Circuit Note

CN-0518

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Rev. 0 | 4 of 8

supplementary stability measure (B1) being above 0, as shown in

Figure 10. This makes the CN0518 unconditionally stable for any

source and load impedance combination.

Figure 10. CN0518 Stability Factor and Measure vs. Frequency

OVERPOWER PROTECTION

An overpower management feature is implemented in CN0518

wherein the RF path of the board is automatically isolated when

the output power reaches the preset threshold. This feature is

performed using an ADL5904 RF power detector.

The ADL5904 provides a resistor-programmable detection thresh-

old, which compares the voltage from an internal envelope detector

with a user-defined input voltage. When the voltage from the enve-

lope detector exceeds the user-defined threshold voltage at the

VIN- pin, an internal comparator captures and latches the event to

a set/reset (SR) flip-flop. Figure 11 shows the overpower protection

circuitry for the CN0518.

Figure 11. CN0518 Overpower Protection Circuit

As shown in Figure 11, the amplified RF input is sampled by a 25.5

dB RF directional coupler. This power is forwarded to the RFIN pin

of the ADL5904, which is then sampled by the internal envelope

detector. The ADL5904 threshold voltage level on VIN- pin is set

by a resistor divider network, which value can be computed using

Equation 1:

VIN−= 3.3×R2R2×R1 + R1×R2R1+R2 ×IBIAS

(1)

where:

VIN- is the voltage level at the VIN- pin of the ADL5904.

R1 is the user-defined resistance value.

R2 is the user-defined resistance value.

IBIAS is the input bias current.

The CN0518 was set at -25 dBm input threshold power, which is

equivalent to 0 dBm RF output to protect any sensitive downstream

equipment connected to the receiver system. Table 1 can be found

in the ADL5904 data sheet and shows the -25 dBm threshold power

corresponds to a VIN- voltage level of 17 mV when operating at 900

MHz.

Table 1. Recommended Typical Values for Threshold Voltage (VIN-) When

Operating Uncalibrated

Input Threshold Power

(dBm)

Threshold Voltage

(mV)

100 MHz

900 MHz

1900 MHz

-25

18

17

16

-24

20

18

17

-23

22

20

19

-22

24

22

21

-21

26

25

23

The threshold level on VIN- is set by a resistor divider. The value of

R1 is primarily chosen to minimize loading on the 3.3 V rail, and R2

is chosen for an output impedance that is insensitive to leakage and

bias currents. Setting R1 to 10 kΩ and using the power threshold

value of 17 mV, the R2 value is calculated using Equation 1:

VIN- = 17 mV

IBIAS = 20 µA

R1 = 10 kΩ

17 mV= 3.3×R2

10 kΩ×R2

+ 10 kΩ×R2

10 kΩ+R2

×20 μA

R2 = 48.8 Ω or 49.9 Ω (using standard resistor value)

Solving Equation 1 using the closest standard resistor value sets

the VIN- voltage level at 17 mV. This user-defined VIN- voltage

is then compared to the output voltage of the internal envelope

detector, which is driven by the RF input level. When the power

exceeds the threshold value, an overpowering event occurs which

opens or isolates the RF path. The series combination of R1 and R2

results in a negligible divider current of 328 μA.

The ADL5904 introduces an error level of up to +2.5 dBm on its RF

threshold power and can vary from device-to-device. If an accurate

threshold power is required, a simple calibration routine must be

performed to compensate for the device-to-device variation. Refer

to the ADL5904 data sheet for more information on the calibration

routine.