Method and system of preventing interference caused by images

A method for reducing interference while frequency converting a periodically modulated radio frequency (RF) signal received by a receiver system to a plurality of intermediate frequency (IF) signals, the receiver system comprising a local oscillator (LO) for generating a plurality of LO signals to mix with portions of the periodically modulated RF signal to provide the plurality of IF signals, an analog to digital converter (ADC) having a predetermined ADC sampling rate for digitizing the plurality of IF signals, and a domain converter for performing discrete Fourier transforms (DFTs) on the digitized IF signals to provide DFT data records corresponding to each of the digitized IF signals, the method comprising:

• identifying tone frequencies of a plurality of tones, determining a required minimum tone spacing between adjacent tones of the plurality of tones in the periodically modulated RF signal using a priori information regarding a period of modulation, and determining a DFT record size of the DFT data records, based on the ADC sampling rate and the required minimum tone spacing, suitable for providing a desired level of protection from image interference;
• determining an improved tone spacing based on the suitable DFT record size;
• and
• determining LO frequencies of the plurality of LO signals based on at least the improved tone spacing, the ADC sampling rate and the tone frequencies, such that, when the plurality of LO signals having the determined LO frequencies are respectively mixed with the portions of the periodically modulated RF signal, tones in the IF signals corresponding to the plurality of tones in the periodically modulated RF signal fall on first DFT bins of the corresponding DFT data records and images created by the mixing fall on second DFT bins of the corresponding DFT data records, the second DFT bins being discarded to substantially prevent the images from interfering with the tones in the IF signals.

A system for acquiring a periodically modulated radio frequency (RF) signal with reduced interference caused by images, the system comprising:

• a local oscillator (LO) for generating a plurality of LO signals;
• a mixer for receiving the periodically modulated RF signal and for mixing portions of the periodically modulated RF signal with the plurality of LO signals, respectively, to provide a plurality of intermediate frequency (IF) signals;
• an analog to digital converter (ADC) for digitizing the plurality of IF signals, the ADC having a predetermined ADC sampling rate;
• and
• a processing unit for processing the digitized IF signals, the processing unit being configured to execute instructions stored on a non-transitory computer readable medium, causing the processing unit to:
• perform discrete Fourier transforms (DFTs) on the digitized IF signals to provide DFT data records respectively corresponding to the digitized IF signals;
• receive tone frequencies of a plurality of tones and tone spacing between adjacent tones of the plurality of tones in the periodically modulated RF signal using a priori information;
• determine a DFT record size of the DFT data records based on at least the tone spacing, the ADC sampling rate, and a desired amount of interference reduction;
• and
• determine LO frequencies of the plurality of LO signals based on at least the DFT record size, the ADC sampling rate and the tone frequencies, such that, when the plurality of LO signals having the determined LO frequencies are respectively mixed with the portions of the periodically modulated RF signal, tones in the IF signals corresponding to the plurality of tones in the periodically modulated RF signal fall on first DFT bins of the corresponding DFT data records and images created by mixing the plurality of LO signals with the portions of the periodically modulated RF signal, respectively, fall on second DFT bins of the corresponding DFT data records,
• wherein the LO generates the plurality of LO signals having the determined LO frequencies,
• wherein the mixer mixes the generated plurality of LO signals with portions of the received periodically modulated RF signal, respectively, to provide the plurality of IF signals,
• wherein the tones of each IF signal fall on first DFT bins of the corresponding DFT data record and the images of each IF signal fall on second DFT bins of the corresponding DFT data record, and
• wherein the processing unit discards the second DFT bins, substantially preventing the images from interfering with the tones in the first DFT bins, and spectrally stitches the first DFT bins of each IF signal to provide data corresponding to the periodically modulated RF signal.