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[Technical article, rfforum Editorial Team]
Analyser datasheets quote TOI / DANL / phase noise at specific mixer levels and attenuator settings. If your measurement uses a different mixer level, the spur-free dynamic range you think you bought is not the one you have.
1. Mixer level is a setting, not a vibe
Most SAs let you set or infer the level at the first mixer (input power minus input attenuator, plus any preamp). Distortion products (TOI-limited) get worse as mixer level rises; noise floor gets worse as attenuator rises or preamp is off. The sweet spot is where distortion and noise meet for your particular test.
2. Why the front-panel "TOI" misleads
The published TOI assumes a stated mixer level (e.g. −10 dBm). Drive the mixer 10 dB harder and third-order products can rise ~30 dB in power — your spur budget collapses even though the instrument still "looks calibrated".
3. Preamp and attenuator traps
Internal preamp improves DANL but usually destroys TOI. External amplification before the SA does the same unless you add enough external attenuation to protect the mixer. Always state mixer level (or atten + preamp state) next to any spur or ACLR screenshot.
4. Practical checklist
1. Pick the distortion product you care about (LO harmonic, two-tone IMD, adjacent channel).
2. Set attenuator so mixer level sits at or below the datasheet TOI condition, then confirm noise is still acceptable.
3. Re-measure with ±5 dB attenuator: if the spur moves 3:1 with input, it is likely analyser IMD; if it stays put in dBc, it may be the DUT.
Discussion: what mixer level do you standardise on for sub-6 GHz spur hunts, and do you write it into the test report every time?
— rfforum 编辑部 / Editorial Team
Analyser datasheets quote TOI / DANL / phase noise at specific mixer levels and attenuator settings. If your measurement uses a different mixer level, the spur-free dynamic range you think you bought is not the one you have.
1. Mixer level is a setting, not a vibe
Most SAs let you set or infer the level at the first mixer (input power minus input attenuator, plus any preamp). Distortion products (TOI-limited) get worse as mixer level rises; noise floor gets worse as attenuator rises or preamp is off. The sweet spot is where distortion and noise meet for your particular test.
2. Why the front-panel "TOI" misleads
The published TOI assumes a stated mixer level (e.g. −10 dBm). Drive the mixer 10 dB harder and third-order products can rise ~30 dB in power — your spur budget collapses even though the instrument still "looks calibrated".
3. Preamp and attenuator traps
Internal preamp improves DANL but usually destroys TOI. External amplification before the SA does the same unless you add enough external attenuation to protect the mixer. Always state mixer level (or atten + preamp state) next to any spur or ACLR screenshot.
4. Practical checklist
1. Pick the distortion product you care about (LO harmonic, two-tone IMD, adjacent channel).
2. Set attenuator so mixer level sits at or below the datasheet TOI condition, then confirm noise is still acceptable.
3. Re-measure with ±5 dB attenuator: if the spur moves 3:1 with input, it is likely analyser IMD; if it stays put in dBc, it may be the DUT.
Discussion: what mixer level do you standardise on for sub-6 GHz spur hunts, and do you write it into the test report every time?
— rfforum 编辑部 / Editorial Team
