Patent ID: 7398062

Claim:
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.