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PCF5077T

Power amplifier controller for GSM and PCN systems

Philips Electronics 

Philips Semiconductors

Power amplifier controller for GSM and

PCN systems

Preliminary specification

PCF5077T

APPLICATION INFORMATION

Direct power control with ramping function (DC = 1)

The circuit offers a useful feature to control power levels

close to the saturation region of the external power

module.

This flexibility consists in the direct control on the power

level by setting bit DC to logic 1.

In this condition, the external control loop is switched off by

disabling the gain path from OP1. The ramping shape of

the signal to be transmitted as well as its final level are

driven only by the internally generated control signal from

the slope generator. In this way transient effects to recover

active components from deep saturation are avoided.

The relative error on the absolute value of output power is

quite limited, as a power amplifier is less sensitive to

temperature variation in its saturated region. However, this

way of operating may increase the phase error.

Increased dynamic range

The PCF5077T is able to control a dynamic range of

30 dBm by switching the gain factor of the sensor amplifier

and the resolution of DAC8. This range corresponds to a

maximum peak-to-peak voltage of 3 V measured at the

sensor diode. Figure 7 shows the voltage at the sensor

diode (VS) versus the output power (P) of the Power

Amplifier (PA) with a directional coupler of 20 dB

attenuation. The maximum voltage of 3 V is reached when

the output power is 35 dBm.

The sensor voltage for power level lower than 13 dBm, as

necessary for GSM Phase 2 and DCS1800, is lower than

200 mV. An 8-bit DAC would not be sufficient to cover the

complete dynamic range. Therefore bits DR0 and DR1 are

used to switch the power range that can be controlled with

the controller (see Table 7).

REDUCED VOLTAGE STEPS OF POWER LEVEL DAC8

(DR0 = 1)

The DR0 bit is used to switch resistor R9 (switch DR0 is

closed) at the integrator input (OP4). The ratio of the DAC8

range to the sensor signal voltage is therefore halved and

the power corresponding to one LSB of DAC8 is reduced

by 3 dB. With this setting the power module can be

controlled more accurately for low output power levels.

GAIN FACTOR OF OP1 (DR1)

Bit DR1 switches (switch DR1 is closed) the ratio of the

capacitances at OP1. The gain factor for the sensor

amplifier is five times higher when DR1 is in high state.

When DR1 = 1, the control loop regulates the output

power of the PA to a lower power level. A dynamic range

of about 10 dBm can be switched by this manner.

Vs : Vpeak is the ratio of sensor signal to slope generator

output voltage effective at the integrator output (OP4).

Table 7 Gain factors

DR1

0

0

1

1

DR0

0

1

0

1

VS : Vpeak

1:1

2:1

5:1

10 : 1

Additional application information

Evaluation kits with software and demonstration board are

available for the PCF5077T together with Philips power

modules BGY206, CGY2010, CGY2020 and CGY2021 for

GSM and PCN, which will provide help for applications.

Very little bus traffic is required for the PCF5077T because

the ramping curves are generated on-chip. VKICK and

VHOME define the start conditions for up-ramping.

VPL determines the power levels. TRIG is the trigger for up

and down-ramping.

The non-linear behaviour of the control curves of the

power modules have a big influence on the loop. Start

conditions in the flat area of the control curve are critical

and need some attention. Initially VINT(O) will be at the

home position. The HPA switches release the regulator.

The integrator is moved into the active part of the control

curve. This is achieved by integrating VKICK. When VINT(O)

has reached the active region of the control curve the loop

is closed and the circuit is able to follow the ramping

function generated by a voltage step to the slope

generator. The step height VPL determines the power of

the transmit burst. Down-ramping is started at the slope

generator input by a voltage step from VPL back to VQRS.

The loop follows the leading function for down-ramping

until the RF sensor measures zero. The reason for VQRS is

to shorten the tail of the slope.

Figures 8 and 9 show the results of measurements on the

up and down-ramping where REF is the reference level of

the power in the time slot, ATTEN is the attenuation of the

input instrument for not to destroy the instrument itself,

RES BW is the resolution bandwidth, VBW is the video

bandwidth, CENTER is the carrier frequency for the burst

that has been measured and SWP is the sweep time used

for the measurement.

1997 Nov 19

15

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