in the following description , reference will be made to a dual - mode mobile telephone comprising a modulation / demodulation circuit constituted by a first dsp signal digital processor that performs tasks of modulation and demodulation when the telephone is connected to the gsm network , and a second modulation / demodulation circuit constituted by a second dsp signal digital processor that performs tasks of modulation and demodulation when the telephone is connected to the umts network . with reference to fig1 , the mobile telephone includes a central unit 2 coupled via a data bus 4 to the first dsp signal digital processor 6 , to the second dsp signal digital processor 8 , and to a synchronization module 10 . the synchronization module 10 is an electronic card allowing the mobile telephone to be synchronized selectively with a base station of the gsm network or with a base station of the umts network . in a preferred embodiment of the invention , the synchronization module 10 includes a first counter 12 coupled to a first digital clock 14 , and a second counter 16 coupled to a second digital clock 18 . the first digital clock 14 is synchronized with the gsm network whereas the second digital clock 18 is synchronized with the umts network . the synchronization module 10 further includes a control buffer register 19 coupled to each of the counter 12 and 16 . in a second embodiment illustrated in fig2 , the mobile telephone comprises a first analog clock 20 actuated and coupled to the first modulation / demodulation circuit 6 through a first digital - analog converter ( cna ) 22 , and a second analog clock 24 actuated and coupled to the second modulation / demodulation circuit 8 through a second digital - analog converter ( cna ) 26 . as described above , at a given moment , one of the modulation / demodulation circuits 6 or 8 is active and the other is inactive . consequently , at that moment , solely communication with the network associated with the active modulation / demodulation circuit ( gsm or umts ) is possible , while the inactive modulation / demodulation circuit can only perform measurements on neighboring cells belonging to the other network ( gsm or umts ). however , a condition for ensuring frequency synchronization with the neighboring base station to be as precise as possible is exactly that the central unit 2 be able to read regularly information on the downlink channels that the mobile is listening . in order to accomplish this synchronization , the method according to the invention includes the following steps : measuring a first frequency difference between the active modulation / demodulation circuit and the corresponding radio access network ( gsm or umts ), estimating a second frequency difference between the clocks of the base stations of the gsm and umts networks , calculating a third frequency difference between the passive modulation / demodulation circuit and the passive access network according to the first and second frequency differences , using this third frequency difference to synchronize the terminal with the passive network . once this synchronization is accomplished , it is possible to evaluate the quality of the communication over the passive network and compare the measured quality of the communication with the current communication quality . if the measured quality of communication is better than the quality of the communication over the active network used by the terminal , the central unit 2 automatically switches the link to the radio access network that has been passive . in operation , the counters 12 and 16 are initialized and then activated by a command generated by the central unit 2 , and then stopped by this central unit 2 . the value of the counter 12 is then read and stored in a gsm_counter memory while that of the counter 18 is then read and stored in a umts_counter memory . if for example the modulation / demodulation circuit 8 of umts is the one that is active , and if the relative frequency error between the 19 . 2 mhz clock 18 ( alternatively 24 ) and the clock of the base station of the current cell in the umts network net_clock is defined by δf u , the frequency error between the clock of the modulation / demodulation circuit 6 and the clock of the current base station denoted as net_clock is calculated by the equation : gsm_clock net_clock = gsm_counter umts_counter × umts_clock ⁢ _ideal + δ ⁢ ⁢ f u umts_clock ⁢ _ideal [ 1 ] if on the contrary , the modulation / demodulation circuit 6 of gsm is the one that is active , and if the relative frequency error between the 13 mhz clock 14 ( alternatively 20 ) and the clock of the base station of the current cell in the gsm network net_clock is defined by δf g , the frequency error between the clock of the modulation / demodulation circuit 8 and the clock of the current base station denoted as net_clock is calculated by the equation : umts_clock net_clock = umts_counter gsm_counter × gsm_clock ⁢ _ideal + δ ⁢ ⁢ f g gsm_clock ⁢ _ideal [ 2 ] the equations [ 1 ] and [ 2 ] thus make it possible to deduce the temporal offset between the base station of the active network and the passive modulation / demodulation circuit . knowing the temporal offset between the base stations of the two radio access networks , the offset between the base station of the passive network and the corresponding modulation / demodulation circuit is deduced . fig3 illustrates the method used for estimating the duration of calibration . the period t cal during which the two counters are active is estimated by the central unit 2 with the desired accuracy δf . this estimation is accomplished , assuming the slower 13 mhz clock to be the “ master ” clock and the faster 19 . 2 mhz clock to be the “ slave ” clock . let the maximum possible time drift during the calibration period t cal be represented by δt max , the 19 . 2 mhz clock starts counting at the rising edge of the first pulse of the 13 mhz clock and stops at the rising edge of the last pulse as illustrated in fig3 . these parameters satisfy the following equation : given that the resolution of the counters is equal to the period of the faster clock , that is the one at 19 . 2 mhz , the maximum time drift that may be tolerated is typically , setting a margin of one supplementary clock period , then the maximum time drift is finally , according to the technical specifications of umts and gsm , the frequency accuracy δf should be within 0 . 1 ppm . consequently , the calibration time may be estimated from the expression : in other words , the counters 12 and 16 should be active for a period of 13 . 52 million cycles of the 13 mhz clock to ensure a frequency accuracy of 0 . 1 ppm . sizes of the counters are selected according to the time period during which calibration is performed . thus , in the above example , the gsm counter 12 should be able to count 13 . 52 million cycles of the 13 mhz clock , which is possible with a counter of 24 bits . similarly , the umts 16 counter should be able to count 19 . 968 million cycles of the 19 . 2 mhz clock for a period of 1 . 04 s , which is possible with a counter of 25 bits . the process of calibration should take place in advance , before the inactive modulation / demodulation circuit takes measurements on neighboring cells . in the above example , calibration should be performed at least 1 . 04 s before measurements on neighboring cells take place .