Nokia C250-Mk2 Car Phone Teardown - Page 1
Forum section: electronics / general

author: sonnytiger
author_rank: Frequent Contributor
timestamp: January 31, 2015, 11:05:08 pm
content: Pretty cool inside, I'd like to do something with it, any suggestions?I guess a start would be to try and figure out how the handset part communicates with the base.

author: SeanB
author_rank: Super Contributor
timestamp: February 01, 2015, 06:20:27 am
content: Car kits were allowed to go up to 25W transmit power, but typically operated at 1W or so most of the time, as they had a good antenna. The control uses the microcontroller on the board to do the transmit receive switching in synchrony with the handset ( they communicate via a 1-2 wire interface on the link, so that the phone knows to turn off it's antenna and to accept audio from the car kit and send audio there, along with then doing the car specific functions like mute the radio, respond to the commands from the car kit for volume and such) so that the GSM/CDMA frames are properly handled. The switching is done partly by the large sealed module, which has a set of PIN diodes to isolate the transmit and receive filters ( which have further pin diodes to select the various GSM/CDMA channels in the 800-900MHz spectrum allocated for GSM/CDMA) and switch rapidly between the receive pass through ( most of the time) for the very short transmit pulse in the allocated time slot for the handset.The Exar chipset is likely the audio hands free kit, having AGC, echo cancellation, bandpass filtering and muting to get the audio from the microphone mounted near the driver and remove the speaker audio ( mostly) and send it up the cable to the handset, with the handset audio coming down another wire to it for the speaker ( it has an amplifier of around 5W there, with a relay to disconnect one car speaker to use for audio, or uses a mute line on the radio to mute it and use a separate speaker) for the other party audio. There is also going to be some circuitry to charge the phone via the cradle, and to sense car state, turning off the charge when the ignition is off unless the phone battery status is low, and general power supply for the parts.Sadly not much use any more, as the 800MHz band is only rarely used these days, most phones using higher bands to get more capacity. Only a few small carriers use it, as the equipment for it, though very expensive new, is cheap as used but working stuff from other operators upgrading systems to increase capacity. Many phones no longer have a 800MHZ Rf section as well. Only bonus with 800MHz is that the cells can be very large, which is an advantage in rural areas where most of the old systems are, as it covers a much larger area but must have a lower customer density to avoid overloading the base station. You can easily have a 60km range from such a cell with little dropout, though at 60km the handset will be transmitting at the full 5W of power. Most customers with that use have fixed high gain directed antennas pointed at the tower and a very local passive dipole joined by ultra low loss coax, using the phone as a semi fixed base station.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: February 01, 2015, 06:34:39 am
content: Wow thank you very much Sean, very informative. So the phone isn't purpose designed for the station? huh. So if I can get my hands on a Nokia phone around the same era I might be able to connect it to the station?The FCC ID also says 3W so I'm not sure. How does it know to increase transmitter power?

author: SeanB
author_rank: Super Contributor
timestamp: February 01, 2015, 07:10:57 am
content: Nokis phones with the multipin connector at the bottom all will work with this unit, provided you have the right cable. Transmit power is controlled by the phone, depending on the base station received signal strength. The receiver has a signal strength, but transmit power is only loosely based on this, the base station runs a control loop in the data control channel to all phones to get handsets to reduce power to a minimum, so that all signals received by it are at roughly the same level, so the AGC of the base station is not swamped. That keeps signal received at about 6-10dB above the noise floor in the base station so it can recover data well, but does not have a large variation in receive power between time slots which the AGC cannot respond to. The phone AGC will be able to control signal well as it only is switched on a few ms before the next receive slot to save power. After receive there is a slot in the frames for it to transmit so swamping the receiver is not a problem, as there is time to recover before it will need to listen again to either data or control information.The audio out of the processor on the board is very low noise, the phone itself actually adds noise so that there is always a small amount of data to send even during mute, as the base station controls do not react well to a non transmitted data block ( no data is sent if there is silence, to save transmit power) so there is always a small compressed block sent, and low level noise does not need any special detection and processing, it is ignored by the human ear. There is a lot done in the system to make it reliable. Your SMS is sent as there is a 140 character block unused in each control block for each phone, so it is used for SMS data to and from the network. There is actually no cost for SMS, it is a part of the spec and the equipment to handle it is required in any case, it is just a cash cow. Low rate data (GPRS) is sent as speech packets with a header bit, so that is why you can still send SMS and get GPRS data even if there is no reliable speech connection. Those packets are always acknowledged, and resent in a slot if they are marked as not received correctly. Speech is just dropped if not correct CRC, as a silence is better in a real time stream than noise.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: February 01, 2015, 05:56:40 pm
content: Wow this is good stuff, thanks!