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LO Harmonic Content Dependency on Mod/Demod Performance

LO Harmonic Content Dependency on Mod/Demod Performance. RFG August 14 th , 2009. How does LO harmonics affect direct conversion solutions?.

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LO Harmonic Content Dependency on Mod/Demod Performance

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  1. LO Harmonic Content Dependency on Mod/Demod Performance RFG August 14th, 2009

  2. How does LO harmonics affect direct conversion solutions? We usually assume a synthesizer presents a sinusoidal waveform which we use to apply to a mixer/mod/demod. More often the synth presents a square-wave with potentially non-50% duty cycle which is rich in harmonic content. How do these LO harmonics influence sideband-suppression/image-rejection?

  3. Consider Square-wave spectral characteristics A perfect 50% duty cycle square wave will have an infinite amount of odd order harmonic content with no even order terms. FFT Result Note odd harmonics only, and the harmonics are -20Log(n) below the fundamental

  4. Results for non 50% duty cycle and softened rising and falling edges With ~4nsec rise fall time and ~3% duty cycle error the HD2 is -20dBc and the HD3 is -10dBc. This is close to the harmonic distortion levels present at the VCO divider outputs of the ADF4350.

  5. Replacing Square-wave generator with multi-tone source Approach provides an accurate harmonically defined waveform which is similar to the output of an integrated PLL/VCO.

  6. Simulation Setup for SSB Modulator with Multi-tone LO source and Perfect Quadrature Note zero phase error in quadrature splitter LO Leakage set for -40dBm Input IF set for 100kHz with LO of 10MHz. Using ideal quadrature hybrids to generate IQ baseband input and LO quadrature

  7. Testing Quadrature Modulator Sideband Suppression with Perfect LO Source LO harmonics with linear modulator and perfect quadrature results in no sideband. LO harmonics do result in nxLO-IF mixing products as expected.

  8. Simulation Setup for SSB Modulator with Multi-tone LO source and Imperfect Phase Quadrature on LO harmonics Note: Quadrature phase error for 2nd and 3rd LO harmonics deliberately set for horrible quadrature (10degrees error applied for 2nd, and 30degrees applied to 3rd)

  9. Testing Quadrature Modulator Sideband Suppression with Imperfect LO Source (phase error only) Similar result as before except sidebands show up around 2xLO and 3xLO. Lesson learned: Phase impairments on the LO path is NOT causing poor sideband suppression at fundamental LO output.

  10. Simulation Setup for SSB Modulator with Multi-tone LO source and Imperfect Magnitude Quadrature on LO harmonics Note: Quadrature mag error for 3rd LO harmonic deliberately set for 1dB of error

  11. Testing Quadrature Modulator Sideband Suppression with Imperfect LO Source (1dB error on 3rd LO harmonic only) Somewhat unexpected result. HD3 magnitude quadrature is important for good sideband suppression.

  12. Simulation Setup for SSB Modulator with Multi-tone LO source and Imperfect Magnitude Quadrature on LO harmonics Note: Quadrature mag error for 2nd LO harmonic deliberately set for 1dB of error

  13. Testing Quadrature Modulator Sideband Suppression with Imperfect LO Source (1dB error on 2nd LO harmonic only) Surprisingly 2nd Harmonic has less impact than 3rd harmonic

  14. Testing Quadrature Modulator Sideband Suppression with Imperfect LO Source (1dB error on 2nd and 3rd LO harmonics) With 1dB of quadrature error at the 2nd and 3rd harmonics in the LO path the sideband suppression is reduced to -49dB. Increasing the error to 3dB results in -40dB of sideband suppression.

  15. Sideband Suppression versus LO Harmonic Quadrature Magnitude Error Note that poor magnitude imbalance on the 3rd LO harmonic is the dominate contributor to poor sideband suppression.

  16. Conclusions • LO Harmonics can degrade Sideband-Suppression/Image-Rejection Performance in Direct Conversion Systems. • The degradation is mainly due to quadrature amplitude mismatch through the typical polyphase structures employed in 1xLO designs (not really due to the phase mismatch as we may have thought). • Using a simple 3rd Order lowpass LC filter with a cut-off of 1.5xfLO improves sideband suppression to ~72dBc even with 10dB of quadrature amplitude mismatch at 2nd and 3rd harmonics. Simple shunt-C series-L shunt-C filters should be enough to suppress LO harmonics for 1xLO mod/demods. • When using 1xLO IQ Mod/Demod Components it is important to filter the LO harmonics for good sideband-suppression/image-rejection. • When using 2xLO digital quadrature designs the magnitude and phase can be better matched over a broad range of frequencies and LO harmonics prove to be less critical.

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