Data Sheet

ADL5920

Rev. B | Page 21 of 26

RF POWER AND RETURN LOSS CALCULATION

Figure 42 shows the voltage measured on VRMSF and VRMSR

when RFIN is swept across its power range at various frequencies

with a 50 Ω termination on RFOUT.

The VRMSR output ideally only responds to power reflected

from the load. However, because of the finite directivity of the

bridge circuit of the ADL5920, the VRMSR voltage starts to

increase as the RF power at RFIN increases. Thereafter, the

VRMSR voltage follows a similar linear in dB response as

VRMSF, although at a much lower level. At a particular

frequency, the difference in output voltage between VRMSF and

VRMSR, where both voltages are following this linear in

dB characteristic, is proportional to the directivity in dB of the

bridge circuit when the load is 50 Ω. As frequency increases, the

vertical difference between the VRMSF and VRMSR traces

decreases, indicating a decrease in directivity.

4.0

0

1.0

2.0

0.5

1.5

2.5

3.0

3.5

–40

–20

–10

10

30

–30

0

20

RF INPUT (dBm)

5GHz VRMSF

3GHz VRMSF

7GHz VRMSF

1GHz VRMSF

10MHz VRMSF

5GHz VRMSR

3GHz VRMSR

7GHz VRMSR

1GHz VRMSR

10MHz VRMSR

Figure 42. VRMSF, VRMSR Output Voltage vs. RF Input at Various Frequencies

When Bridge Driven from RFIN and RFOUT Terminated with 50 Ω

Use the following equation to calculate the idealized output

(7)

where:

Slope is the change in output voltage divided by the dB change

in input power.

Intercept is the calculated input power level (in dBm) at which

the output voltage is equal to 0 V. Note that Intercept is an

extrapolated theoretical value, not a measured value.

(8)

with the RFIN pin terminated with 50 Ω.

Because slope and intercept vary from device to device and vs.

frequency, calibration must be performed to achieve high

accuracy.

In general, calibration is performed by applying two or more

the ADL5920 and measuring the corresponding output voltages

linear operating range of the device.

With a two-point calibration, calculate the slope and intercept

as follows:

(9)

(10)

After the slope and intercept are calculated and stored in

nonvolatile memory during equipment calibration, use the

following equation to calculate the unknown input power based

on the output voltage of the detector:

(11)

Perform a separate calibration to establish the slope and intercept

of the reverse path. Alternatively, because the forward and

reverse path bridge circuits and rms detectors are matched

closely, use the slope and intercept from the forward path

calibration to convert the VRMSR voltage to the equivalent

dBm RF power. Using this methodology, use the following

(12)

(13)

Note that insertion loss has a negative sign for a passive load.

differential outputs.

Insertion Loss (dB)

(15)

To calculate the directivity of the bridge circuit, place a 50 Ω

dB is then given by the following equation:

(16)