System and method for reducing intra-channel interference
View Patent ↗The present invention provides a method for determining a local oscillator frequency for a baseband converter, comprising obtaining characteristics of a channel comprising a plurality of radio signals at corresponding radio frequencies; using the channel characteristics to select a local oscillator frequency to use in baseband conversion of the plurality of radio signals of the channel to form a first baseband converted signal and a second baseband converted signal; and programming a local oscillator to operate the selected local oscillator frequency; wherein the local oscillator frequency is selected such that an image rejection ratio of the baseband converted second signal to an image of the baseband converted first signal is above a predetermined threshold.
1. A method of receiving a channel, comprising:
receiving a radio signal comprising a plurality of channels, a channel of the plurality comprising a first signal and a second signal;
converting the radio signal to an information bearing signal using a local oscillator, the information bearing signal comprising a converted first signal and a converted second signal, the local oscillator having a local oscillator frequency that is iteratively selected until an image rejection ratio of the converted second signal to an image of the converted first signal is above a predetermined threshold; and
digitally shifting the frequency of the information bearing signal by an amount approximately equal to the difference between a channel center frequency and the local oscillator frequency.
2. The method of claim 1 , wherein the local oscillator frequency is iteratively selected from a range of possible frequencies, the range of possible frequencies having an upper bound and a lower bound determined according to the channel characteristics.
3. The method of claim 2 , wherein the channel comprises a television channel; and wherein the lower bound is determined according to a predetermined amount of tolerable attenuation of an audio signal of the channel; and the upper bound is determined according to a predetermined amount of tolerable interference from a video signal of an adjacent channel.
4. The method of claim 1 , wherein a new local oscillator frequency is selected if the local oscillator undergoes a predetermined amount of oscillator drift.
5. The method of claim 1 , wherein the channel occupies a 6 MHz range of radio spectrum between 30 MHz and 900 MHz, the potential local oscillator frequency comprises a frequency approximately 3 MHz greater than a lowest frequency of the channel, and the selected local oscillator frequency comprises a frequency 0.25 MHz less than the potential local oscillator frequency.
6. The method of claim 5 , wherein:
the channel comprises an NTSC television channel;
the first signal comprises an luma signal having a carrier frequency 1.25 MHz greater than the lowest frequency of the channel;
the second signal comprises a chroma signal having a carrier frequency approximately 3.58 MHz greater than the carrier frequency of the luma signal; and
the channel further comprises an audio signal having a carrier frequency 4.5 MHz greater than the carrier frequency of the luma signal.
7. An apparatus for channel tuning, comprising:
a signal receiver configured to receive a radio signal comprising a plurality of channels, a channel of the plurality comprising a first information bearing signal and a second information bearing signal;
a local oscillator configured to provide a local oscillator frequency that is iteratively selected such that an image rejection ratio of the second information bearing signal to an image of the first information bearing signal is above a predetermined threshold;
an analog frequency converter coupled to the antenna and the local oscillator and configured to output a converted radio signal comprising the first information bearing signal and the second information bearing signal; and
a digital frequency converter configured to shift the converted radio signal by a frequency approximately equal to the frequency difference between the local oscillator frequency and a center channel frequency.
8. The apparatus of claim 7 , wherein a new local oscillator frequency is selected if the local oscillator undergoes a predetermined amount of oscillator drift.
9. The apparatus of claim 7 , wherein the channel comprises a television channel; and wherein the local oscillator frequency is further configured such that attenuation of an audio signal of the channel remains below a predetermined attenuation threshold.
10. The apparatus of claim 9 , wherein the local oscillator is further configured such that interference from a luma signal of an adjacent channel remains below a predetermined adjacent channel interference threshold.
11. The apparatus of claim 7 , wherein the channel occupies a 6 MHz range of radio spectrum between 30 MHz and 900 MHz, and the local oscillator frequency comprises a frequency approximately 0.25 MHz less than a center channel frequency.
12. The apparatus of claim 11 , wherein:
the channel comprises an NTSC television channel;
the first signal comprises an luma signal having a carrier frequency 1.25 MHz greater than the lowest frequency of the channel;
the second signal comprises a chroma signal having a carrier frequency approximately 3.58 MHz greater than the carrier frequency of the luma signal; and
the channel further comprises an audio signal having a carrier frequency 4.5 MHz greater than the carrier frequency of the luma signal.
13. The apparatus of claim 12 , wherein the image rejection ratio is less than or equal to approximately 57 decibels.