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LT5503
(Rev.:RevA)

1.2GHz to 2.7GHz Direct IQ Modulator and Mixer

Linear Technology 

LT5503

Applications Information

Pin 16 should be connected externally to VCC through a

low value series resistor (47Ω typical). To assure proper

output power control, a good, local high frequency AC

ground for Pin 16 is essential. The MODOUT port of the

VGA is an open collector configuration. An inductor with

high self resonance frequency is required to connect

Pin 17 to VCC as a DC return path, and as a part of the

output matching network. Additional matching components

are required to drive a 50Ω load as shown in Figure 1. The

amplifier is designed to operate in Class A for low distor-

tion performance. The typical output 1dB compression

point (P1dB) is –­ 3dBm at 2.45GHz. When the differential

baseband input voltages are higher than 1VP‑P, the VGA

operates in Class AB mode, and the distortion performance

of the modulator is degraded. The logic control inputs do

not draw current when they are low. They draw about 2µA

each when high.

Mixer LO1 Port

The mixer LO1 input port is the linear input to the mixer. It

consists of an active balun amplifier designed to operate

over the 1.4GHz to 2.4GHz frequency range. There is a lin-

ear relationship between LO1 input power and MIXRFOUT

power for LO1 input levels up to approximately –­20dBm.

After that, the mixer output begins to compress. When

operated in the recommended ­–14dBm to –8dBm input

power range, the mixer is well compressed, which in turn

creates a stable output level for the modulator input. As

shown in Figure 1, a simple lowpass matching network

is required to match this pin to 50Ω. This pin is internally

biased, therefore an AC-coupling capacitor is required.

Mixer LO2 Port

The mixer LO2 port is designed to operate in the 50MHz

to 1000MHz range. The first stage is a limiting amplifier.

This stage produces the correct output levels to drive the

internal divider circuit reliably, with LO2 input levels down

to –20dBm. The output of the divider then drives another

stage, which in turn switches the nonlinear inputs of the

double-balanced mixer. Note that the mixer output will

produce broadband noise if the LO2 signal level is too low.

The input amplifier is designed for a good match over the

entire frequency range. The only requirement (Figure 1)

is an external AC-coupling capacitor.

Mixer Output Ports (MX+/MX–)

The mixer output is a differential open collector configura-

tion. Bias current is supplied to these two pins through

the center tap of a balun as shown in Figure 1. Simple

lowpass matching is used to transform each leg of the

mixer output to 25Ω for the balun’s 50Ω input impedance.

The balun approach provides the highest output power

and best LO1 suppression, but is not absolutely neces-

sary. It is also possible to match each output to 50Ω and

couple power from one output. The unused output should

be terminated in the same characteristic impedance. In

this case, output power is approximately 2dB lower and

LO1 suppression degrades to approximately 15dBc. A

schematic for this approach is shown in Figure 6 where

inductors LB+ and LB– supply bias current to the mixer’s

differential outputs, and resistor RTERM terminates the

unused output.

MX+ 10

11 MX–

C5

MIXRFOUT

L5

LB+

VCC

LB–

RTERM

51Ω

C9

CBYPASS

L6

C6

5503 F05

L5,L6

C5, C6

C9

Figure 5. 50Ω Mixer Output Matching Without a Balun

1.9GHz

5.6nH

1.8pF

15pF

For more information www.linear.com/LT5503

2.4GHz

2.7nH

0.68pF

8.2pF

5503fa

15

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