LTC1758-1 查看數據表（PDF） - Linear Technology

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LTC1758-1

RF Power Controllers with 250kHz Control Loop Bandwidth and 40dB Dynamic Range

Linear Technology 

LTC1758-1/LTC1758-2

APPLICATIO S I FOR ATIO

to allow the autozero to cancel the step. Slow DAC rise

times will extend this time by the additional RC time

constants.

Another factor that affects power ramp sidebands is the

DAC signal to PCTL. The bandwidth of the LTC1758 may

not be low enough to adequately filter out steps associated

with the DAC. If the baseband chip does not have an

internal filter, it is recommended that a 1-stage external

filter be placed between the DAC output and the PCTL pin.

Resistor values should be kept below 2k since the PCTL

input resistance is 90k. A typical filter scheme is shown in

Figure 2.

The power control ramp should be started in the range of

1µs to 10µs after TXEN is asserted high.

LTC1758

2k

DAC

PTCL

330pF

1758 F02

Demo Board

Figure 2

The LTC1758 demo board is available upon request. The

demo board has a 900MHz and an 1800MHz RF channel

controlled by the LTC1758. Timing signals for TXEN are

generated on the board using a 13MHz crystal reference.

The PCTL power control pin is driven by a 10-bit DAC and

the DAC profile can be loaded via a serial port. The serial

port data is stored in a flash memory which is capable of

storing eight ramp profiles. The board is supplied preloaded

with four GSM power profiles and four DCS power profiles

covering the entire power range. External timing signals

can be used in place of the internal crystal controlled

timing. A variety of RF power amplifiers are available.

LTC1758 Control Loop Stability

The LTC1758 provides a stable control loop for several RF

power amplifier models from different manufacturers

over a wide range of frequencies, output power levels and

VSWR conditions. However, there are several factors that

can improve or degrade loop frequency stability.

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 LTC1758

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 VSWR conditions. The oscillation fre-

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 3.3µF.

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.

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