The receiver is a direct conversion, dualband linear receiver. Received RF-signal from the antenna is fed via RX/TX switch to 1st RX SAW filter and LNA's (low noise amplifier), separate branches for EGSM900 and DCS1800. Gain selection control of LNA's comes from HAGAR IC. Gain step is activated when RF-level at the antenna is about -45 dBm.
After the LNA the amplified signal (with low noise level) is fed to bandpass filter (2nd RX SAW filter). RX bandpass filters defines how good supperssion of blocking signals outside receive band and the protection against spurious responses.
After the bandpass filters the signals are fed to baluns which converts the single ended signal to ballanced. The balanced signal is fed to the RF input of Hagar. Differential RX signal is amplified and mixed directly down to BB frequency in HAGAR. Local signal is generated with external VCO. VCO signal is divided by 2 (DCS1800) or by 4 (EGSM900). PLL and dividers are in HAGAR-IC.
From the mixer output to ADC input RX signal is divided into I- and Q-signals. Accurate phasing is generated in LO dividers. After the mixer DTOS amplifiers convert the differential signals to single ended. DTOS has two gain stages. The first one has constant gain of 12dB and 85kHz cut off frequency. The gain ofsecond stage is controlled with control signal g10. If g10 is high (1) the gain is 6dB and if g10 is low (0) the gain of the stage is -4dB.
The active channel filters in HAGAR provides selectivity forchannels (-3dB @ +/-100 kHz typ.). The integrated base band filter is an active-RC-filter with two off-chip capacitors. Large RC-time constants are needed in the channel select filter ofthe direct conversion receiver. These are produced with large off-chip capacitors. The Baseband filter consists of two stages, DTOS and BIQUAD. The DTOS is a differential to single-ended converter having 8dB or 18dB gain. The BIQUAD is modified Sallen-Key Biquad.
Integrated resistors and capacitors are tunable. These are controlled with a digital control word via Hagar serial interface. The correct control words that compensate for the process variations of integrated resistors and capacitors and of tolerance of off chip capacitors are found during RX filter calibration.
Next stage in the receiver chain is the AGC-amplifier, also integrated into HAGAR. The AGC has digital gain control via serial mode bus from COBBA IC. The AGC-stage provides gain control range (40 dB, 10 dB steps) for the receiver and also the necessary DC compensation. One 10 dB AGC step is implemented in DTOS stages.
DC compensation is made during DCN1 and DCN2 operations (controlled via serial bus). DCN1 is carried out by charging the large external capacitors in the AGC stages to a voltage which cause a zero dc-offset. DCN2 set the signal offset to constant value (VREF RX 1.2 V). The VREF RX signal (from COBBA GJP) is used as a zero level to RX ADCs.
Single ended filtered I/Q-signal is then fed to ADCs in COBBA-IC. Input level for ADC is 1.4Vpp max.
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