Transmitter

Transmitter chain consists of final frequency IQ-modulator, dualband power amplifier and a power control loop.

I- and Q-signals are generated by baseband also in COBBA-ASIC. After post filtering (RC-network) they go into IQ-modulator in HAGAR. LO-signal for modulator is generated by VCO and is divided by 2 or by 4 depending on system mode, EGSM/DCS1800. After modulator the TX-signal is amplified and buffered. There are separate outputs for both EGSM and DCS1800. HAGAR TX output level is 5 dBm.

Next TX signals are converted to single ended by baluns. Then TX signals are amplified and buffered in discrete buffers. In EGSM branch there is a SAW filter after the balun to attenuate unwanted signals and wideband noise from the Hagar IC.

The final amplification is realized with dualband power amplifier. It has one 50 ohm input and two 50 ohm outputs. There is also a gain control, which is controlled with a power control loop in HAGAR. PA is able to produce over 2 W (3 dBm input level) in EGSM band and over 1 W (6 dBm input level) in DCS1800 band into 50 ohm output. Gain control range is over 35 dB to get desired power levels and power ramping up and down.

Harmonics generated by the nonlinear PA are filtered out with the diplexer inside the RX/ TX switch-module.

Power control circuit consists of discrete power detector (common for EGSM and DCS1800) and error amplifier in HAGAR. There is a directional coupler connected between PA output and RX/TX switch. It is a dualband type and has input and outputs for both systems. Dir. coupler takes a sample from the forward going power with certain ratio. This signal is rectified in a schottky-diode and it produces a DC-signal after filtering.

This detected voltage is compared in the error-amplifier in HAGAR to TXC-voltage, which is generated by DA-converter in COBBA. TXC has got a raised cosine form (cos4 - function), which reduces switching transients, when pulsing power up and down. Because dynamic range of the detector is not wide enough to control the power (actually RF output voltage) over the whole range, there is a control named TXP to work under detected levels. Burst is enabled and set to rise with TXP until the output level is high enough, that feedback loop works. Loop controls the output via the control pin in PA to the desired output level and burst has gotthe waveform of TXC-ramps. Because feedback loops could be unstable, this loop is compensated with a dominating pole. This pole decreases gain on higher frequencies to get phase margins high enough. Power control loop in HAGAR has two outputs, one for each freq. band.

DIR.COUPLER

DIR.COUPLER

RF IN

DOMINATING POLE

Figure 21: Transmitter

RF IN

DOMINATING POLE

Figure 21: Transmitter

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