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In the color coded RF spur chart shown on this slide, the black lines with a slope of one indicate the IF output fundamental for the range of RF/LO = 0 to RF/LO = 2. The range of RF/LO < 1 (where LO is higher than RF), is referred to as high side injection. Likewise, the range of RF/LO > 1 (where LO is lower than RF) is referred to as low side injection. The benefits of low side injection can be seen as there is much lower probability of a mixer harmonic spur occurring near the IF output when LO < RF. To find the mixer harmonic content on the IF output, set the vertical red line to the RF/LO ratio. In the case shown, the two red lines in the graph have been put at RF/LO = 1.2 and at 0.8. Following the red line up in the situation where RF/LO = 1.2, it can be seen that the 2nd and 3rd harmonics of the IF output occur at 0.4 and 0.6 on the vertical scale. The 2x3 and 3x4 spurs fall at the same frequencies. If the goal is set the same as the IF, but using high side injection, the RF/LO ration would be 0.8. In this case, the vertical red line indicator shows that the 4x3 spur will fall directly on the IF fundamental and the 3x2 spur will fall at 2 * IF. If the RF/LO ratio falls lower than 0.8, both the 3x2 and 4x3 spurs will fall close to the IF fundamental. The sweet spur for frequency planning is to use low side injection, with RF/LO between 1 and 1.2. This particular plot shows only mixing components resulting from up to 4 * LO and 4 * RF. RF spur charts found on the web often show mixing components up to 6x LO and RF components.
