LT5503 datasheet(15/20 Pages) LINER

15 / 20 page

LT5503

APPLICATIO S I FOR ATIO

Pin 16 should be connected externally to VCC through a

low value series resistor (47

Ω 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 compo-

nents are required to drive a 50

Ω load as shown in

Figure 2. The amplifier is designed to operate in Class A for

low distortion performance. The typical output 1dB com-

pression 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 2

µA 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

linear 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 2, a simple lowpass matching network

is required to match this pin to 50

Ω. 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 2) 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 2. Simple lowpass

matching is used to transform each leg of the mixer output

to 25

Ω 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 50

Ω 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.

5503 F06

RTERM

Ω

LB+

MX+

MX–

LB–

VCC

CBYPASS

MIXRFOUT

Figure 6. 50

Ω Mixer Output Matching Without a Balun

1.9GHz

2.4GHz

L5,L6

5.6nH

2.7nH

C5, C6

1.8pF

0.68pF

15pF

8.2pF

LT5503 Datasheet(PDF) 15 Page - Linear Technology

LT5503 Datasheet(HTML) 15 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL