LMH6560MTX datasheet(19/23 Pages) NSC

19 / 23 page

Application Notes (Continued)

This effect works to both sides because the circuit will not

generate radiation but the circuit is also not sensible if ex-

posed to a certain radiation level. The same is also notice-

able when placing components flat on the printed circuit

board. Standard through hole components when placed up-

right can act as antennae causing electric fields which can

be picked up by a nearby upright component. If placed

directly at the surface of the pcb this influence is much lower.

The Effect of Variation For e

When using pcb material the e

r has a certain shift over the

used frequency spectrum, so if it is necessary to work with

very accurate trace impedances, one must take into account

the frequency region for which the design is to be functional.

Figure 14 http://www.isola.de gives an example of what the

drift in e

r will be when using the pcb material produced by

Isola. If working at frequencies of 100MHz then a 50

Ω trace

has a width of 3.04mm for standard 1.6mm FR4 pcb mate-

rial, and the same trace needs a width of 3.14mm. for

frequencies around 10GHz.

Routing Power Traces

Power line traces routed over a pcb should be kept together

for best practice. If not a ground loop will occur which may

cause more sensitivity to radiation. Also additional ground

trace length may lead to more ringing on digital signals.

Careful attention to power line distribution leads to improved

overall circuit performance. This is especially valid for analog

circuits which are more sensitive to spurious noise and other

unwanted signals.

As demonstrated in Figure 15 the power lines are routed

from both sides on the pcb. In this case a current loop is

created as indicated by the dotted line. This loop can act as

an antenna for high frequency signals which makes the

circuit sensitive to R

F radiation. A better way to route the

power traces can be seen in the following setup. (see Figure

16).

In this arrangement the power lines have been routed in

order to avoid ground loops and to minimize sensitivity to

noise etc. The same technique is valid when routing a high

frequent signal over a board which has no ground plane. In

that case is it good practice to route the high frequency

signal alongside a ground trace. A still better way to create a

pcb carrying high frequency signals is to use a pcb with

ground a ground plane or planes.

20064256

FIGURE 13.

20064257

FIGURE 14.

20064258

FIGURE 15.

20064259

FIGURE 16.

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