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GSM phones structure

When switching on the phone, the low-frequency part starts working (i.e. the processor), its external activity demonstration are the lighted screen, music sounds or simply squeak and LED blinking (in outdated models). But this is only the outward of the question and while the phone sings and dances, the processor is occupied with reading of the SIM-card information and configuring phone's radio part.

The receiver switches on under the processor control and starts listening immediately to the GSM range air, searching the information about around networks. Such information is broadcasted continuously by all networks. If there is the native SIM-card network (or roaming network), the phone chooses a base station with optimum signal and begins information interchange between the phone and the network. The phone tunes its frequency and time base under supplying it base station and requests permission to registration.

After SIM-card check the phone is registered in the network and if nothing more interesting doesn't happen around, starts dozing. It isn't absolutely clearly, why this condition is calls “ deep sleep ”, it is similar to the deep sleep just as the sleep of the person who jumps each five minutes and runs to the door with a cry “Who is it? ”, but, having understood that there is nobody, falls asleep again. In case with the phone, it wakes up for a moment to find out, whether there is incoming call. The deep sleep is very important for battery longevity.

In terminology GSM phone (terminal or mobile equipment) is only a part of mobile station, the SIM-card is its second part. Frankly speaking, I don't know, who uses this terminology. As the experience shows, it is absolutely unnecessary - one friend of mine calls any hardware "thing". I'll make a remark, this lady is the project manager in mobile communication company. I'll add for the completeness that she is quite good manager.

In the phone is usually distinguished high-frequency (Radiofrequency, RF) and low-frequency (BaseBand, BB) parts. The conditional border passes by IQ-signals (In-phase, Quadrature) - see pic. 1.

At the receiver output IQ-signals are low-frequency (restituted) signals which bear information. They are worked up by the processor and the information reaches the user in the sound or visual form. The situation is diametrically opposite at the transmitter input - in this case this is encode information which you want to transfer, for example voice during phone conversation.

The transceiver is controlled completely by the processor which, programs synthesizer frequencies, sets gain factor and ranges, controls the amplifier and the aerial switch, adjusts the quartz generator frequency and is engaged in many useful things.

Traditionally in the radio part (further - the radio) are discerned the receiver, the transmitter, the stabilized power sources, the frequency synthesizers block, the transmitting and the quartz generator.

Until recently the receiver and the transmitter (except for the output amplifier) were made as one microcircuit (this microcircuit is frequently called the transceiver), frequency synthesizers - as the other, and the transmitting generator and the amplifier - as the microassembly. The band-pass filters of the reception section are made with the help of the superficial acoustic waves technology. The driving quartz generator has the standard frequency of 13 or 26 MHz.

Already now the transceiver, the synthesizers and the transmitting generator are integrated in one microcircuit, and the amplifiers are produced with the built - in amplification controller. The following step will be the integration of the amplifier and the aerial switch in one case. All the progressive mankind in the person of radio engineers waits with impatience and trembling for the moment when the well known Japanese company will put this chipset into mass production.

The low-frequency part can be subdivided into the digital-to-analog interface, the stabilized power sources, the battery charging circuit, the flash-memory and the processor with data transmission ports (UART, IrDA), SIM, keyboard and screen interfaces, the data and address bus.

The receivers are produced by the superhet circuit or the direct converting circuit, the last one becomes more popular and already is now widely extended. As an example there is photo of the high-frequency part of dual band GSM module with superhet receiver. Any GSM phone will be constructed by one of these circuits and will have the same elements, but, the choice of the concrete details and their placement on the board depends on the designer engineer.

For the prevention of the various phone parts influence on each other, it is closed with the metal shield. Apply integral shields or construction with lateral walls and cover. There are also more exotic decisions when the shield is stamped from rather soft material and fastened, instead of soldering to the printed-circuit-board.

In the pic. # 3 is shown the same module as in pic. #2, but with shielding walls.

On the return side of the board is installed low-frequency processing part.

The full size of the device is 45х31х5.5 mm. When using the direct converting receiver, more integrated transceiver and the amplifier with built-in amplification detector - controller, it is also possible to place the low-frequency section on the one board side with the size of 45х31 mm and to reduce the device thickness up to 3.2 mm.

The entire device is assembled under the superficial installation circuit, when the solder is applied as the paste on the printed-circuit-board, and details "are simply pasted" from above. Then the heating is made and the paste turns to metal. The circle seen in the middle of the shielding structure in pic. # 3 is needed for the pick-and-place machine.

Now the standard components size is 0402 - 1х0.5mm, the resistors and the condensers of 0201 - 0.6х0.3mm size are more often applied.

After the assembly the phone is calibrated and tested. By all accounts, the phones of one first-rate Finnish manufacturer aren't calibrated due to very stable design. I think it's unlikely, because the components manufacturers frequently surprise the developers with the wide scatter of parameters which always is «within the limits».

It is not less difficult developing stably working test equipment, than a mobile phone.

I repeatedly received letters with questions about the European phones certification. I'll take the occasion and briefly describe the situation. In order to the phone would be allowed to sale, it should pass through a series of tests described in ETSI TS 151010-1 document, it's known among engineers under the name "11.10". This document contains about 4300 pages. All new phones pass through these tests. I'll make a reservation, that if the simply minor alteration in design of electronics takes place, or of the phone case, the procedure becomes simpler. The phone will be certificated entirely, therefore any changes made without the knowledge or approval of the manufacturer can result in badly working device.

This also concerns to the software replacement. I have already mentioned the calibration of phone. In the beginning the standard transceiver adjustments are set up in the phone, but during calibration they are replaced on corresponding to the individual device. At correct software replacement these individual data are taken from phone and after replacement are saved back or the memory area where these data are stored is not used. Any initiative can result that in your will be only the standard data which can be completely bad for your concrete device.

Sergey Senin (sergei.senin@ubinetics.com)
Translated by Andreas Von Horn (andreas@mobile-review.com)

Published — 03 June 2003

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