LTC1757A-1 LINER [Linear Technology], LTC1757A-1 Datasheet - Page 10

• Manufacturer Part Number: LTC1757A-1
• Description: Single/Dual Band RF Power Controllers
• Manufacturer: LINER [Linear Technology]

LTC1757A-1/LT1757A-2
APPLICATIO S I FOR ATIO
transfer functions are often not available and must be
generated by the user. Loop oscillations are most likely to
occur in the midpower range where the external 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 losses supplied by the directional coupler will
improve phase margin. The larger the directional coupler
loss the more stable the loop will become. However, 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 Gain
The external loop gain contributed by the RF channel and
directional coupler network should be measured in a closed
loop configuration. A voltage step is applied to PCTL and
the change in V
voltage is 0.85 • PCTL and the external gain contributed by
the RF power amplifier and directional coupler network is
0.85 • V
configuration accounts for the nonlinear detector gain that
is dependent on RF input voltage and frequency.
The LTC1757A unity gain bandwidth specified in the data
sheet assumes that the net 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
gain with the loop closed, apply a 100mV step to PCTL
from 300mV to 400mV. V
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 gain contributed by the RF power amplifier and
directional coupler network can be calculated by dividing
the 85mV change at the RF pin by the 42mV change at the
V
mately 2. The loop bandwidth extends to 2 • BW1. If BW1
is 400kHz, the loop bandwidth increases to approximately
800kHz. External gains exceeding 6 may cause loop fre-
quency stability problems.
PCA
BW1 = 0.35/rise time
pin. The net external gain would then be approxi-
PCTL
/ V
PCA
VPCA
PCA
(or V
. Measuring gain in the closed loop
changed from 1.498V to 1.540V to
PCB
PCA
) is measured. The detected
(or V
PCB
) will increase to
PCA
or V
PCB