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LT5570IDD

Fast Responding, 40MHz to 2.7GHz Mean-Squared Power Detector

Linear Technology 

LT5570

APPLICATIONS INFORMATION

The LT5570 is a mean-squared RF power detector, capable

of measuring an RF signal over the frequency range from

40MHz to 2.7GHz, independent of input waveforms with

different crest factors such as CW, CDMA, WCDMA, TD-

SCDMA and WiMAX signals. A wide dynamic range is

achieved with very stable output within the full temperature

range from –40˚C to 85˚C.

RF Inputs

The differential RF inputs are internally biased at

VCC – 1.224V. The differential impedance is about 200Ω.

These pins should be DC blocked when connected to

ground or other matching components. The impedance

vs. frequency of the differential RF input is detailed in the

following table.

Table 1. RF Differential Input Impedance

FREQUENCY DIFFERENTIAL INPUT

(MHz)

IMPEDANCE (Ω)

40

204 –j 0.6

MAG

0.606

100

204 –j 1.8

0.606

200

204 –j 3.6

0.606

400

203.5 –j 7.3

0.606

600

202.8 –j 10.9

0.605

800

201.8 –j 14.5

0.604

1000

200.6 –j17.9

0.603

1200

199.1 –j21.3

0.602

1400

197.3 –j24.7

0.601

1600

195.4 –j27.9

0.599

1800

193.2 –j31.1

0.598

2000

190.8 –j34.2

0.596

2200

188.2 –j37.4

0.593

2400

185.3 –j40.4

0.591

2600

181.9 –j43.5

0.589

2800

178.3 –j46.4

0.586

3000

174.4 –j49.3

0.582

S11

ANGLE (°)

–0.1

–0.3

–0.5

–1.1

–1.6

–2.1

–2.7

–3.2

–3.8

–4.4

–5.0

–5.6

–6.2

–6.9

–7.6

–8.4

–9.2

The LT5570’s differential inputs are optimally driven from

a fully balanced source. When the signal is from a single-

ended 50Ω source, conversion to a differential signal is

required to achieve the maximum dynamic range. This

is best achieved using a 1:4 balun to match the internal

200Ω input impedance as shown in Figures 1 and 3. This

impedance transformation results in 6dB voltage gain. At

high frequency, additional LC elements may be needed

for input impedance matching due to the parasitics of the

transformer and PCB trace.

The approximate RF input power range of the LT5570 is

60dB at frequencies up to 900MHz, even with high crest

factor signals such as a 4-carrier W-CDMA waveform.

However the minimum detectable RF power level degrades

as the input RF frequency increases.

Due to the high RF input impedance of the LT5570, a

narrow band L-C matching network can be used for the

conversion of a single-ended to balanced signal as well.

By this means, the sensitivity and overall linear dynamic

range of the detector remain the same, without using an

RF balun.

The LT5570 can also be driven in a single-ended con-

figuration. Figure 5 shows the simplified circuit of this

single-ended configuration. The DEC Pin is preferably ac-

coupled to ground via a capacitor rather than left floating.

RF

INPUT

IN+

1nF

DEC

1nF

IN–

1007

LT5570

1007

1007

5570 F05

Figure 5. Single-Ended Input Configuration

5570f

11

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