ADL5920ACPZ-R2 datasheet(17/26 Pages) AD

17 / 26 page

Data Sheet

ADL5920

Rev. B | Page 17 of 26

THEORY OF OPERATION

The ADL5920 contains a symmetric and bidirectional resistive

bridge plus two identical rms detectors that provide both

forward and reverse power indications at the VRMSF and

VRMSR pins, respectively. A detailed description of the theory

of operation can be found in the Analog Dialogue article, An

Integrated Bidirectional Bridge with Dual RMS Detectors for

RF Power and Return Loss Measurement.

The device provides return loss and VSWR indication at the

VDIFF+ and VDIFF− outputs, where

VDIFF = (VDIFF+) – (VDIFF−) = VRMSF − VRMSR

(1)

The bridge has an insertion loss (IL) of 0.9 dB below about 1 GHz

when the source and load impedances are 50 Ω (the ADL5920

is only intended to be used in 50 Ω systems). The insertion loss

increases with increasing frequency to 1.9 dB at 6 GHz. Note

that insertion loss in dB is

IL = −20log10|S21| = −20log10|VRFOUT/VRFIN|

(2)

where:

VRFOUT is the RFOUT voltage.

VRFIN is the RFIN voltage.

As the source or load impedance deviates from 50 Ω, the

VRMSF and VRMSR outputs indicate this deviation via a

reduction in the separation of these two voltages. For example,

with a fixed signal level applied to the RFIN port, as the load

resistance on the RFOUT port varies from a short-circuit

condition to an open circuit condition, only the VRMSR signal

changes. The VRMSF output stays constant. The voltage

difference indicates the return loss and reflection coefficient of

the load and indicates the directivity of the structure when

RLOAD = RSOURCE = 50 Ω.

The two rms detectors are architecturally similar to the ADL5906

but are internally dc-coupled to operate down to dc. The detectors

provide linear in dB outputs and thereby give a direct indication in

dBm of the applied forward and reverse signals. Due to their

linear in dB response, the output voltages represent the coupled

and isolated port voltages in dB and thereby their difference

directly indicates directivity or return loss, which is an

advantage over simple diode detectors that produce a linear in

volt output. The detector slope of each detector output voltage

vs. PIN is approximately 60 mV/dB. Because both detectors are

identical, the difference in output voltage with a perfectly

matched source and load (50 Ω RSOURCE and RLOAD) is the

directivity of the bidirectional bridge and is calculated as

follows:

Directivity = ((VRMSF − VRMSR)/Slope) (dB)

(3)

Directivity is defined as follows:

Directivity (dB) = Coupling (dB) − Isolation (dB)

= 20log10(C/I)

(4)

Where the isolation (I) and coupling (C) factors are positive

numbers, and isolation is a smaller value than C.

In the default, single-supply and ac-coupled connection (see

Figure 38), the ADL5920 device directivity is greater than 30 dB

for frequencies below 400 MHz, as shown in Figure 5, which

shows as a constant difference voltage for the largest input

powers. When the signal is applied to the RFIN port (by definition

in the forward direction), the resulting VDIFF, VRMSF – VRMSR,

is approximately constant at frequencies less than 100 MHz.

However, as the input signal level reduces, eventually, the

rejected side limits at the noise and offset floor, and the VRMSR

output stays constant while the VRMSF output keeps decreasing

until this output also reaches the noise and offset floor. To

determine the inherent directivity of the ADL5920 measurement

system, apply a large enough input signal level to reliably

determine the isolated port voltage, which is best achieved

through a PIN sweep of around 100 MHz.

ADL5920ACPZ-R2 数据表(PDF) 17 Page - Analog Devices

ADL5920ACPZ-R2 数据表(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

数据表下载