Spurious-free flexible frequency converter and a satellite communication repeater architecture

1. A method for frequency conversion within a satellite-based repeater system, the method comprising:

receiving an uplink radio frequency signal having a frequency f RF ;

generating a first and at least a second local oscillator signal having frequencies f LO1 and f LO2 , respectively;

at a first frequency conversion stage, mixing said uplink signal with said first local oscillator signal to generate an intermediate signal having a frequency f MF ; and

at a second frequency conversion stage, mixing said intermediate signal with said at least second local oscillator signal to generate a downlink signal having a frequency f IF ;

wherein the generation and mixing of said signals are performed such that:

f LO1 and f LO2 >f RF and f IF

f MF =f LO1 −f RF , and

f IF =f LO2 −f MF .

2. The method according to claim 1 , wherein

receiving the uplink signal comprises receiving an uplink signal comprising a first block of channels within a certain bandwidth;

mixing said uplink signal with said first local oscillator signal comprises generating an intermediate signal comprising an intermediate block of channels; and

mixing said intermediate signal with said at least second local oscillator signal comprises generating a downlink signal comprising a second block of channels within the same bandwidth and with the same internal relations between the channels as of the input signal.

3. The method according to claim 1 , wherein

receiving the uplink signal comprises receiving an uplink signal comprising a first block of channels within a certain bandwidth;

generating the local oscillator signals comprises generating more than two local oscillator signals;

mixing said uplink signal with said first local oscillator signal comprises generating an intermediate signal comprising an intermediate block of channels; and

mixing said intermediate signal with said at least second local oscillator signal comprises splitting the intermediate signal into a plurality of signal paths and filtering the signal in each path with an individually chosen filter (F 11-1N ) to obtain separate intermediate signals having frequencies f MF1-N at the outputs of said filters (F 11-1N ), and mixing each of the separate intermediate signals independently with a separate local oscillator signal to generate a plurality of separate downlink signals.

4. The method according to claim 1 , wherein generating the local oscillator signals comprises:

generating a low frequency signal with frequency f XOR ;

providing the low frequency signal as an input signal to:

a fixed PLL loop comprising a fixed high frequency source of frequency f XO1 being phase and frequency locked to f XOR ;

a first and at least a second programmable PLL loop comprising a first and a second independent synthesized microwave oscillator of frequency f XO2 and f XO3 , respectively, said oscillators being phase and frequency locked to a suitable frequency;

mixing the output signal from the fixed loop with the output signal from the first programmable PLL loop to obtain a first local oscillator signal having a frequency f LO1=f XO1±f XO2 ; and

mixing the output signal from the fixed PLL loop with the output signal from the at least second programmable PLL loop to obtain at least a second local oscillator signal having a frequency f LO2 =f XO1 ±f XO3 .

5. A local oscillator circuit for generating local oscillator signals, comprising:

a low frequency source for generating a low frequency signal;

a fixed PLL loop comprising a fixed high frequency source, said fixed PLL loop being arranged to receive the signal from the low frequency source as input and said fixed high frequency source being phase and frequency locked to the frequency of the received signal;

a first and at least a second programmable PLL loop comprising a first and a second independent synthesized microwave oscillator, respectively, said first and at least second programmable PLL loop being arranged to receive the signal from the low frequency source as input, and said independent synthesized oscillators being phase and frequency locked to a suitable frequency;

a mixer arranged to receive the outputs from the fixed PLL loop, the first programmable PLL loop, and the at least second programmable PLL loop and mix the output from said fixed PLL loop with the outputs from the at least two programmable PLL loops to generate at least two local oscillator signals, and

connection circuitry arranged to connect said local oscillator circuit to a frequency converter and to provide one of said local oscillator signals as input to a first down-conversion stage in said frequency converter, and to provide the at least second local oscillator signal as input to a second up-conversion stage in said frequency converter.

6. A satellite-based repeater system, comprising:

a frequency converter arranged to receive an uplink radio frequency signal, said frequency converter comprises:

a first mixer arranged to receive the uplink signal and a first local oscillator signal as input signals, and output an intermediate signal as the difference between the first local oscillator signal and the uplink signal; and

at least a second mixer arranged to receive the processed intermediate signal and at least a second local oscillator signal as input signals, and output at least one downlink signal as the difference between the at least second local oscillator signal and the intermediate signal, and

a local oscillator circuit according to claim 5 for generating said at least two local oscillator signals.