What is the fractional spur problem in a fractional-N synthesizer and how do I mitigate it?
Fractional-N Spurious Mitigation
Fractional spurs are the primary disadvantage of fractional-N synthesis compared to integer-N synthesis. Modern delta-sigma fractional-N synthesizers have greatly reduced spur levels through advanced modulator design, but the problem remains for demanding applications.
| Parameter | Passive Diode | Active FET | Subharmonic |
|---|---|---|---|
| Conversion Loss/Gain | 5-9 dB loss | 0-10 dB gain | 8-12 dB loss |
| LO Drive Level | +7 to +17 dBm | -5 to +5 dBm | +5 to +13 dBm |
| IP3 (typical) | +15 to +30 dBm | +5 to +20 dBm | +10 to +20 dBm |
| Noise Figure | 5-9 dB (= conv. loss) | 8-15 dB | 9-14 dB |
| LO-RF Isolation | 25-45 dB | 15-35 dB | 20-40 dB |
Conversion Architecture
When evaluating the fractional spur problem in a fractional-n synthesizer and how do i mitigate it?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Spurious Performance
When evaluating the fractional spur problem in a fractional-n synthesizer and how do i mitigate it?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Design Trade-offs
When evaluating the fractional spur problem in a fractional-n synthesizer and how do i mitigate it?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Implementation Considerations
When evaluating the fractional spur problem in a fractional-n synthesizer and how do i mitigate it?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Frequently Asked Questions
Are all fractional values equally bad?
No. The spur level depends heavily on the fractional value: F/M = 1/2: the strongest spur (at f_ref/2). The modulator alternates between N and N+1 every other cycle, creating a strong tone. F/M = 1/4, 1/3: also strong spurs at f_ref/4 and f_ref/3. These simple fractions have short periods, concentrating energy in a few discrete tones. F/M = 17/100: weak spurs. The longer period spreads the energy across many small tones. F/M = irrational (approximated by a large M): the weakest spurs. The modulator pattern appears nearly random. Best practice: avoid simple fractions. Choose the reference frequency and M value so that the operating frequencies correspond to complex fractional values.
How do modern synthesizer ICs handle fractional spurs?
Modern fractional-N synthesizer ICs (Analog Devices ADF4356/ADF5356, Texas Instruments LMX2594, Renesas 8V97003) include: 3rd or 4th order MASH delta-sigma modulators, built-in dithering options, automatic charge pump calibration, and spur cancellation circuits (some ICs include a dedicated DAC that injects a cancellation signal at the known spur frequencies). These features reduce fractional spurs to -60 to -90 dBc for most fractional values, which is adequate for the majority of communications applications.
What spur levels are acceptable?
Depends on the application: cellular base station: < -65 dBc at any offset (to meet the transmitted spurious emission mask). Radar: < -50 dBc (spurs create false targets). Test equipment (signal generators): < -70 dBc (to avoid corrupting the DUT measurement). Military communications: < -60 dBc (to avoid detection). Research/scientific: < -80 dBc (for demanding spectral purity requirements). If the required spur level cannot be achieved with a fractional-N synthesizer: use an integer-N synthesizer with a lower reference frequency (no fractional spurs, but coarser frequency resolution and potentially higher in-band noise).