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Created: Dec. 2024
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Introduction
Introduction
The tinySA has a built-in function that can automatically calculate the OIP3 [1]. When activated, this function prompts the user for the fundamental frequencies, which it then uses to set the frequency sweep. The function also activates trace averaging (4x) and continuously adjusts the internal attenuator to ensure accurate level reading. The internal attenuator has range of 0-32 dB and plays an important role in minimizing internally produced distortion.
As the tinySA lacked the linearity of big-box spectrum analyzers, when measuring a DUT with high OIP3, the tinySA may read a value that is artificially low because it is constrained its own system IP3 (or input IP3). For example, using -4 dBm input level, we observed a minimum (worst case) system IP3 of 18.7 dBm at 10 MHz (fig. 1). This value is right in the ballpark of the tinySA's specifications (fig. 2)
Fig. 1: The tinySA exhibits a relatively poor system IP3 of 18.7 dBm (upper side-band) when fed with 4 dBm fundamental tones
Fig. 2: IIP3 specifications for tinySA (left), tinySA Ultra (middle) & tinySA Ultra plus (right) [2].
Fig. 3: Higher system IP3 can be achieved by increasing the fundamental signal level (red trace). However, the improvement afforded by this method is limited because signal levels above 5 dBm exceeds the tinySA's accurate measurement range.
Material & method
Material & method
Big box analyzers typically use mechanical step attenuators which have very high linearity. In contrast, the tinySA likely uses a semiconductor attenuator for cost & size reduction, but it is likely a lot less linear than its mechanical counterpart. So, it is possible that the attenuator is responsible for some of the system IP3. Hence, it will be interesting to find out if replacing part of the internal attenuator with an external one can improve system IP3.
Fig. 4: Setup for measuring the tinySA's system IP3. Although the tinySA has an internal attenuator, an external one is added in order to evaluate the effect on the overall non-linearity (IP3).
Results
Results
Using a combination of internal & external attenuators can significantly improve system IP3 (fig. 5). When the total attenuation is fixed at 30 dB (light blue bars), splitting it to external 20 dB and internal 10 dB (20+10) increases IP3 ~5 dB over using all internal attenuation (0+30).
Even greater improvement can be achieved by increasing the total attenuation to 50 dB (green bars). Using external 30 dB and internal 20 dB (30+20), the system IP3 is improved to 38 dBm; i.e. an improvement of ~9 dB over the reference case of 0+30.
Fig. 5: Using a combination of internal & external attenuators can significantly improve system IP3. The best system IP3 is achieved using external 30 dB and internal 20 dB attenuation - this improves IP3 ~ 9 dB over the reference case (30 dB internal). In all cases, the input signals are fixed at 9.0 +/- 0.5 dBm
Using external 20 dB and internal 10 dB (20+10) attenuation, the best system IP3 (~34 dBm) is achieved when the input signals are in the 7~15 dBm range (fig. 6).
Fig. 6: For optimum system IP3, the input signal range should be kept within 7~15 dBm
Using external 20 dB and internal 30 dB (20+30) attenuation, the system IP3 is better than 34 dB over the HF 5~30 MHz range (fig. 7).
Fig. 7: Over 5~30 MHz, the system IP3 >= 34 dBm using ~9.9 dBm input signals
Fig. 8: Over 5~20 MHz, the system IP3 >= 36 dBm using ~15.1 dBm input signals
Using external 20 dB and internal 30 dB (20+30) attenuation, the system IP3 is better than 42 dB at 10 MHz (fig. 9).
Fig. 9: system IP3 >42 dBm at 10 MHz.
Conclusion
Conclusion
The tinySA's system IP3 can be significantly improved through manipulating the input signal level and the total attenuation.
References
References
[1] "Measuring Third Order Intermodulation", https://tinysa.org/wiki/pmwiki.php?n=Main.IIP3
[2] "Pre-sale, tinySA ULTRA plus", https://groups.io/g/tinysa/topic/109845037#msg18712
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