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:

• 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

Glossary¶

• 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¶