LT5504

6

5504i

1:

63.56

Ω

–j98.05

Ω

900.00MHz

2:

26.69

Ω

–j42.90

Ω

1.90GHz

3:

28.88

Ω

–j27.76

Ω

2.50GHz

1

2

3

START: 100MHz

STOP:3GHz

5504 F01

The LT5504 consists of the following sections: RF/IF

limiters, limiting mixer, RF/IF detectors, LO buffer ampli-

fier, 3rd-order integrated low pass filter (LPF), output

interface and bias circuitry.

An RF signal ranging from 800MHz to 2.7GHz is detected

by the RF and IF detectors using a proprietary technique.

The down-converted IF signal is band limited by the on-

chip LPF, reducing broadband noise, and thus an ultrawide

dynamic range signal can be measured. The RF measuring

receiver is essentially a logarithmic voltage detector. The

measured output voltage is directly proportional to the RF

signal voltage. An internal temperature compensation

circuit results in a highly temperature-stable output volt-

age.

RF Limiter

The differential input impedance of the RF limiter is shown

in Figure 1. A 1:1 input transformer can be used to achieve

50

Ω broadband matching with an 82Ω shunt resistor

(R1) at the inputs as shown in Figure 5.

APPLICATIO S I FOR ATIO

Figure 2. RF Input Matching Network at 1900MHz

Figure 3 shows the output voltage vs RF input power

response for these two input terminations. The voltage

gain of the single-ended-to-differential conversion circuit

is:

GAIN

LOG

R

dB

IN

==

20

50

3

•,

Thus, the output voltage curve in this case is shifted to the

left by about 3dB.

900

12.0

3.9

1900

3.3

3.3

2500

2.7

2.2

2700

2.4

1.5

Figure 1. Differential RF Input Impedance

The 1:1 RF input transformer can also be replaced with a

narrow band single-ended-to-differential conversion cir-

cuit using three discrete elements as shown in Figure 2.

Their nominal values are listed in Table 1. Due to the

parasitics of the PCB, these values may require adjust-

ment.

MATCHING NETWORK

RF

INPUT

3.3pF

3.3pF

3.3nH

5504 F02

2.5

2.0

1.5

1.0

0.5

0

PIN (dBm)

–80 –70 –60 –50 –40 –30 –20 –10

10

5504 F03

0

WITH SINGLE-ENDED-TO-

DIFFERENTIAL INPUT CIRCUIT

WITH 1:1 INPUT TX

Figure 3. The Output Voltage vs RF Input Power