Low power spectrally pure offset local oscillator system
The present disclosure is directed to a dual output path LNA that can be used to break the tradeoff between the output impedance and linearity of an LNA without the problems of a programmable output impedance LNA. In an embodiment, the dual output path architecture includes an LNA driving a low level of impedance in a low voltage gain path, thus achieving high linearity in the presence of large blockers, and driving a high level of impedance in a high voltage gain path to increase the LNA's voltage gain and minimize performance degradation due to a noisier, low power receiver front-end chain following the LNA. The present disclosure is further directed to a local oscillator (LO) offset circuit with low power and reduced spur generation.
1. A transmitter back-end comprising:
a frequency synthesizer configured to generate an offset local oscillator (LO) signal;
an LO offset circuit configured to adjust a frequency of the offset LO signal to provide a desired LO signal;
a back-end mixer configured to mix a baseband signal and the desired LO signal to provide an up-converted signal; and
a power amplifier (PA) configured to amplify the up-converted signal output by the back-end mixer;
wherein the LO offset circuit comprises:
an offset mixer configured to frequency shift the offset LO signal to provide a frequency shifted offset LO signal; and
a filter configured to filter an unwanted portion of the frequency shifted offset LO signal.
2. The transmitter back-end of claim 1 , wherein the LO offset circuit further comprises:
a first divide by 2 circuit configured to divide down a frequency of the offset LO signal by a factor of two before the offset LO signal is processed by the offset mixer.
3. The transmitter back-end of claim 2 , wherein the LO offset circuit further comprises:
a second divide by 2 circuit configured to divide down a frequency of the frequency shifted offset LO signal by a factor of two to provide the desired LO signal.
4. The transmitter back-end of claim 1 , wherein the offset LO signal has a frequency of 3.2 GHz.
5. The transmitter back-end of claim 4 , wherein the desired LO signal has a frequency of 2.4 GHz.
6. The transmitter back-end of claim 1 , wherein the unwanted portion of the frequency shifted offset LO signal has a frequency of 1.6 GHz, and a wanted portion of the frequency shifted offset LO signal has a frequency of 4.8 GHz.
7. The transmitter back-end circuit of claim 6 , wherein the filter notches out the frequency of 1.6 GHz in the frequency shifted offset LO signal and passes the frequency of 4.8 GHz in the frequency shifted offset LO signal.
8. The transmitter back-end circuit of claim 7 , further comprising a second divide by 2 circuit configured to divide down the 4.8 GHz frequency in the frequency shifted offset LO signal by a factor of two to provide the desired LO signal having a frequency of 2.4 GHz.
9. The transmitter back-end circuit of claim 1 , wherein the frequency of the offset LO signal is greater than a frequency of the desired LO signal.
10. The transmitter back-end of claim 1 , wherein the frequency of the offset LO signal is 4/3 of the frequency of the desired LO signal.
11. The transmitter back-end of claim 1 , wherein the filter is a bi-resonant LC filter having at least one capacitor and at least one inductor configured to notch out the unwanted portion and pass a wanted portion of the frequency shifted offset LO signal.
12. The transmitter back-end of claim 1 , wherein the desired LO signal is not a sub-multiple of the offset LO signal.
13. A transmitter back-end comprising:
a frequency synthesizer configured to generate an offset local oscillator (LO) signal;
an LO offset circuit configured to adjust a frequency of the offset LO signal to provide a desired LO signal;
a back-end mixer configured to mix a baseband signal and the desired LO signal to provide an up-converted signal; and
a power amplifier (PA) configured to amplify the up-converted signal output by the back-end mixer,
wherein the frequency of the offset LO signal is 4/3 of the frequency of the desired LO signal.
14. A transmitter back-end comprising:
a local oscillator (LO) offset circuit configured to receive an offset LO signal and adjust a frequency of the offset LO signal to provide a desired LO signal, wherein a frequency of the offset LO signal is 4/3 that of a frequency of the desired LO signal;
a back-end mixer configured to mix a baseband signal and the desired LO signal to provide an up-converted signal; and
a power amplifier (PA) configured to amplify the up-converted signal output by the back-end mixer;
wherein the LO offset circuit comprises:
a first divide by 2 circuit configured to divide down a frequency of the offset LO signal by a factor of two to generate an oscillator signal;
an offset mixer configured to mix the oscillator signal with the offset LO signal to provide a frequency shifted offset LO signal having a desired harmonic and an undesired harmonic; and
a filter configured to filter out the undesired harmonic of the frequency shifted offset LO signal.
15. The transmitter back-end of claim 14 , further comprising:
a second divide by 2 circuit configured to divide down the desired harmonic of the frequency shifted offset LO signal by a factor of 2 to provide the desired LO signal.
16. The transmitter back-end of claim 15 , wherein the filter is a bi-resonant LC filter having at least one capacitor and at least one inductor configured to notch out the undesired harmonic of the frequency shifted offset LO signal and pass the desired harmonic of the frequency shifted offset LO signal.
17. The transmitter back-end of claim 15 , wherein a frequency of the offset LO signal is 3.2 GHz, and a frequency of the desired LO signal is 2.4 GHz.
18. The transmitter back-end of claim 17 , wherein a frequency of the desired harmonic of the frequency shifted offset LO signal is 4.8 GHz, and a frequency of the undesired harmonic of the frequency shifted offset LO signal is 1.6 GHz.
19. A method of operating a back-end transmitter, comprising:
generating an offset local oscillator (LO) signal;
adjusting a frequency of the offset LO signal to provide a desired LO signal, wherein a frequency of the desired LO signal is ¾ the frequency of the offset LO signal;
mixing a baseband signal and the desired LO signal to provide an up-converted signal; and
amplifying the up-converted signal;
wherein adjusting the frequency of the offset LO signal comprises:
dividing down the frequency of the offset LO signal by a factor of two to generate an oscillator signal;
mixing the oscillator signal with the offset LO signal to provide a frequency shifted offset LO signal having a desired harmonic and an undesired harmonic;
filtering out the undesired harmonic of the frequency shifted offset LO signal and passing the desired harmonic of the frequency shifted offset LO signal; and
dividing down the desired harmonic of the frequency shifted offset LO signal by a factor of 2 to provide the desired LO signal.
20. The method of claim 19 , wherein:
the frequency of the offset LO signal is 3.2 GHz;
the frequency of the desired LO signal is 2.4 GHz;
a frequency of the desired harmonic of the frequency shifted offset LO signal is 4.8 GHz; and
a frequency of the undesired harmonic of the frequency shifted offset LO signal is 1.6 GHz.