Typical Ti Calypso baseband modem design

This is the standard Ti Calypso-based modem design used in many phones, including the Compal-built Motorola C1xx phones and
also the Openmoko GTA01 (Neo1973) and GTA02 (FreeRunner) and possibly a number of other older FIC GSM products.

It is a typically a dual- or tri-band GSM modem design with or without support for GPRS.

Block Schematic

The block layout of this modem looks like this: calypso-block.png

  • Yellow: Clocks
  • Red: Digital Serial interfaces like SPI
  • Green: Analog I/Q differential baseband data
  • Magenta: RF signals
  • Blue: TSP, the time sequence port, sometimes parallel, sometimes serial

Circuit Description

This is only a simplified version, ignoring the time constraints, sequencing of power-on/off events, AFC, AGC and APC.

GSM Rx Path

  • the GSM signal from a BTS is picked up by the antenna
  • it reaches the Antenna Switch Module (ASM), typically a diode or MEMS switch
    • the Antenna Switch is configured to connect the antenna to one of the GSM/DCS/PCS Rx paths
  • the RF signal goes through Rx SAW filters to remove any out-of-band frequencies
  • the RF signal reaches the TRF6151/Rita zero-IF GSM Transceiver, where it is
    • amplified and further filtered
    • mixed with the frequency of the TRF6151-builtin VCO
    • exported as analog I/Q signals
  • The analog I/Q is input into the TWL3025/Iota ABB, where it is
    • sampled by an ADC
    • sent as serial stream of I+Q samples to the DBB via the BSP
  • In the DBB, the signal is
    • received on the RIF (Radio Inter Face) and DMA'ed into DSP API RAM
    • processed by the HardwareCalypsoDSP DSP core inside the Calypso DBB
    • converted into results (e.g. a MAC block) that is sent to the ARM via API RAM
  • The ARM in the Calypso DBB then runs the GSM protocol stack

GSM Tx Path

  • The ARM inside the Calypso DBB
    • generates some data (e.g. a MAC block) to be transmitted
    • writes this data plus associated commands in the API RAM
  • The DSP inside the Calypso DBB
    • executes this command during the next TDMA frame interrupt
    • performs forward error correction, interleaving, encryption (optional)
    • sends the GSM burst bits via the BSP to the ABB
  • The TWL3025/Iota ABB
    • receives burst bits via BSP and stores them in the burst buffer
    • runs those burst bits through a hardware GSMK modulator when triggered by BULENA on the TSP
    • outputs an Analog I/Q baseband GMSK signal to the TRF6151
  • The TRF6151/Rita Transceiver
    • mixes the analog I/Q baseband signal with the VCO frequency
    • sends the resulting GSM-band frequency to the RF3166
  • The RF3166 RF Power Amplififer
    • amplifies the signal according to the analog level of the APC
    • forwards the amplified signal to the Antenna Switch
  • The ASM4532 antenna switch
    • connects the PA output with the antenna for the duration of the burst


  • ABB: Analog Base Band
  • AFC: Automatic Frequency Correction (tuning of the VTXCO by ABB)
  • APC: Automatic Power Correction (Tx Power envelope from ABB to PA)
  • BSP: Baseband Serial Port, like SPI
  • DBB: Digital Base Band, like SPI
  • USP: uController Serial Port
  • VCO: Voltage Controlled Oscillator
  • RFCLK: A 26MHz master clock generated by the Transceiver
  • CLK13M: 13MHz system clock provided by DBB
  • CLK32K: A 32.768kHz RTC clock signal

Actual implementation in Motorola C123


Updated by zecke almost 8 years ago · 12 revisions

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