GSM Activity Scanner - background: GSM Interface

The I_GSM interface is the interface with the modified Philips Savvy Dual Band. The GSM is modified by soldering small wires to certain via's and combining the signals using a small AND-port IC. This reduces the number of signals to transport and the IC provides some buffering of the signals. The resulting signals are available on the connector shown on the picture (click picture to enlarge).

The signals are available during all operations of the GSM, which means: during power-up, during scanning the available networks, during logging on to the network, during standby, during an outgoing call, during an incoming call etc.

The "scanning the available networks" phase is used by the GSM Activity Scanner. Nice to know is that this phase is never completed when there is no SIM card in the GSM, this means that the phone will keep on scanning and an connected GSM Activity scanner will be able to keep on processing the results :-)

The IQ data provided on the I_GSM interface is for the receive path only, so only GMSK burst from basestation to the GSM are available on the interface.

Signals on the 10 pins header:

IQ_CK: Clock signal for IQ waveform samples
GSM_D: combined tuning data and IQ sample data
GSM_E: combined tuning Enable and IQ Framing signal (active low)
F_CK: Clock signal for tuning data
RESERVED: One pin on the connector is reserved
GND: 5 pins are grounded to provide shielding during transport from GSM to the device connected to it

A Step-by-Step description can be found in the file "Adding a GSM Interface to a Philips Savvy DB GSM" [PDF]

Extract of the step-by-step description:
Location of the via's and pads:

Notes:
- P_GND, P_VCC are situated on the pads of the EEPROM
- GND is situated anywhere on the edge
- IQ_D, IQ_CK, F_D, F_CK, F_E are situated on via’s
- IQ_FS is situated on a SMD/SMT resistor

Schematic modifications:

Example Data:

Tuning cycle. Area I: rest, Area II & III: tuning sequences, Area IV: rest

IQ sample. Area I: rest, Area II: I Data (15 bits), Area III: Q Data (15 bits), Area IV: I data next sample, etc.

Example Waveforms:

Visualised I and Q Waveforms of a GMSK Burst. Click picture to enlarge

[TXT] IQ sample values for two GMSK bursts (in ascii format)

See also: [wanted: Philips Savvy Dual Band GSM's]