Proper Routing Makes For Many Happy Return Paths

Here’s a question for you: when your PCB has a ground plane layer, where do return signals flow? It seems like a trick question, but as [Kristof Mulier] explains, there’s more to return path routing than just doing a copper pour and calling it a day.

Like so many other things in life, the answer to the above question is “it depends,” and as [Kristof] ably demonstrates in this concise article, the return path for a signal largely depends on its frequency. He begins by explaining current loop areas and how they factor into the tendency for a circuit to both emit and be susceptible to electromagnetic noise. The bigger the loop area, the worse things can get from a noise perspective. At low frequencies, return signals will tend to take the shortest possible path, which can result in large current loop areas if you’re not careful. At higher frequencies, though, signals will tend to follow the path of minimal energy instead, which generally ends up being similar to the signal trace, even if it has a huge ground plane to flow through.

Since high-frequency signals naturally follow a path through the ground plane that minimizes the current loop, that means the problem takes care of itself, right? It would, except that we have a habit of putting all kinds of gaps in the way, from ground plane vias to isolation slots. [Kristof] argues that this can result in return paths that wiggle around these features, increasing the current loop area to the point where problems creep in. His solution? Route all your signal return paths. Even if you know that the return traces are going to get incorporated into a pour, the act of intentionally routing them will help minimize the current loop area. It’s brilliantly counterintuitive.

This is the first time we’ve seen the topic of high-frequency return paths tackled. This succinct demonstration shows exactly how return path obstructions can cause unexpected results.

Thanks to [Marius Heier] for the tip.

The Perils Of Return Path Gaps

The radio frequency world is full of mysteries, some of which seem to take a lifetime to master. And even then, it seems like there’s always something more to learn, and some new subtlety that can turn a good design on paper into a nightmare of unwanted interference and unexpected consequences in the real world.

As [Ken Wyatt] aptly demonstrates in the video below, where you put gaps in return paths on a PCB is one way to really screw things up. His demo system is simple: a pair of insulated wires running from the center pins on BNC jacks and running along the surface of a piece of copper-clad board to simulate a PCB trace. The end of each wire is connected to the board’s ground plane through a 50 ohm resistor, with one wire running over a narrow slot cut into the board. A harmonics-rich signal is fed into each trace while an H-field EMC probe connected to a spectrum analyzer is run along the length of the trace.

With the trace running over the solid ground plane, the harmonics are plentiful, as expected, but they fall off very quickly away from the trace. But over on the trace with the gapped return trace it’s a far different story. The harmonics are still there, but they’re about 5 dBmV higher in the vicinity of the gap. [Ken] also uses the probe to show just how far from the signal trace the return path extends to get around the gap. And even worse, the gap makes it so that harmonics are detectable on the unpowered trace. He also uses a current probe to show how common-mode current will radiate from a long conductor attached to the backplane, and that it’s about 20 dB higher with the gapped trace.

Hats off to [Ken] for this simple explanation and vivid reminder to watch return paths on clock traces and other high-frequency signals. Need an EMC probe to check your work? A bit of rigid coax and an SDR are all you needContinue reading “The Perils Of Return Path Gaps”

A Look Inside A 70-GHz Electromechanical Attenuator

It might not count as “DC to daylight,” but an electromechanical attenuator that covers up to 70 GHz is pretty close, and getting a guided tour of its insides is quite a treat.

Perhaps unsurprisingly, this one comes to us from [Shahriar] at “The Signal Path,” where high-end gear most of us never get a chance to work with goes for one last hurrah after it releases the magic smoke. And indeed, that appears to be exactly what happened to the Rohde & Schwarz 75 dB step attenuator, a part that may have lived in the front end of one of their spectrum analyzers. As one would expect from such an expensive component, the insides have some pretty special engineering. The signal is carried through the five attenuation stages on a narrow strip of copper. Each stage uses a solenoid to move the strip between either a plain conductor or a small Pi pad with a specified attenuation. The attention to detail inside the cavity is amazing, with great care taken to maintain the physical orientation of the stripline to prevent impedance mismatches and unwanted reflections.

The Pi pads themselves are fascinating, too, especially under [Shahriar]’s super-duper microscope. All of them were destructively removed from the cavity before getting to him, but it’s still pretty clear what’s going on. That’s especially true with the 5-dB pad, which bears clear signs of the overload that brought on the demise of the whole attenuator. We suppose a repair would have been feasible if it had been just the one pad that needed replacement, but with all of them broken, it’s off to the scrap bin. Or to the recycler — there appears to be plenty of gold in there.

We thought this was a fantastic look under the covers of an exquisitely engineered part. Too bad it didn’t rate the [Shahriar] X-ray treatment, as this multimeter repair or this 60-GHz phased array did. Oh, well — maybe next time.

Continue reading “A Look Inside A 70-GHz Electromechanical Attenuator”

SMA Connector Footprint Design For Open Source RF Projects

When you first start out in the PCB layout game and know just enough to be dangerous, you simply plop down a connector, run a trace or two, and call it a hack. As you learn more about the finer points of inconveniencing electrons, dipping toes into the waters of higher performance, little details like via size, count, ground plane cutouts, and all that jazz start to matter, and it’s very easy to get yourself in quite a pickle trying to decide what is needed to just exceed the specifications (or worse, how to make it ‘the best.’) Connector terminations are one of those things that get overlooked until the MHz become GHz. Luckily for us, [Rob Ruark] is on hand to give us a leg-up on how to get decent performance from edge-launch SMA connections for RF applications. These principles should also hold up for high-speed digital connections, so it’s not just an analog game.

Continue reading “SMA Connector Footprint Design For Open Source RF Projects”

Don’t Let The Baluns Float Over Your Head

Most ham radio operators will build an antenna of some sort when they first start listening or transmitting, whether it’s a simple dipole, a beam antenna like a Yagi, or even just a random wire vertical antenna. All of these will need to be connected feedline of some sort, and in the likely event you reach for some 50-ohm coax cable you’ll also need a balun to reduce noise or unwanted radiation. Don’t be afraid of extra expenses when getting into this hobby, though, as [W6NBC] demonstrates how to construct an “ugly balun” out of the coax wire itself (PDF).

The main purpose of a balun, a contraction of “balanced-unbalanced” is to convert an unbalanced transmission line to a balanced one. However, as [W6NBC] explains, this explanation obscures much of what baluns are actually doing. In reality, they take a three-wire system (the coax) and convert it to a two-wire system (the antenna), which keeps all of the electrical noise and current on the shield wire of the coax from interfering with the desirable RF on the interior of the coax.

This might seem somewhat confusing on the surface, as coax wires only have a center conductor and a shield wire, but thanks to the skin effect which drives currents to the outside of the conductor, the shield wire effectively becomes two conductors when taking into account its inner and outer surfaces. At these high frequencies the balun is acting as a choke which keeps these two high-frequency conductors separate from one another, and keeps all the noise on the outside of the shield wire and out of the transmitter or receiver.

Granted, the world of high-frequency radio circuits can get quite complex and counter-intuitive and, as we’ve shown before, can behave quite unexpectedly when compared to DC or even mains-frequency AC. But a proper understanding of baluns and other types of transformers and the ways they interact with RF can be a powerful tool to have. We’eve even seen other hams use specialty transformers like these to make antennas out of random lengths and shapes of wire.

Continue reading “Don’t Let The Baluns Float Over Your Head”

Low-Cost RF Power Sensor Gets All The Details Right

Dirty little secret time: although amateur radio operators talk a good game about relishing the technical challenge of building their own radio equipment, what’s really behind all the DIY gear is the fact that the really good stuff is just too expensive to buy.

A case in point is this super-low-cost RF power sensor that [Tech Minds (M0DQW)] recently built. It’s based on a design by [DL5NEG] that uses a single Schottky diode and a handful of passive components. The design is simple, but as with all things RF, details count. Chief among these details is the physical layout of the PCB, which features a stripline of precise dimensions to keep the input impedance at the expected 50 ohms. Also important are the number and locations of the vias that stitch the ground planes together on the double-sided PCB.

While [Tech Minds]’ first pass at the sensor hewed closely to the original design and used a homebrew PCB, the sensor seemed like a great candidate for translating to a commercial PCB. This version proved to be just as effective as the original, with the voltage output lining up nicely with the original calibration curves generated by [DL5NEG]. The addition of a nice extruded aluminum case and an N-type RF input made for a very professional-looking tool, not to mention a useful one.

[Tech Minds] is lucky enough to live within view of QO-100, ham radio’s first geosynchronous satellite, so this sensor will be teamed up with an ADC and a Raspberry Pi to create a wattmeter with a graphical display for his 2.4-GHz satellite operations.

Continue reading “Low-Cost RF Power Sensor Gets All The Details Right”

JFET Stands In For Triode In This Infinite Impedance Detector

An “Infinite Impedance Detector” might sound a little like something that [Zaphod Beeblebrox] would use to zip around the galaxy. It’s not, of course, but it is an interesting and useful demodulator for AM radio signals, as [Sebastian Westerhold] over at Baltic Labs explains in the brief but well-done video below.

If you’ve ever browsed through schematics of old vacuum tube radios, [Sebastian]’s JFET-based detector circuit might look strangely familiar. That’s because this demodulator is about as close to a direct translation between a vacuum tube circuit and a silicon circuit as possible. In fact, [Sebastian] even used literature from the triode version of this detector to figure out the values for some of the components. The only active component is a BF256B JFET; the rest are a small handful of resistors and caps. Construction is in the ever-popular ugly style.

The test setup is simple — a function generator set to 455 kHz and modulated with a 1,000 Hz sine wave. The detector demodulates the audio signal very cleanly, judging by the oscilloscope traces. Just for fun, [Sebastian] also tried a 10.7 MHz carrier with a 1,500 Hz audio modulation, and that worked fine too. He also tried a variation on the circuit with an IF transformer on the input. That circuit works just about the same as the transformerless version, although it does provide a little gain.

Earth-shattering stuff? Probably not. But it does show the fun you can have with a scrap of PCB and a few components, and seems like it could easily be the kind of project that would take you down the RF rabbit hole. Thanks to [Sebastian] for sharing this one with us.

Continue reading “JFET Stands In For Triode In This Infinite Impedance Detector”