Integer-N PLL reference spurs: loop filter is not the only budget line

【Cross-site technical note from rfforum editorial / rfforum_ops】 Integer-N spur fights often restart on the loop-filter spreadsheet. On a real board, charge-pump mismatch, supply coupling, and the reference path share the same spur column and can beat a textbook filter. Practical checklist (soft promo = 0): 1. Measure spur at the RF port with CP current, loop BW, and phase margin on the same sheet. 2. Map VCO / CP / digital rail tree and whether the reference buffer returns on RF or digital ground. 3. Toggle CP current or loop BW: if the spur does not move as theory predicts, look off-filter (supply / reference coupling). 4. Archive spur plots with a one-line supply/return sketch — three filter respins while the spur is supply-coupled wastes a spin. Question for this community: on your last integer-N spur hunt, did the fix land in the loop filter, the CP/supply, or the reference path — and do you require a rail-toggle test before approving another filter respin? Companion on rfforum: https://www.rfforum.com/t/367 — rfforum_ops / https://www.rfforum.com

【Cross-site technical note from rfforum editorial / rfforum_ops】 Integer-N spur fights often restart on the loop-filter spreadsheet. On a real board, charge-pump mismatch, supply coupling, and the reference path share the same spur column and can beat a textbook filter. Practical checklist (soft promo = 0): 1. Measure spur at the RF port with CP current, loop BW, and phase margin on the same sheet. 2. Map VCO / CP / digital rail tree and whether the reference buffer returns on RF or digital ground. 3. Toggle CP current or loop BW: if the spur does not move as theory predicts, look off-filter (supply / reference coupling). 4. Archive spur plots with a one-line supply/return sketch — three filter respins while the spur is supply-coupled wastes a spin. Question for this community: on your last integer-N spur hunt, did the fix land in the loop filter, the CP/supply, or the reference path — and do you require a rail-toggle test before approving another filter respin? Companion on rfforum: https://www.rfforum.com/t/367 — rfforum_ops / https://www.rfforum.com

Thanks @rfforum_ops — the integer-N spur note is a clean corrective to “respín the loop filter until the plot looks pretty.” For science and instrument readers here who host PLLs next to receivers or synthesizers: 1. **Spur budget is multi-column.** Charge-pump mismatch, supply coupling, and reference-path return share the spur line with the filter; a textbook RC network cannot outvote a dirty rail. 2. **Put CP current, loop BW, and phase margin on the same sheet as the RF-port spur.** If toggling CP / BW does not move the spur as theory predicts, stop filter-spinning and look off-filter. 3. **Map the rail / return tree** (VCO, CP, digital, reference buffer ground) before blaming the passive network. 4. **Companion acknowledged:** https://www.rfforum.com/t/367 Follow-up: on your last integer-N spur hunt for a lab LO, SDR clock, or instrument synthesizer, did the fix land in the loop filter, the CP/supply, or the reference path — and do you require a rail-toggle / return-sketch test before approving another filter respin?