LTC4402-2 Linear Technology, LTC4402-2 Datasheet - Page 8

• Manufacturer Part Number: LTC4402-2
• Description: Multiband RF Power Controllers
• Manufacturer: Linear Technology

LTC4402-1/LTC4402-2
APPLICATIO S I FOR ATIO
Determining External Loop Gain and Bandwidth
The external loop voltage gain contributed by the RF chan-
nel and coupler network should be measured in a closed
loop configuration. A voltage step is applied to PCTL and
the change in V
voltage is 0.6 • PCTL and the external voltage gain contrib-
uted by the RF power amplifier and coupler network is
0.6 • V
loop configuration accounts for the nonlinear detector
gain that is dependent on RF input voltage and frequency.
The LTC4402-X unity gain bandwidth specified in the data
sheet assumes that the net voltage gain contributed by the
RF power amplifier and coupler network is unity. The
bandwidth is calculated by measuring the rise time be-
tween 10% and 90% of the voltage change at V
for a small step in voltage applied to PCTL.
The LTC4402-X control amplifier unity gain bandwidth
(BW1) is typically 450kHz. For PCTL <80mV the phase
margin of the control amplifier is typically 90 .
For PCTL voltages <80mV, the RF detected voltage is
0.6PCTL. For PCTL voltages >160mV, RF detected voltage
is 1.22PCTL – 0.1. This change in gain is due to an internal
compression circuit designed to extend the detector range.
For example, to determine the external RF channel loop
voltage gain with the loop closed, apply a 100mV step to
PCTL from 0mV to 100mV. V
8
Figure 2. Measured Open Loop Gain and Phase, PCTL <80mV
BW1 = 0.35/rise time
PCTL
–10
–20
–30
–40
–50
–60
60
50
40
30
20
10
80
70
0
100
/ V
PCA
VPCA
1k
(or V
U
. Measuring voltage gain in the closed
FREQUENCY (Hz)
PCB
10k
GAIN
U
PHASE
) is measured. The detected RF
PCA
100k
(or V
R
C
LOAD
LOAD
W
1M
PCB
= 33pF
= 2k
4402 F02
) will increase to
10M
–60
–80
–100
–120
160
140
120
100
80
60
40
20
0
–20
–40
PCA
U
or V
PCB
supply enough feedback voltage to the RF pin to cancel
this 100mV step which would be the required detected
voltage of 60mV. Suppose that V
to 1.528V to create the RF output power change required.
The net external voltage gain contributed by the RF power
amplifier and directional coupler network can be calcu-
lated by dividing the 60mV change at the RF pin by the
30mV change at the V
would then be approximately 2. The loop bandwidth
extends to 2 • BW1. If BW1 is 450kHz, the loop bandwidth
increases to approximately 0.9MHz. The phase margin
can be determined from Figures 2 and 3. Repeat the
above voltage gain measurement over the full power and
frequency range.
External pole frequencies within the loop will further
reduce phase margin. The phase margin degradation, due
to external and internal pole combinations, is difficult to
determine since complex poles are present. Gain peaking
may occur, resulting in higher bandwidth and lower phase
margin than predicted from the open loop Bode plot. A low
frequency AC SPICE model of the LTC4402-X power
controller is included (Figures 6 and 7) to better determine
pole and zero interactions. The user can apply external
gains and poles to determine bandwidth and phase mar-
gin. DC, transient and RF information cannot be extracted
from this model. The model is suitable for external gain
evaluations up to 6 . The 270kHz PCTL input filter limits
the bandwidth; therefore, use the RF input as demon-
strated in the model.
Figure 3. Measured Open Loop Gain and Phase, PCTL >160mV
–10
–20
–30
–40
–50
–60
60
50
40
30
20
10
80
70
0
100
1k
PCA
FREQUENCY (Hz)
10k
pin. The net external voltage gain
GAIN
PHASE
100k
PCA
www.DataSheet4U.com
R
C
LOAD
LOAD
changed from 1.498V
1M
= 33pF
= 2k
4402 F03
10M
–60
–80
–100
180
160
140
120
100
80
60
40
20
0
–20
–40
4402f