CN-0082 Datasheet, PDF (2/4 Pages) Analog Devices – Creating a Constant Envelope Signal Using the ADL5331 RFVGA and AD8319 Log Detector

CN-0082
amplifier output) until the level at the RF input corresponds to
the applied setpoint voltage. GAIN settles to a value that results
in the correct balance between the input signal level at the
detector and the setpoint voltage.
The basic connections for operating the ADL5331 in an AGC
loop with the AD8319 are shown in Figure 1. The AD8319 is a
1 MHz to 10 GHz precision demodulating logarithmic
amplifier. It offers a detection range of 45 dB with ±0.5 dB
temperature stability. The VOUT pin of the AD8319 controls the
GAIN (gain control) pin of the ADL5331. When the AD8319 is
in controller mode, as it is in this application, VOUT on the
AD8319 can drive the ADL5331 GAIN pin over its full linear
range of 0 V to 1.4 V. Under very low power RF in conditions,
outside the linear control range of the loop, VOUT on the
AD8319 may be driven to its maximum value very close to
VPOS. To avoid overdrive recovery issues with the ADL5331
GAIN input, a voltage divider can be placed between VOUT on
the AD8319 and GAIN on the ADL5331. This may have a slight
effect on the overall speed of the loop, for instance, when the
input power to the ADL5331 is stepped.
A coupler/attenuation of 23 dB is used to match the desired
output power range from the VGA to the linear operating
range of the AD8319. In this case, the desired output power
range of the VGA is −15 dBm to +15 dBm. With the given
attenuator/coupler, the range of power to the AD8319 RF input
is −8 dBm to −38 dBm, within the specified range of −3 dBm to
−43dBm for a ±1 dB error.
The detector’s error amplifier uses CLFP, a ground-referenced
capacitor pin, to integrate the error signal (in the form of a
current). A capacitor must be connected to CLFP to set the
loop bandwidth and to ensure loop stability.
Figure 2, Figure 3, and Figure 4 show the transfer function of
the ADL5331 output power vs. the AD5621 DAC code for a
100 MHz sine wave with an input power of 0 dBm, −10 dBm,
and −20 dBm. Note that the power control of the AD8319 has a
negative sense. Decreasing the DAC code, which corresponds
to demanding a higher signal from the ADL5331, tends to
increase GAIN.
In order for the AGC loop to remain in equilibrium, the
AD8319 must track the envelope of the ADL5331 output signal
and provide the necessary voltage levels to the ADL5331’s gain
control input. Figure 5 shows an oscilloscope screenshot of the
AGC loop in Figure 1. A 100 MHz sine wave with 50% AM
modulation is applied to the ADL5331. The output signal from
the ADL5331 is a constant envelope sine wave with amplitude
corresponding to a setpoint voltage at the AD8319 of 1.5 V.
Also shown is the gain control response of the AD8319 to the
changing input envelope.
Circuit Note
20
5.0
POWER OUT
10
STRAIGHT LINE
ERROR
2.5
0
–10
0
–20
–2.5
–30
–40
1024
1524
2024
2524
AD5621 DAC CODE
3024
–5.0
3524
Figure 2. ADL5331 Power Out vs. AD5621 DAC Code with
RF Input Signal = 0 dBm
20
5.0
POWER OUT
10
STRAIGHT LINE
ERROR
2.5
0
–10
0
–20
–2.5
–30
–40
–5.0
1280
1780
2280
2780
3280
3780
AD5621 DAC CODE
Figure 3. ADL5331 Power Out vs. AD5621 DAC Code with
RF Input Signal = −10 dBm
20
5.0
POWER OUT
10
STRAIGHT LINE
ERROR
2.5
0
–10
0
–20
–2.5
–30
–40
–5.0
1280
1780
2280
2780
3280
3780
AD5621 DAC CODE
Figure 4. ADL5331 Power Out vs. AD5621 DAC Code with
RF Input Signal = −20 dBm
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