Signal Integrity and Ghost Signals
The tinySA Ultra is a powerful tool, but users often encounter signals at multiples of actual frequencies. While these may appear to be harmonics, they are frequently images created by the device's architecture when operating in "Ultra" mode.
The Problem: Spurs vs. Harmonics
- Below 900 MHz: Spurs are rarely an issue due to the device's design. "Ghost" signals in this range are usually true harmonics and can be avoided by keeping the input level low enough.
- Above 900 MHz: In Ultra mode, the device is more susceptible to image frequencies. For example, a strong mobile signal near 2 GHz can create a "false" signal at 4 GHz.
Recommended Setup: A Banded Approach
General-purpose broadband antennas and simple attenuation are often insufficient for clean readings. A segmented frequency strategy is highly recommended.
1. Low Frequency (Below 900 MHz)
It is impossible to achieve a perfectly "flat" broadband antenna. Instead, use multiple antennas for better accuracy:
- Sub-250 MHz: Use multiple untuned loop antennas.
- 250 MHz to 900 MHz: Use a dedicated broadband antenna for this window.
- Note: If you suspect harmonics in this range, the simplest solution is to increase your physical distance from the transmitter. IF the suspected signal disappears quicker it is probably a harmonic
2. Ultra Mode (1 GHz to 6.5 GHz)
To prevent broadband signals (like mobile phones) from creating false images, use antennas or bandpass filters that are "deaf" to signals outside your target range. Because false signals are generally 2 GHz from the real signal, use 2 GHz segments:
- 1–3 GHz: Covers 2.4 GHz Wi-Fi and mobile bands.
- 3–4.5 GHz: Isolates mid-band signals.
- 4.5–6.5 GHz: Targets 5 GHz Wi-Fi.
3. High Frequency (Above 6.5 GHz)
- 6.5–12 GHz: Focus only on specific microwave links (e.g., 10 or 12 GHz) as needed.
- Above 15 GHz: Not recommended for standard use. The tinySA Ultra’s sensitivity drops significantly here. Measuring 18 GHz+ requires specialized external Low-Noise Amplifiers (LNAs) and precision filters.
Summary Table
| Frequency Range | Primary Concern | Recommended Solution |
|---|
| < 900 MHz | Harmonics | Untuned loops; increase distance from source |
| 900 MHz – 6.5 GHz | Mirrors or ghosts | Band-specific antennas/filters (2 GHz windows) |
| 6.5 GHz – 12 GHz | Low Sensitivity | Dedicated antenna/filter pairs for known signals |
| > 12 GHz | High Noise Floor | External LNAs and specialized equipment required |
Conclusion
Do not rely on a single broadband antenna. By ensuring the tinySA is "blind" to out-of-band signals through the use of filters and tuned antennas, you eliminate false signals before they reach the mixer.
