What is the spur chart for a mixer and how do I use it to identify problematic spurious products?
Spur Chart Usage
The spur chart data is typically provided by the mixer manufacturer in the data sheet, or measured by the system designer using a two-tone test setup. The chart is organized as a matrix with RF harmonic order (m) on one axis and LO harmonic order (n) on the other. Each cell contains the relative power of that m×n product in dBc (dB below the desired 1×1 product).
| 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 |
- 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
Frequently Asked Questions
Do I check all m×n combinations?
In practice, check orders up to m+n ≤ 7. Products with m+n > 7 are typically below -60 dBc and are usually below the system noise floor. Focus on the low-order products (2×1, 1×2, 3×1, 1×3) first as these are the strongest and most likely to fall in-band.
How does LO power affect the spur chart?
Higher LO power generally improves the spur chart by providing harder diode switching and better balance. The spur chart at Level 17 is typically 3-10 dB better (lower spur levels) than at Level 7. Always use the spur chart measured at the actual operating LO level.
Can I improve the spur chart?
Using a triple-balanced mixer improves even-order suppression by 10-15 dB beyond a DBM. Adding external filtering to reject strong RF harmonics before the mixer prevents them from entering the mixing process. Pre-filtering the RF reduces spurs at the cost of insertion loss.