Charlie ZL2CTM has been pushing out a series of excellent videos on his YouTube channel that take you through the circuit design, making and testing of superhet SSB transceiver modules. Yesterday my eyebrow involuntarily raised ever so slightly at his use of Veroboard as the substrate for his latest 8MHz crystal filter. Veroboard? At radio frequencies? Veroboard et les fréquences radio ne sont pas compatibles!
Veroboard is a remarkable product, continuously available and literally unchanged since my youth… I’ve been using it for over 40 years. It is phenolic board, punctuated by a grid of 0.1 inch holes, with bonded copper strips running its length. To use it, you push thru-hole component leads through and solder them to the tracks, cutting the copper over a hole to break a conducting bus as required.
Breaking a copper track can be done just by twisting a sharp drill bit centered on a hole. This operation can easily be done by hand. If you use a drill you’ll sooner or later go right through the board. As I’ve done a hundred times. You don’t want to know about the times my finger has been on the other side! 😲😭🤕
Tribal knowledge holds that the small amounts of capacitance between adjacent copper strips makes Veroboard unsuitable for use at radio frequencies. Well it obviously worked for Charlie. It made me wonder, what might the capacitance be? Then it dawned on me how easy it would be to measure it.
I soldered two leaded component tails to adjacent tracks on a scrap of Veroboard and measured the capacitance on my LC Meter. On a piece of board 6cm long I got 4.2pF.
What about two tracks separated by a track? 2.7pF. What if I tested the longest piece of Veroboard I could find? On a piece 18cm long I measured 11.8pF between adjacent tracks.
I dropped Charlie a note on YouTube.
VK3HN> I was wondering if the Veroboard would introduce capacitance by virtue of the parallel conductors. Soldered pigtails to the end of adjacent tracks and measured 4 pF. Wonder if this is material at 8MHz? Best way to tell would be to make up a second filter ugly style and compare. Probably not worth doing but an interesting thought!
ZL2CTM> That could very well be the case. I do try and break tracks where they are not needed in an effort to prevent outputs from influencing inputs. I’ll probably leave it for now and compensate it in software.
I found a forum thread discussing this topic. People suggested grounding all unused tracks to form a patchwork ground plane, trimming tracks as short as possible, and even applying excessive soldering iron heat to lift unused tracks.
In summary, random capacitance of a few pF scattered around a module might not be an issue up to (say) 10MHz, and sensible techniques like grounding as much copper as possible should help. I wouldn’t do it with a gain stage, like a grounded gate JFET or dual gate MOSFET RF amplifier, even at low radio frequencies, where a hint of stray capacitance can initiate VHF oscillations. But for low or zero gain low radio frequency or audio circuits, Veroboard might be a reasonable choice, if you want to avoid cooking up a custom PCB or working Manhattan/Ugly.