stripline

3 Articles

Making PCB Strip Filter Design Easy To Understand

October 28, 2024 by 4 Comments

We’ve always been fascinated by things that perform complex electronic functions merely by virtue of their shapes. Waveguides come to mind, but so do active elements like filters made from nothing but PCB traces, which is the subject of this interesting video by [FesZ].

Of course, it’s not quite that simple. A PCB is more than just copper, of course, and the properties of the substrate have to be taken into account when designing these elements. To demonstrate this, [FesZ] used an online tool to design a bandpass filter for ADS-B signals. He designed two filters, one using standard FR4 substrate and the other using the more exotic PTFE.

He put both filters to the test, first on the spectrum analyzer. The center frequencies were a bit off, but he took care of that by shortening the traces slightly with a knife. The thing that really stood out to us was the difference in insertion loss between the two substrates, with the PTFE being much less lossy. The PTFE filter was also much more selective, with a tighter pass band than the FR4. PTFE was also much more thermostable than FR4, which had a larger shift in center frequency and increased loss after heating than the PTFE. [FesZ] also did a more real-world test and found that both filters did a good job damping down RF signals across the spectrum, even the tricky and pervasive FM broadcast signals that bedevil ADS-B experimenters.

Although we would have liked a better explanation of design details such as via stitching and trace finish selection, we always enjoy these lessons by [FesZ]. He has a knack for explaining abstract concepts through concrete examples; anyone who can make coax stubs and cavity filters understandable has our seal of approval.

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A Look Inside A 70-GHz Electromechanical Attenuator

March 15, 2024 by 4 Comments

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.

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A Microwave Frequency Doubler

October 18, 2021 by 14 Comments

It is an age-old problem. You have a 2.5 GHz source and you want it at 5 GHz. You need a frequency doubler. [All Electronics Channel] has an interesting video that talks not only about the theory of such a device but shows a practical one made with copper strips on a blank PCB substrate.

A fun thing about microwaves is that even little strips of copper are circuit elements since the wavelength at 2.5 GHz is only 12cm. That means a quarter-wave stub is only 3 cm — just over an inch.

The construction technique used is simple and, as he points out, experimenting with a real circuit will give you much more feel for how these circuits work than just reading and working out the math.

The multiplier drives an amplifier into nonlinearity which, of course, generates harmonics. Then a bandpass filter selects the second harmonic. If you haven’t dealt with stub circuits before, you might want to read up on how a piece of copper connected at one end can act like an inductor, a capacitor, or even a tuned circuit.

If you want more detail on the copper tape technique, we can help. If you don’t want to double frequency, maybe you would prefer to try voltage.

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