LT5503

16

APPLICATIO S I FOR ATIO

EVALUATION BOARD

Figure 7 shows the circuit schematic of the evaluation

board. The MODRFIN, MODRFOUT and MIXRFOUT ports

are matched to 50

Ω at 2.45GHz. The LO1IN port is

matched to 50

Ω at 2.1GHz and the LO2IN port is internally

matched.

A 390

Ω resistor is used to reduce the quality factor (Q) of

the modulator output and deliver an output power of

–3dBm typically. A lower value resistor may be used if the

desired output power is lower. For example, the output

power will be 3dB lower if a 200

Ω resistor is used.

Inductors with high self-resonance frequency should be

used for L1 to L6.

For simpler evaluation in a lab environment, the evaluation

board includes op amps to convert single-ended I and Q

input signals to differential . The op amp configuration has

a voltage gain of two; therefore the peak baseband input

voltage should be halved to maintain the same RF output

power. The op amp configuration shown will maintain

acceptable differential balance up to 10MHz typically. It is

also possible to bypass the op amps and drive the

modulator’s differential inputs directly by connecting to

the four oversized vias on the board (V1, V2, V3 and V4).

Figure 7 also shows a table of matching network values for

designs centered at 1.9GHz and1.2GHz.

Figure 8 shows the evaluation board with connectors and

ICs. Figure 9 shows the test set-up with the upconverting

mixer and IQ modulator connected in a transmit configu-

ration. Refer to the demo board

DC365A Quick Start Guide

for detailed testing information.

RF Layout Tips:

• Use 50

Ω impedance transmission lines up to the

matching networks, use of a ground plane is a must.

• Keep the matching networks as close to the pins as

possible.

• Surface mount 0402 outline (or smaller) parts are

recommended to minimize parasitic inductances and

capacitances.

• Isolate the MODOUT pin from the LO2 input by putting

the LO2 transmission line on the bottom side of the

board.

• The only ground connection is through the exposed pad

on the bottom of the package. This exposed pad must

be soldered to the board in such a way to get complete

RF contact.

• Low impedance RF ground connections are essential

and can only be obtained by one or more vias tying

directly into the ground plane.

broadband capacitors to prevent instability.

• Separate power supply lines should be used to isolate

the MODIN signal and other stray signals from the

MODOUT line. If possible, power planes should be

used.

• Avoid use of long traces whenever possible. Long RF

traces in particular can lead to signal radiation and

degraded isolation, as well as higher losses.