The Distances Chart: A New Approach to Spurs Calculation

A new method for mixer spurs calculation is presented, which has been found easier and more straightforward to use than the traditional spurs chart approach when dealing with the frequency plan optimization in broadband double conversion transceivers suitable for applications as multiband, multistandard and Software Defined Radio (SDR). Instead of plotting the spurs at the mixer’s output, it shows the distance from the wanted RF input to other unfiltered input frequencies (named RFmn interferers) that will appear, through the mixing process, at a minimum “guard” distance—user specified—from the borders of the IF output band. The main benefit of this method, as compared to the spur chart, is that the LO sweep is implicit in the results and the spur lines are static, thus permitting the optimization of the RF and IF filter bands and the LO sweep band. This method assumes that the IF is a design variable and that a second conversion is used to get to/from a lower IF frequency more suitable for A/D or D/A conversion.

From: Vol. 53 | No. 2 | February 2010 | Page 86

by José Luis Flores, AT4 Wireless, Málaga, Spain

When dealing with frequency conversion processes, it is necessary to know the type and level of the non-desired spurs appearing at the output of the frequency conversion device. Power level calculations rely on mixer behavior (that is the nonlinear transfer function from V_in to V_out) that is dependant on specific mixer technology and design.

These levels are commonly referred to the level of the wanted output signal and are expressed as rejections in decibels (positive values) or gains (negative values). Analytical expressions for intermodulation product (IMP) suppressions can be found1 for the case of doubly balanced diode ring mixers; they take into account the balun imbalances, diode mismatches and diode turn-on voltages. The proper setting of these parameters can require a deep knowledge of the internal mixer circuitry and technology, not often available.

The rejections to IMPs are also dependant on the relative levels of oscillator and input signals and they vary with frequency. Many manufacturers of microwave mixers provide spur rejection tables in their datasheets, corresponding to conversions between a mid-to-high RF and a low IF, usually of 100 MHz. In conversions to/from much higher intermediate frequencies, as those considered in this study, it may be necessary to obtain new spur tables from measurement, prior to their use in a microwave simulator. Also in a first approach, the analytical results from Henderson¹ could be used.

Regardless of the output spectrum component levels in a particular frequency conversion, their frequency values can be obtained in a much more generic way: All the convolutions between harmonics of the oscillator and input signals will appear at the mixer output. Thus, for single tone RF and LO inputs to a mixer, the IF output will consist of a set of spectral lines with positive frequencies expressed as |mRF ± nLO|, that will be named |IF_mn| spurs. For the three common frequency conversion types—RF+LO, LO-RF and RF-LO—the desired output signals correspond to IF₁₁, IF_-11 and IF_1-1.

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