LTC1957-1EMS8#TR Linear Technology, LTC1957-1EMS8#TR Datasheet - Page 10

• Manufacturer Part Number: LTC1957-1EMS8#TR
• Description: IC RF PWR CONTRLLR SINGLE 8-MSOP
• Manufacturer: Linear Technology

Specifications of LTC1957-1EMS8#TR

• Rf Type: Cellular, GSM, GPRS, PCS, Wireless Modem, TDMA
• Frequency: 850MHz ~ 2GHz
• Features: Single Output
• Package / Case: 8-TSSOP, 8-MSOP (0.118", 3.00mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: LTC1957-1EMS8TR

APPLICATIO S I FOR ATIO
LTC1957-1/LTC1957-2
Determining External Loop Gain and Bandwidth
The external loop voltage gain contributed by the RF chan-
nel and directional coupler network should be measured in
a closed loop configuration. A voltage step is applied to
PCTL and the change in V
detected voltage is 0.6 • PCTL and the external voltage gain
contributed by the RF power amplifier and directional
coupler network is 0.6 • V
age gain in the closed loop configuration accounts for the
nonlinear detector gain that is dependent on RF input
voltage and frequency.
The LTC1957 unity gain bandwidth specified in the data
sheet assumes that the net voltage gain contributed by the
RF power amplifier and directional coupler 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 LTC1957 control amplifier unity gain bandwidth (BW1)
is typically 370kHz. The phase margin of the control
amplifier is typically 86 .
10
Figure 3. Measured Open Loop Gain and Phase, PCTL < 640mV
BW1 = 0.35/rise time
–40
–50
–60
–10
–20
–30
80
70
60
50
40
30
20
10
0
100
1k
U
FREQUENCY (Hz)
GAIN
10k
U
PCA
PCTL
PHASE
(or V
100k
/ V
R
C
PCB
LOAD
W
LOAD
VPCA
1M
) is measured. The
= 33pF
= 2k
1957 F03
. Measuring volt-
10M
140
120
100
80
60
40
20
–20
–40
–60
–80
–100
180
160
0
PCA
U
or V
PCB
For PCTL voltages <650mV, the RF detected voltage is
0.6PCTL. For PCTL voltages >650mV, RF detected voltage
is 1.18PCTL – 0.38. This change in gain is due to an
internal compression circuit designed to extend the detec-
tor range.
For example, to determine the external RF channel loop
voltage gain with the loop closed, apply a 100mV step to
PCTL from 300mV to 400mV. V
to supply enough feedback voltage to the RF pin to cancel
this 100mV step which would be the required detected
voltage of 60mV. V
create the RF output power change required. The net
external voltage gain contributed by the RF power ampli-
fier and directional coupler network can be calculated by
dividing the 60mV change at the RF pin by the 30mV
change at the V
then be approximately 2. The loop bandwidth extends to
2 • BW1. If BW1 is 370kHz, the loop bandwidth increases
to approximately 740kHz. The phase margin can be deter-
mined from Figures 3 and 4. Repeat the above voltage gain
measurement over the full power and frequency range.
Figure 4. Measured Open Loop Gain and Phase, PCTL > 640mV
–20
–40
–50
–60
–10
–30
80
70
60
50
40
30
20
10
0
100
PCA
pin. The net external voltage gain would
1k
PCA
changed from 1.498V to 1.528V to
FREQUENCY (Hz)
GAIN
10k
PHASE
100k
PCA
R
C
LOAD
LOAD
(or V
1M
= 33pF
= 2k
1957 F04
PCB
10M
) will increase
180
160
140
120
100
80
60
40
20
0
–20
–40
–60
–80
–100