LTC1757A-1EMS8 Linear Technology, LTC1757A-1EMS8 Datasheet - Page 10

• Manufacturer Part Number: LTC1757A-1EMS8
• Description: IC CTRLR RF POWER SINGLE 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1757A-1EMS8

• Rf Type: Cellular, GSM, 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

LTC1757A-1/LTC1757A-2
APPLICATIO S I FOR ATIO
1) The additional voltage gain supplied by the RF power
amplifier increases the loop gain raising poles normally
below the 0dB axis. The extra voltage gain can vary
significantly over input/output power ranges, frequency,
power supply, temperature and manufacturer. RF power
amplifier gain control transfer functions are often not
available and must be generated by the user. Loop oscil-
lations are most likely to occur in the midpower range
where the external voltage gain associated with the RF
power amplifier typically peaks. It is useful to measure the
oscillation or ringing frequency to determine whether it
corresponds to the expected loop bandwidth and thus is
due to high gain bandwidth.
2) Loop voltage losses supplied by the directional coupler
will improve phase margin. The larger the directional
coupler loss the more stable the loop will become. How-
ever, larger losses reduce the RF signal to the LTC1757A
and detector performance may be degraded at low power
levels. (See RF Detector Characteristics.)
3) Additional poles within the loop due to filtering or the
turn-on response of the RF power amplifier can degrade
the phase margin if these pole frequencies are near the
effective loop bandwidth frequency. Generally loops using
RF power amplifiers with fast turn-on times have more
phase margin. Extra filtering below 16MHz should never
be placed within the control loop, as this will only degrade
phase margin.
4) Control loop instability can also be due to open loop
issues. RF power amplifiers should first be characterized
in an open loop configuration to ensure self oscillation is
not present. Self-oscillation is often related to poor power
supply decoupling, ground loops, coupling due to poor
layout and extreme V
quency is generally in the 100kHz to 10MHz range. Power
supply related oscillation suppression requires large value
ceramic decoupling capacitors placed close to the RF
power amp supply pins. The range of decoupling capacitor
values is typically 1nF to 3.3 F.
5) Poor layout techniques associated with the directional
coupler area may result in high frequency signals bypass-
ing the coupler. This could result in stability problems due
to the reduction in the coupler loss.
10
U
SWR
U
conditions. The oscillation fre-
W
U
Determining External Loop Voltage 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 0.85 • PCTL and the external voltage
gain contributed by the RF power amplifier and directional
coupler network is 0.85 • V
age gain in the closed loop configuration accounts for the
nonlinear detector gain that is dependent on RF input
voltage and frequency as well as RF channel gain peaking.
The LTC1757A 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 LTC1757A control amplifier unity gain bandwidth
(BW1) is typically 400kHz. The phase margin of the control
amplifier is typically 86 .
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 85mV. 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 85mV change at the RF pin by the 42mV
change at the V
then be approximately 2. The loop bandwidth extends to
2 • BW1. If BW1 is 400kHz, the loop bandwidth increases
to approximately 800kHz. The phase margin can be deter-
mined from Figure 4. Repeat the above voltage gain
measurement over the full power and frequency range.
BW1 = 0.35/rise time
PCA
pin. The net external voltage gain would
PCA
changed from 1.498V to 1.540V to
PCA
PCTL
(or V
PCA
/ V
PCB
(or V
VPCA
) is measured. The
PCB
. Measuring volt-
) will increase
PCA
or V
PCB