AN4016

Circuit description

Doc ID 022523 Rev 1

5/17

2

Circuit description

The Input RF network must be carefully designed respecting the correct electrical symmetry,

because it is affected by driving high level signals (Pin ~ 20 W), and is made up of:

1.

Balun transformer T1,

λ / 4-25 Ohm transmission line type @ 123 MHz, needed to

lower the 50 Ohm RF input impedance to 12.5 Ohm, and is realized in a stripline

technique on a 2-layer substrate (Roger 4350B, with a thickness of 20 +20 mils: see

Figure 5) and is fed by a suspended microstrip line ('line bridge' in Figure 3). Moreover,

T1, being a quasi one-dimensional RF structure, can be mapped on the PCB without

compromising the electrical symmetry. T1, finally, is loaded from R7 and R29 in order to

dampen reflected waves from the gates and for stability purposes.

2.

Two in-phase power splitters (L4, L8, C16, C18, C20) and (L12, L16, C36, C41, C45)

simply decrease the impedance level (2 Ohm), and more importantly, allow the gates of

each STAC4932 to be kept isolated.

3.

RF decoupling filters, fed through the VG1 and VG2 connectors (Figure 3) need to bias

each STAC4932B gate. They are essentially LC multi-section filters with capacitors of

several technologies (tantalum, ceramic) to improve effective broadband RF isolation.

Independent voltage dividers act on the 4 gates (R4, R32, R16, R33, R17, R34, R31, R35)

to assure broadband RF stability, while the lower value series resistors (R6, R8, R10, ...)

need to dampen mismatching reflections on the gate impedance and then mitigate any

asymmetries on the gate impedance value.

The output RF network acts on the DMOS drains, in order to achieve optimal impedance by

means of the RF transformer T2, and also to properly feed high DC current filtered at

Vd=100 V, through the output biasing network directly via the primary winding of T2.

The transformer T2 (ratio 4:1) is designed on the top/bottom layers (see Figure 7) using

substrate Roger 4350B of 60 mils thickness in suspended broadside coupled strips and acts

as a composite transmission line transformer in balanced to unbalanced mode. The RF

output (type N-female connector) is directly connected to the winding output strip of T2 (see

top view in Figure 7) through an air suspended microstrip-line (50 Ohm): in this way, the

current (differential) generated on the primary winding strip (on the top layer) between the

two STACs is moved from T2 versus unbalanced RF output by the ground of the plate

copper carrier (see Figure 8) without further wave discontinuity, therefore avoiding losses

and creating a reliable design to support very high RF output power.

The transformer T2 has been designed using commercially available SW (ADS, HFSS) and

continues the refinement between electromagnetic and circuit simulation: T2, in fact, uses a

lamped capacitor (C25, C26, C23 caps group on winding top strip, and C37, C42 caps

group on the bottom side strip) to tune the proper impedance for DMOS drains.

In particular, the output biasing network (acts through the center tap of the winding top strip

of T2) uses several multilayer ceramic capacitors, and also adds the following electrical

functions:

1.

Dampens voltage overshoot generated by each transient effected by pulsed RF

modulation: that is the group L10, R13, C29, C30, C33.

2.

Two test points can be inserted between two calibrated Rm resistors for current /

voltage monitoring.

3.

Lamp LED D1, for safety purposes.