TSH690 datasheet(13/14 Pages) STMICROELECTRONICS

13 / 14 page

TSH690

13/14

Figure 8 : PCB component side (not to scale)

Routing Requirements

Supply voltages : as well as any RF design

boards, decoupling of supply voltages requires

carefully routing. So, the output stage features a

µF tank capacitor to smooth current peaks and

2 decoupling capacitors of 1nF and 100pF avoid-

ing a RF feedback to the supply.

The oscillator biasing (pin 7) requires a RF choke

self inductance of 56nH in series with R1 and also

a grounded decoupling capacitor of 1nF.

Ground : The ground routing has to be done in the

manner of decreasing resistive and inductive par-

asitics effects to guarantee the best equipotential

of the electrical ground node. By using microstrip

lines, bottom and top ground planes must be con-

nected via through holes in respect with a distance

shorter than 10 times the running wavelength. In

practice,

with

standard

epoxy

substrate

(Er~4,5), to run at about 450MHz, distance be-

tween 2 vias must not exceed ~3cm. By using a

standard 2.54mm PCB grid, design takes profit to

avoid harmonics radiation.

Improvements

In comparison with the first diagram proposal, 2nd

& 3rd harmonics levels can be reduced respec-

tively as low as -27dBc and -34dBc by replacing

L2 with a parallel tank circuit (LC) of 10nH, 13pF.

In such a condition, fundamental output level is

slightly degraded of less than 0.5dB keeping a

good 50

Ω impedance matching.

Figure 9 : PCB solder side (not to scale)

Figure 10 : Silk screen (not to scale)

TSH690 Datasheet(PDF) 13 Page - STMicroelectronics