Many comments and practical tips in my book are from my own experience, especially from my own mistakes. Two such cases are discussed in this blog.
Grounding in RF Circuits
In Section 8.6 of the book, an effect of the ground via location on signal integrity is discussed for a case where a signal trace transitions between two layers. I learned this lesson first hand when working on a reference design using an early generation of WiFi chipset which consisted of two separate ICs for the base-band and RF blocks respectively. The signal trace considered in the book was actually the clock line from the base-band chip to the RF chip. The PCB space was very tight due to the constraint by the required form factor.
In our initial layout, the ground via was placed a few millimeters away from the signal via because of a component near the signal via. Later it was found that the sensitivity of one of the channels was considerably worse than all the other channels. Luckily we were able to probe the IF signal (after down conversion) in the receiving path. In sweeping through receiving channels, we observed a large spurious signal at the exact channel where the sensitivity was severely degraded. Further, the spurious frequency turned out to be an exact harmonic of the clock frequency. This observation convinced us that the clock signal was somehow coupled to the receiving path, causing the degradation in sensitivity. By examining the clock signal path closely we determined that the via transition was the most likely place where the signal integrity was compromised. Once we re-spun the board by placing the ground via closer to the signal via, the spurious was indeed remarkably reduced, bringing the sensitivity within the specification.
Today, WiFi chipsets are almost all in the form of SoC (system on chip), and the board designer no longer needs to deal with a clock line connecting the base-band and radio sections. Nevertheless, the concept that the ground is part of a transmission line and as such should be treated carefully in PCB layout is definitely still valid.
While the sketch of the ground RF current in Fig. 8.14 is convenient for illustration, a notion of a stray EM field caused by an imperfection of ground structure is more suitable for the reader to visualize a process where the stray field is coupled to a signal line. This undesirable coupling is an additional condition for any imperfect ground structure to cause a signal integrity problem. The coupling mechanism is usually difficult to predict and is often of narrow band nature due to a resonance associated with parasitic effects. This explains why only one channel was severely affected by the clock signal in the case discussed above. It also explains that in practice a poorly implemented grounding does not always cause an EMC problem. For the specific case of ground-plane transition, some imperfection always exists. It is generally difficult to quantify its impact on signal integrity, which is one of the reasons that RF design is popularly considered an ART.
Coherent Signals in Power Combiners
The second example is about a power combiner discussed in Section 9.2.3.2, which explains that the parameters of a combiner (such as insertion loss, etc.) are specified under a condition of the two input signals being incoherent. When they are coherent, the combiner behaves quite differently. I learned this lesson many years ago in attempting to do a two-tone measurement. In my initial setup I used a signal generator with dual output along with a Wilkinson combiner to supply the required two-tone signal to the DUT. But I quickly found that the measurement results did not make sense. After verifying the combiner, it was clear that the problem was associated with the signal generator. Upon reading the manual more closely, I realized that although the two output signals from the generator were synthesized independently they were phase-locked to the same reference oscillator. In other words, the two signals are coherent.
Mouqun Dong is author of RF Circuits and Applications for Practicing Engineers. For more information or to buy, click here.
