Transmission system with high frequency stability

1. External unit placed on the subscriber side of a transmission system using at least one reference subcarrier placed at various locations in a transmission frequency band, the transmission frequency band furthermore comprising at least one data-carrier downlink channel, wherein the external unit comprises:

a reference oscillator for generating a frequency reference signal;

a local oscillator for generating a first transposing signal for transposing the transmission frequency band into a transposed frequency band;

a frequency synthesizer for providing a frequency transposition signal coupled to the reference oscillator;

additional transposition means for mixing and filtering the transposed reference subcarrier with the frequency transposition signal generating an additionally transposed reference subcarrier in a predefined frequency band;

synchronization means for comparing the phase of the additionally transposed reference subcarrier and the frequency signal, carrying out a frequency feedback control signal to synchronize the local oscillator; and

a latching device placed between the local oscillator and the synchronization means so that the latching device drives the local oscillator until a transposed image of the reference subcarrier appears in the desired frequency span.

2. Unit according to claim 1 , wherein the additional transposition means comprise a very selective filter.

3. Radio frequency transmission system comprising at least one base station and at least one subscriber device, the base station using a single oscillator to perform a downward transposition of signals towards the frequency band transmitted by radio and possibly an upward transposition of signals from the frequency band received by radio, the subscriber device comprising an internal unit and an external unit which are linked by a cable, said external unit placed on the subscriber side of a transmission system operating in a transmission frequency band and in a reception frequency band using at least one reference sub carrier placed at various location in a transmission frequency band, the reception frequency band furthermore comprising at least one data-carrier uplink channel, wherein said external unit comprises:

a reference oscillator for generating a frequency reference signal;

a local oscillator for generating a signal for transposing the transmission frequency band into a transposed frequency band, the transposed frequency band including the transposed reference sub carrier;

a first transposition means for transposing the transmission frequency band into a transposed frequency band, the transposed frequency band including the transposed reference sub carrier;

second transposition means for transposing the received frequency band into the uplink frequency band;

a frequency synthesizer for providing a frequency transposition signal coupled to the reference oscillator;

additional transposition means for mixing and filtering the transposed reference sub carrier with the frequency transposition signal generating an additionally transposed reference sub carrier in a predefined frequency band;

synchronization means for comparing the phase of the additionally transposed reference sub carrier and the frequency reference signal, carrying out a frequency feedback control signal to synchronize the local oscillator; and

a latching device placed between the local oscillator and the comparator means.

4. System according to claim 3 , wherein said additional transposition means comprise a very selective filter.

5. System according to claim 3 , wherein the second transposition means comprise a third and a fourth frequency transposition means, and wherein the third and the fourth transpositions are done in an infradyne manner in the one case and in a supradyne manner in the other case.

6. External unit placed on the subscriber side of a transmission system operating in a transmission frequency band and in a reception frequency band using at least one reference sub carrier placed at various location in a transmission frequency band, the reception frequency band furthermore comprising at least one data-carrier uplink channel, wherein the external unit comprises:

a reference oscillator for generating a frequency reference signal;

a local oscillator for generating a signal for transposing the transmission frequency band into a transposed frequency band, the transposed frequency band including the transposed reference sub carrier;

first transposition means for transposing the transmission frequency band into a transposed frequency band, the transposed frequency band including the transposed reference sub carrier;

second transposing means for transposing the received frequency band into the uplink frequency band;

a frequency synthesizer for providing a frequency transposition signal coupled to the reference oscillator;

additional transposition means for mixing and filtering the transposed reference sub carrier with the frequency transposition signal generating an additionally transposed reference sub carrier in a predefined frequency band;

synchronization means for comparing the phase of the additionally transposed reference sub carrier and the frequency reference signal, carrying out a frequency feedback control signal to synchronize the local oscillator; and

a latching device placed between the local oscillator and the synchronization means so that the latching device drives the local oscillator until a transposed image of the reference subcarrier appears in the desired frequency span.

7. Unit according to claim 6 , wherein said additional transposition means comprise a very selective filter.

8. Unit according to claim 6 , wherein the second transposing means comprise a third and a fourth frequency transposition means, and wherein the third and the fourth transpositions are done in an infradyne manner in the one case and in a supradyne manner in the other case.