your signal
management assignment. Overall recommended target values for return loss are minus
25 dB at the highest frequency of interest (usually the worse-case data point), which
converts to about 1.1 VSWR.
Traditional PCB design has been driven by "smaller, faster and cheaper". At RF
frequencies on a PCB, "faster" does not always allow for "smaller" due to some realities
of RF signal management design:
1. The primary way to manage unwanted crosstalk is by ground plane management,
trace-to-trace spacing, and/or reduction of stub inductance.
2. The primary way to reduce return loss is to match impedance. This involves
effectively managing the dielectric materials and spacing between the active trace
and the ground, particularly in transitions.
Since interconnect points are the weakest link in the electronic chain, each should be
challenged and solved as their electromagnetic properties become the dominant
engineering issue with the use of RF frequencies. The three major categories of
interconnect in a board system are chip-to-board, within the PCB, and getting the signal
on and off the PCB from an external device.
Chip-to-PCB
Within the chip itself performance is secure and processing speeds are already well into
the 1 GHz range. Pentium IV, Itranium, and even faster chips with huge input/output
interconnection counts are already being introduced or designed. At the recent Wireless
Workshop in Sedona, AZ (now called GHz Interconnect Workshop – go to
www.az.ww.com) one of the most stimulating topics being discussed was various known
and proposed ways of dealing with rising I/O count and frequency. The basic problem is
that interconnect density has become so high that the fundamental particle size of the
materials is becoming the limit. An innovative answer put forward was use of a very local
wireless transmitter built into the chip for the purpose of moving data to adjacent board
devices.
Regardless of where this takes us, it was clear to that audience that IC design is far ahead
of PCB platform design with respect to the use of high frequencies.
Within the PCB
Techniques and guidelines for high frequency PCB design do exist:
* To reduce return loss, miter corners on transmission line traces (see Figure One).
* Utilize high performance dielectric board laminates with tightly controlled dielectric
constant values. This allows engineered management of the electromagnetic field that
is moving through the dielectric adjacent to the trace itself.
* Complete PCB design specifications regarding high precision etching (usually helped
by specifying one-half ounce copper, tolerancing the trace width to +/-0.0007 overall,
managing the undercut and cross sectional view of the trace geometry, and specifying
the plating condition of the side-walls of the trace itself). These steps result in overall
management of the geometry and plated surface of the trace (conductor), important
due to skin effect, a phenomenon associated with microwave frequency. See Figure
Two.
* Avoid using leaded components due to stub inductance of the protruding lead. At
these frequencies, surface mount components is strongly preferred.
* On signal vias, avoid pth technology in sensitive board areas due to the unwanted stub
inductance of the hole. (Imagine a pth on a 20-layer board to connect signal layers 1
and 3 , the "stub" is the pth itself radiating onto layers 4-19).
* Provide generously for ground planes. Stitch them together with mode suppression
holes to inhibit the 3D electromagnetic fields covering the board.
* Select electroless nickel/immersion gold instead of HASL for plating. This surface
offers better surface properties for high frequency currents (see “skin effect” explained
in Figure Two). In addition, this highly solderable plating involves less lead and is
better for our kids and the planet that they live on.
* Soldermask prevents the unwanted flow of solderpaste. However, applying
soldermask all over the surface of the board effectively alters the flow of
electromagnetic energy in a microstrip design due to coverage of uncertain thickness
and unknown dielectric. Instead, use only solder "dams" as soldermask.
If these issues are unfamiliar to you, tap into the rich knowledge base of a microwave
board design engineer experienced in the military segment. You can discuss your price
point boundary conditions with them suggesting, for instance, that use of copper-backed
coplanar microstrip design is more cost effective than stripline, and that this matters to
you. These talented engineers may be unaccustomed to cost limits, but their skill set is
complex. Attempting to develop young "green" engineers that are inexperience with RF
effects and how to effectively deal with them may prove to be a long-term project.
Other solutions are appearing such as improved computer models that offer RF effects
built in to the software.
PCB to Outside World
Imagine that we solve all the signal management problems on the board and in the
interconnects to the discrete components soldered to them. What about getting the signal
on and off the board into a wire (copper or fiber) for connection to a device some
distance away? As an innovator in coax technology, our company has been working on
this with some important results (see examples in Figure Three).
Also, take a look at the electromagnetic fields represented in Figure Four. In this case,
we are managing a transition from microstrip to coax. In coax, the ground plane is
circular (braid) and evenly spaced. In microstrip this is changed to a ground plane under
the active trace. This introduces certain fringe effects that need to be understood,
predicted, and considered in the final design. Certainly, this mismatch is a source of
return loss and must be minimized to avoid additional noise and signal interference.
Managing impedance in the board, up to the surface level of the board, through a solder
joint, into a connector, and back out via coax is not a trivial design problem. Further,
impedance is a moving target that can vary with frequency and become harder to manage
with rising frequency. Moving signals over larger bandwidths (broadband) using higher
frequencies seems to be an established design issue for the immediate future. Even fairly
narrow-band applications, such as moving uncompressed CATV data files or voice-over-
IP data files, are starting to look like broadband applications with the use of frequency
stacking (block conversion).
Conclusion
PCB platform technology needs to play "catch up" ball to get to where the integrated
circuit people are now. Continuing rapid advances are needed in the area of high
frequency signal management in the PCB and in getting the signal on and off the PCB.
Whatever exciting innovation ensues, my prediction is that bandwidth use will continue
to be higher than ever, and use of high frequency signals will be the enabling
technology to achieve this.