Datasheet

AD8317
Rev. B | Page 14 of 20
INLO
INHI
GAIN
OPLO
OPHI
DIRECTIONAL
COUPLER
ATT ENUATOR
VPSx COMx
ADL5330
+5
V
+5V
+5
V
COMM
VOUT VPOS
VSET INHI
INLOCLPF
AD8317
LOG AMP
DAC
RF OUTPUT
SIGNAL
4.12kΩ
10kΩ
SETPOINT
VOLTAGE
1nF
47nF
47nF
120nH
120nH
100pF
100pF
100pF
100pF
TADJ
18kΩ
52.3Ω
RF INPUT
SIGNAL
05541-030
Figure 30. AD8317 Operating in Controller Mode to Provide Automatic Gain Control Functionality in Combination with the ADL5330
Figure 31 shows the transfer function of the output power vs.
the setpoint voltage over temperature for a 900 MHz sine wave
with an input power of −1.5 dBm. Note that the power control
of the AD8317 has a negative sense. Decreasing V
SET
, which
corresponds to demanding a higher signal from the ADL5330,
increases 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.
–50
–40
–30
–10
0
10
20
30
OUTPUT POWER (dBm)
ERROR (dB)
–20
–4
–3
0
1
2
3
4
–1
–2
0.2 0.4 0.6 0.8 1.0 1.4 1.8
2
.
0
SETPOINT VOLTAGE (V)
1.61.2
05541-031
Figure 31. ADL5330 Output Power vs. AD8317 Setpoint Voltage, P
IN
= −1.5 dBm
For the AGC loop to remain in equilibrium, the AD8317 must
track the envelope of the ADL5330 output signal and provide
the necessary voltage levels to the ADL5330 gain control input.
Figure 32 shows an oscilloscope screenshot of the AGC loop
depicted in Figure 30. A 100 MHz sine wave with 50% AM
modulation is applied to the ADL5330. The output signal from
the VGA is a constant envelope sine wave with amplitude corre-
sponding to a setpoint voltage at the AD8317 of 1.5 V. Also
shown is the gain control response of the AD8317 to the
changing input envelope.
CH1 200mV
CH3 50.0mVΩ
A CH2 820mV
05541-032
M2.00ms
T 640.00µs
1
3
2
AM MODULATED INPUT
AD8317 OUTPUT
ADL5330 OUTPUT
Ch2 200mV
Figure 32. Oscilloscope Screenshot Showing an AM Modulated Input Signal
and the Response from the AD8317