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Why FR-4 Is Often Inadequate for RF Design

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Note: Ed Troy, owner of Aerospace Consulting, LLC, was kind enough to offer a few of his articles for posting on RF Cafe. With more than 30 years in the electronics communications design field, Ed has a lot of valuable knowledge to impart to us mortals ;-) This second paper discusses why using a less expensive substrate material for a production printed filter design can be much more costly in the long run. As the old says goes, a picture (in this case a screen capture) is worth a thousand words. He's a highly rated airplane pilot as well, which counts for something in my book. Stay tuned for others...

Why FR-4 Is Often Inadequate for RF Design
Using Simulation Tools to Troubleshoot an LC Filter Design
Why Using Spectral Domain Simulation is Vitally Important When Designing Complex RF, Microwave, and Wireless Systems
Using Simulation Tools to Debug an Oscillating Amplifier

Why FR-4 Is Often Inadequate for RF Design

By Ed Troy,  Aerospace Consulting, LLC

Many times, designers want to use FR-4 for RF and microwave circuit designs. After all, it is less expensive than other materials, like Rogers 4350, and much less expensive than reinforced PTFE composite boards such as Rogers RT/Duroid 5870. However, sometimes it is necessary to spend the money for higher quality circuit board material and processing to get good, repeatable designs.

In this paper, to demonstrate the danger of using FR-4 for some designs, I will design a bandpass filter to work in the 2.4 GHz FCC Part 15 band which is typically used for WiFi, Bluetooth, ZigBee, cordless phones, and many other applications. The filter will be designed using the microwave filter design tool that is part of the Keysight (formerly Agilent) Genesys suite of RF and microwave design software. The pass band is defined as 2.4 to 2.483 GHz. I set an insertion loss goal of 3 dB. The filter program first designs a filter which is close to the desired performance, but it must be optimized to get the performance that is closest to the desired performance. In the case of the first filter, I used FR-4 with a dielectric constant of 4.5, a loss tangent of 0.010, and a thickness of 59 mils. The metallization is 1 oz. rolled copper. Figure 1 shows the final, optimized performance.



Optimized Response of 2.4 GHz Bandpass Filter on FR-4 - RF Cafe
Figure 1 - Optimized Response of 2.4 GHz Bandpass Filter on FR-4

As we can see, it did not come very close to meeting the rejection goals, and the insertion loss is pretty horrendous at about 5 dB. Next, we will look at the performance of that filter as the dielectric constant of the FR-4 varies over the "normal" range for FR-4, which is about ± 1.


Performance of Filter Shown in Figure 1 - RF Cafe
Figure 2 - Performance of Filter Shown in Figure 1 When Dielectric Constant of FR-4 Varied from 3.5 to 5.5


Clearly, this filter would be useless in any kind of a production environment. In fact, the only way you could have any hope of producing a filter that even came close to working would be to measure the dielectric constant of your board material before etching the copper to be sure it was, indeed, 4.5. Figure 2 only shows the various S21, or insertion loss, plots. It does not show return loss, S11, but it is obviously going to be all over the place, as well. I did not show it only because it would uselessly clutter up the graph. Also, this Monte Carlo simulation did not take into account variations in the thickness of the board, over-etch or under-etch of the copper pattern, or variations in the loss tangent. Those factors would only make the situation worse. Next, I designed the same filter using Rogers RT/Duroid 5870. This is a very high quality microwave dielectric material. It has a dielectric constant of 2.33 ±0.02. It has a loss tangent of 0.0012 (which is an order of magnitude better than that of the FR-4) and a thickness of 62 mils. Again, I used 1 oz. rolled copper for the metallization. (Actually, the dielectric constant of 62 mil RT5870 is 2.333 according to a calculator program from Rogers Corporation called MWI-2014, but I am ignoring that for this paper. However, if you wanted to actually build this filter, it would be prudent to use that value in you design effort.)

Figure 3 shows the simulated performance of the optimized filter. This is clearly a huge improvement. The insertion loss is about 3 dB, 2 dB better, and the rejection is much better.


Simulated Performance of Optimized Filter on RT/Duroid 5870 Material - RF Cafe
Figure 3 - Simulated Performance of Optimized Filter on RT/Duroid 5870 Material


Now for the real test, and a critical reason for choosing materials with a more consistent dielectric constant. I performed a Monte Carlo analysis on this filter, again varying only the dielectric constant. But, in this case, we only had to vary it over a range of ±0.02, instead of ±1. The result is dramatically better, as shown in figure 4.


Monte Carlo Analysis of RT/Duroid Filter - RF Cafe
Figure 4 - Monte Carlo Analysis of RT/Duroid Filter


This is a filter that might actually work for a real product that could be reliably re-produced. Of course, this filter does not meet the pass band and stop band requirements, either, but it is sure a lot closer. But, those limits were arbitrarily set. In a real design, this might be adequate. If a better pass band or stop band response was required, then the filter would need more poles. This would make it larger, and it would increase the insertion loss, somewhat, but those are the trade-offs that are necessary when designing microwave filters. Alternatively, other materials, with even lower loss, could be used. This would tend to sharpen the stop band rejection and decrease the insertion loss. Thus, it might be possible to meet the design goals without having to go to more poles. If smaller physical size was required, a material with a higher dielectric constant could be used.

Figure 5 shows the results of an electromagnetic simulation using Momentum GXF from within Genesys on the same graph as the original optimized linear simulation of the RT/Duroid filter. It clearly shows that before attempting to actually manufacture this filter, the design would have to be optimized within an EM simulation program to obtain proper performance. (EM simulation results are in green and orange). Figure 6 shows what the filter would look like. This circuit board is about 4.8" x 3.9".


RT/Duroid 5870 Linear and EM Simulation Predictions - RF Cafe
Figure 5 - RT/Duroid 5870 Linear and EM Simulation Predictions


Physical Appearance of RT/Duroid 5870 Bandpass Filter - RF Cafe
Figure 6 - Physical Appearance of RT/Duroid 5870 Bandpass Filter


To download an Adobe Acrobat copy of this paper, click here.


Copyright 2015, Aerospace Consulting LLC. All rights reserved.


About Aerospace Consulting, LLC

Aerospace Consulting was started in 1982 as a part-time business. The founder, Ed Troy, had just changed jobs from working for a company that made telemetry transmitters and other military communications equipment to a company that produced electronic article surveillance equipment. While working at ICI Americas, Ed had an opportunity to do some design consulting with a company in New York that made hardware related to anti-submarine warfare. Additionally, Narco Avionics needed help in redesigning some DME hardware because the transistor used in their high power pulse transmitter was no longer available and a new amplifier had to be designed and developed. With permission from ICI America's management, Ed started consulting on a part-time basis for these companies that were clearly not competitors of ICI's EAS business. At the same time, Ed, a pilot with commercial, multi-engine, and instrument ratings, was brokering aircraft in his free time. So, given the work for 2 aerospace-related companies and the fact that he was brokering aircraft, Aerospace Consulting seemed a natural name for the company. Today, it might seem a bit of a misnomer since most of Aerospace Consulting's customers are not aerospace companies (although some are), but the name has not been changed. In the 25 years since going full-time, Aerospace Consulting has grown dramatically both in terms of capabilities as well as the types of businesses served. Aerospace Consulting also works with companies, both large and small, that have plenty of RF, microwave, and wireless facilities and employees, but they just need some extra help to get through an especially busy time.





Posted  March 11, 2015

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