ADL5330

Rev. A | Page 16 of 24

INLO

INHI

GAIN

OPLO

OPHI

DIRECTIONAL

COUPLER

ATTENUATOR

VPOS

COMM

ADL5330

+5V

+5V

+5V

COMM

VOUT

VPOS

VSET

INHI

INLO

CLPF

AD8318

LOG AMP

DAC

RF INPUT

SIGNAL

RF OUTPUT

SIGNAL

412

Ω

1k

Ω

SETPOINT

VOLTAGE

220pF

1nF

1nF

120nH

120nH

100pF

100pF

100pF

100pF

Figure 37. ADL5330 Operating in an Automatic Gain Control Loop in Combination with the AD8318

Figure 38 shows the transfer function of the output power vs.

the VSET voltage over temperature for a 900 MHz sine wave

with an input power of −1.5 dBm. Note that the power control

of the AD8318 has a negative sense. Decreasing VSET, which

corresponds to demanding a higher signal from the ADL5330,

tends to increase GAIN.

The AGC loop is capable of controlling signals just under the

full 60 dB gain control range of the ADL5330. The performance

over temperature is most accurate over the highest power range,

where it is generally most critical. Across the top 40 dB range of

output power, the linear conformance error is well within

±0.5 dB over temperature.

–4

4

3

2

1

0

–1

–2

–3

SETPOINT VOLTAGE (V)

2.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

30

20

10

0

–10

–20

–30

–40

–50

Figure 38. ADL5330 Output Power vs. AD8318 Setpoint Voltage,

The broadband noise added by the logarithmic amplifier is

negligible.

In order for the AGC loop to remain in equilibrium, the

AD8318 must track the envelope of the ADL5330 output signal

and provide the necessary voltage levels to the ADL5330’s gain

control input. Figure 39 shows an oscilloscope screenshot of the

AGC loop depicted in Figure 37. A 100 MHz sine wave with

50% AM modulation is applied to the ADL5330. The output

signal from the ADL5330 is a constant envelope sine wave with

amplitude corresponding to a setpoint voltage at the AD8318 of

1.5 V. Also shown is the gain control response of the AD8318 to

the changing input envelope.

AD8318 OUTPUT

Ω

CH3 250mV

Ω

M2.00ms

A CH4

1.80V

1

3

T

0.00000s

T

T

ADL5330 OUTPUT

AM MODULATED INPUT

Figure 39. Oscilloscope Screenshot Showing an AM Modulated Input Signal