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Dynamic Range improvements using Option LNP vs RF preamp

Question asked by pxaphil on Apr 27, 2010
Topic: Low Noise Path (Opt LNP) for improved dynamic range in certain use cases
LNP provides dynamic range improvement for signal measurements near -20dBm input level.

Background: The PXA provides an optional Low Noise Path (Option LNP) which bypasses some of the lossy elements in the analyzer’s RF input signal path, improving sensitivity without activation of the internal preamplifier (Option P03, P08, P13 and P26).

Question: What is the advantage of option LNP (Low Noise Path) over using the internal pre-amplifiers to improve sensitivity for high dynamic range measurements?

Answer: Firstly it is important to note that LNP is only available above 3.6 GHz frequencies (PXA high band operating region is from 3.6 GHz up to 26.5 GHz).
The LNP and pre-amplifier can both be used to improve the sensitivity of the analyzer, but not simultaneously. Using the PXA’s internal preamplifier provides the ultimate improvement in DANL, however activation of the preamplifier will also reduce the 1 dB gain compression point of the analyzer. Using the low noise path provides about 4 to 12 dB of DANL improvement with the same amount of degradation of the compression point, thus keeping the compression-to-noise dynamic range constant. In contrast, using the preamplifier improves the DANL by more, up to 17 dB at some frequencies, but degrades the compression point even more still, over 30 dB at some frequencies, giving poorer compression-to-noise dynamic range. This same situation applies to third-order dynamic range too. A typical use case where the low noise path can be beneficial is in situations where low-level signals are being measured in the presence of a large-signal main RF carrier.  For examples, consider the cases of harmonic measurements or intermodulation distortion testing. 

For intermodulation distortion, the situation is different. Figure 2 shows the area of improved dynamic range when using Option LNP in a measurement dominated by third-order intermodulation distortion.   For signal measurements in the –20 dBm range requiring highest possible dynamic range, using the Low Noise Path can provide approximately 8 dB of improvement.  Thus, the maximum available dynamic range did not improve from using the LNP, but the range of power levels over which the maximum dynamic range can be achieved is broadened.

The same effect seen in Figure 2 with intermodulation distortion will also occur when the measurement limitation is due to front-end compression, as might happen with extremely narrow radar pulse signals. The LNP allows a broader range of input powers for maximum dynamic range before the preamp must be activated.

An example is shown in Figure 1, using the Low Noise Path (shown in Yellow) with Delta markers to show the effective difference between a 10 GHz fundamental tone at –20 dBm amplitude, and measuring the second harmonic at 20 GHz.  If you turn the preamplifier on to measure the low-level 2nd harmonic signal, and add sufficient step attenuation to counteract the preamplifier gain (20 to 26 dB), the noise level is increased while the available dynamic range is actually reduced due to the non-linear effects of the preamp.   The fundamental is causing distortion in the analyzer’s front end, reducing the dynamic range by over 30 dB for this particular measurement! (The input levels and attenuation used in this example was chosen to highlight the effects of gain compression.) For harmonic distortion, the LNP bypasses semiconductor switching elements that dominate the distortion, so dramatic improvements in dynamic range can be achieved.

For more information and figures, please view the attached file.  

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