Mapping Mixer Spurs Into the Wanted IF Band

A mixer produces more than the desired sum or difference frequency. Harmonic combinations of its input signals can fall into the wanted output band at particular tuning points. Map those intersections before selecting an IF filter or assuming a spur table guarantees rejection across the full frequency plan.

Write the wanted conversion explicitly

Record RF, LO, and IF ranges and the intended high-side or low-side injection. The Marki Microwave guide to mixer spurs discusses evaluating spurious performance for a particular frequency plan and cautions that tabulated suppression is not a universal result at every frequency.

Include tuning steps and any alternate conversion modes. A spur can be absent at the center frequency yet enter the passband near one end of the range.

Enumerate relevant combinations

Calculate candidate products from integer combinations of RF and LO frequencies within the orders relevant to the mixer and application. Compare their output frequencies with the wanted IF bandwidth, not only its center.

For a simple illustration, if the wanted IF is the magnitude of RF minus LO, another combination such as twice RF minus twice LO appears at twice that difference. Whether it matters depends on the filter and receiving bandwidth. Other combinations can coincide with the wanted band at different tuning relationships.

Separate frequency prediction from amplitude

The arithmetic locates a possible spur; it does not determine its level. Amplitude depends on mixer structure, port drive, frequency, bias where applicable, and input power.

Use data measured under comparable conditions and note whether the published table is typical, averaged, or bounded. Avoid treating a suppression figure at one RF level as unchanged after increasing the input substantially.

Measure the critical intersections

Test tuning points where predicted products approach or enter the wanted band. Keep LO drive within the selected mixer’s specified range and characterize source harmonics so they are not mistaken for products generated by the mixer.

Vary one input frequency or level deliberately to verify the suspected product’s behavior. Use suitable filtering and analyzer settings to avoid overload and to resolve the component from the wanted signal.

Save the frequency map, relevant orders, drive levels, source purity evidence, and measured spur amplitudes. A useful plan identifies difficult channels and the rejection available there. A clean spectrum at one convenient conversion frequency does not establish a spur-free tuning range, and a frequency calculator alone cannot guarantee the amplitude margin required by the complete receiver.