Page 416 - Complete Wireless Design

Wireless Issues

Wireless Issues 415

Metallic shielding of synthesizers and oscillators to prevent coupling energy
into other circuits, or to prevent other circuits from injecting energy into the
frequency sources, is vital. Any such EMI propagating out of a frequency
source can decrease the isolation between stages and/or cause harmonic or
mixing products to form in the rest of the radio’s stages, while EMI entering a
synthesizer or VCO can create undesirable spurs. Traces leaving a frequency
source should be shielded by placing them at a lower PCB layer (in stripline
form), which is especially important from high-power frequency sources, such
as those used to feed diode mixer conversion stages. However, when shielding
of the top layer is employed to prevent microstrip traces from radiating, the
characteristic impedance of the microstrip can be greatly affected by the top
and side shield enclosure if it is placed too close to the track.
Temperature-reducing perforated shielding should be utilized with caution—
and never when a design is to operate above 2 GHz—as harmonics of the fun-
damental signal frequency can easily escape such an enclosure.
Lack of shielding or proper layout can cause a filter to become virtually use-
less if the signal to be filtered is able to propagate around the filter itself. This
can be especially problematic with SAW devices because of their high initial
insertion loss, which allows EMI to pass almost unfiltered.
Traces for transistors, inductors, mixers, and RFIC’s, and even the compo-
nents themselves, can all radiate; so self-shielded parts should be used when-
ever required.
Tough EMI radiation problems can be mitigated by applying RF-absorbing
foam or rubber over an offending circuit (at 400 MHz and above), or placed
within the shield itself to prevent the waveguide effect. (The waveguide effect
is undesirable low-loss coupling through a long shield structure that creates a
waveguide-like transmission line.)
In most wireless PCB situations it is prudent to ground all external cases,
shields, and septums straight to the ground plane in order to reduce EMI
emissions.
Even though placing shielding over a radiating circuit—or shielding a cir-
cuit that is being adversely affected—will help EMI substantially, any such
electromagnetic emissions can still travel through the dielectric of the PCB
and influence susceptible circuits. A method of attenuating this type of EMI is
to utilize a “fence” of through-hole vias, which are located around the affected
circuit, from the top of the PCB to the bottom ground plane. At very high fre-
quencies, or if high-amplitude harmonics of the fundamental are present, then
vias will have to be placed in a relatively dense pattern.
Preventing EMI from corrupting the DC power supply, and thus the entire
wireless device, is critical. A generous amount of decoupling capacitors that
are capable of shunting the entire spectrum of possible EMI frequencies—
along with a high-frequency choke—should be placed close to all active
devices. This will prevent unwanted oscillations and/or contamination of
our desired signal (see “Coupling/Decoupling of Amplifiers”), and is espe-
cially important for VCOs and LNAs, which must be heavily decoupled from

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