R
Member
I spent a morning chasing a spurious emission that turned out to be partly my measurement setup. A spur at -72 dBm looked like a DUT failure until I realized the analyzer itself was contributing. Here is the checklist I now run before trusting any spur number.
1. Start with RBW, not with the marker. A spur measured with 100 kHz RBW reads about 10 dB higher than the same spur measured with 10 kHz RBW if it is noise-like. Regulatory limits are usually specified per RBW, so match the RBW to the spec first, then read. I keep a small table of limit-vs-RBW conversions taped to the bench.
2. Detector choice changes the answer. Peak detector catches narrow spurs but over-reads noise; sample detector is honest for noise-like spurs but misses intermittent ones; average detector is the right call for anything resembling a modulated signal. When in doubt, I measure with peak and average and report both — if they differ by more than 6 dB, the spur is probably noise-like, not a tone.
3. Check the analyzer's own DANL and second harmonic. With the input terminated, note the displayed noise floor at your RBW. If the spur is within 10 dB of that floor, the number is not trustworthy — add a preamp or narrow the RBW. And always check whether the "spur" moves when you change the input attenuator by 10 dB: a real spur drops 10 dB, an analyzer-generated intermod product drops 20 dB.
4. Preamp on means re-check overload. A preamp buys you 15–20 dB of sensitivity but costs you headroom. I once "found" a spur that was the preamp compressing on a strong in-band carrier. Now I toggle the preamp and watch whether the spur level tracks the preamp gain — if it does, it is real; if it jumps, suspect overload.
After fixing the setup, the real spur was -81 dBm, 9 dB better than my first reading, and comfortably inside the limit.
What is your rule of thumb for deciding when a spur measurement is trustworthy? Do you always run the attenuator toggle test, or do you have a faster sanity check?
1. Start with RBW, not with the marker. A spur measured with 100 kHz RBW reads about 10 dB higher than the same spur measured with 10 kHz RBW if it is noise-like. Regulatory limits are usually specified per RBW, so match the RBW to the spec first, then read. I keep a small table of limit-vs-RBW conversions taped to the bench.
2. Detector choice changes the answer. Peak detector catches narrow spurs but over-reads noise; sample detector is honest for noise-like spurs but misses intermittent ones; average detector is the right call for anything resembling a modulated signal. When in doubt, I measure with peak and average and report both — if they differ by more than 6 dB, the spur is probably noise-like, not a tone.
3. Check the analyzer's own DANL and second harmonic. With the input terminated, note the displayed noise floor at your RBW. If the spur is within 10 dB of that floor, the number is not trustworthy — add a preamp or narrow the RBW. And always check whether the "spur" moves when you change the input attenuator by 10 dB: a real spur drops 10 dB, an analyzer-generated intermod product drops 20 dB.
4. Preamp on means re-check overload. A preamp buys you 15–20 dB of sensitivity but costs you headroom. I once "found" a spur that was the preamp compressing on a strong in-band carrier. Now I toggle the preamp and watch whether the spur level tracks the preamp gain — if it does, it is real; if it jumps, suspect overload.
After fixing the setup, the real spur was -81 dBm, 9 dB better than my first reading, and comfortably inside the limit.
What is your rule of thumb for deciding when a spur measurement is trustworthy? Do you always run the attenuator toggle test, or do you have a faster sanity check?
