System Specialist at AWR
Understanding and Correctly Predicting Critical Metrics for Wireless RF Links
February 14th, 2013 by Joel Kirshman
Understanding and correctly predicting cellular, radar, or satellite RF link performance early in the design cycle has become a key element in product success. The requirements of today’s complex, high performance wireless devices are driving designers to assess critical measurements—noise figure (NF), 1dB gain compression (P1dB), third order intermodulation distortion versus output power (IM3dBc), and signal-to-noise ratio (SNR)—long before manufacturing begins. Traditional modeling methods such as rules of thumb and spreadsheet calculations (Friis equations) give limited insight on the full performance of an RF link in next-generation wireless products. This white paper highlights the advantages of using specialized RF system simulation software to accurately predict critical metrics for wireless RF links.
Simulation Software—A Novel Approach
A more modern approach is to use a tool such as AWR’s Visual System Simulator™ (VSS) software to determine system specifications (Figure 2). This tool is built specifically to exceed the capabilities of the traditional spreadsheet method and offer optimization features such as budget analysis and spur analysis. With this approach, designers can start from a spreadsheet interface to define the components (mixer, amplifier, etc.), whether file- or circuit-based, go on to define the circuit measurements such as cascaded noise figure and cascaded P1db, and also automatically generate a system diagram. This method provides far greater insight into what is happening. RF behavioral, circuit-based, and file-based models are available in VSS that account for voltage standing wave ratio (VSWR) effects and frequency dependence, as well as support yield analysis and optimization.
Example 1 – Accounting for PSD at the Input of a mixer
As Figure 3 shows, the RF link starts with the continuous wave (CW) source, followed by the amplifier, the filter, the attenuator, and finally the conversion. Looking at the low order (LO) path, there is a CW tone, an attenuator, a model representing a cable, the amplifier, and then it goes directly into the LO. When the analysis is run in the VSS software, a significant difference in the cascaded NF can be noted between the traditional spreadsheet, which shows 4.64dB, and the specialized software, which shows 11.45dB.
What’s happening to this link that is causing the discrepancy? The math was done correctly and entered into the spreadsheet, so the expectation is that the NF should be 4.6dB. The design was built and the analysis done, but the VSS simulation does not return anything close to 4.6dB. Why? The VSS tool is much more sophisticated than a spreadsheet and has lots of capabilities and measurements, so the next step would be to try doing a further analysis of the LO link.
The analysis done here is to make the PSD measurement at the very beginning of the LO link. It can be seen in Figure 4 that the PSD at the input of the mixer’s LO is -138.6dBm/Hz. This is due to the fact that there is an amplifier prior to the LO input of the mixer. When there is gain and NF in the amplifier, what does that do to thermal noise? The NF goes up, as shown in Figure 4. Typical spreadsheet equations do not account for this, so while designers can follow the book and do everything right, the VSS software tool provides greater insight on the link.
What’s the solution to this problem? Place a filter after the amplifier and, as shown in Figure 5, the noise density at the input of the mixer goes down to 174dBm/Hz and the software gives the expected measurement of 4.63dB.
Example 2 – Accounting for Reflections
Going down to the circuit level, what happens if a particular component level is switched or if the inductance of one of the inductors in this filter is tuned? As the S11 response in the reflection is changed, once again the impact on NF can be seen (Figure 7). The filter’s response changes with inductance value, and the resulting NF of the link fluctuates as well. Changes in S11 and S21 result in changes in cascaded measurements.
Two cases have now been demonstrated where the VSS simulation tool has differentiated from the typical spreadsheet. In the first case, the PSD running through the LO path causes the NF to go up due to the higher PSD in thermal, and in the second case, tuning or optimizing on the inductance value and changing the S11 changes the NF.
The final example (Figure 8) is a little more complicated. The goal here is to measure the ratio of a third order intermodulation product to the carrier IM3dBc.
First, the typical spreadsheet process is used to define the components, and then the system is built in the software using a budget analysis tool such as VSS RFB™ (RF Budget Analysis) (Figure 9).
Once again, it can be seen that VSS does not agree with the standard spreadsheet measurement. The spreadsheet calculation is 39.577dBm and the VSS calculation is 30.845dBm (29.9dBm – 0.945dBm). Why do the two methods differ?
Let’s look more closely. The third order intermodulation product is measured at the output of the second mixer and compared to the spreadsheet (Figure 10). In the software, the intermodulation product is -92dBm, and the spreadsheet is calculating -97dBm. Only the ratio of that tone to the third order intermodulation product is being measured. To get to -97dBm, the previous value for the third order intermodulation is -101dBm, and -105dBm has been voltage combined to get to -97dBm. The software is calculating -92dBm—what is going on?
AWR’s VSS software offers a spur analysis tool called RFI™, short for RF Inspector, which enables users to understand the contributions to a particular spur and what causes the spur to be at that particular value—indicating there is something contributing to that third order intermodulation product to make it higher than expected, something that is folding into the third order intermodulation frequency (Figure 11).
Note there are no frequencies labeled, but there is a combination of Tones A, B, C in the index of one on the first LO, the combination of Tone A minus B minus C plus the LO is causing that value of -105dBm to be -99dBm. That is what the software calculated. Something folded over into that third order intermodulation product.
So, the voltage combined -95.5dBm + -101dBm equals -92dBm. What is coming into that mixer? At this point, RFI reveals that Tones C, A, and B are combined with the LO (Figure 12), and that value fell onto the third order modulation product. So how can the combination of Tones C, A and B in the LO be removed?
The mathematics in the spreadsheet can’t tell us what to do, but VSS’s spur analysis does. It suggests that a filter be placed to eliminate unwanted tones (Figure 13).
A filter can be placed at that point to eliminate any of the Tones—A, B, or C. The LO can’t be eliminated, but once one of the tones is suppressed, the software gives exactly what the spreadsheet said: -101dBm from the previous and -106dBm from VSS. (Note: if designers are worried, they can dive into -106dBm to see what contributed to that to make a difference from the -105dBm.)
So once again the software was able to find the problem and offer a solution. A filter is added, two of the tones are reduced, the measurement is made, and the results are now reconciled (Figure 14).
Category: Visual System Simulator