A Wire Is an Inductor (EE Tip #126)

I’m confident you know that you should keep wires and PCB tracks as short as possible. But I’m also sure that you will underestimate this problem fairly frequently.

Remember that 1 cm of a 0.25-mm-wide PCB track is roughly equivalent to an inductance of 10 nH. If this 10 nH is paired with, say, a 10-pF capacitor, that gives a resonant frequency as low as 500 MHz, which is easily below the third or fifth harmonics of the clock frequencies commonly seen on modern high-speed digital boards. Similarly, a 1-cm-long track will jeopardize the performances of any RF system such as a 2.4-GHz transceiver. There is only one solution: keep tracks and wires as short as possible. If you can’t, then use impedance-matched tracks.

Remember this rule especially for the ground connections: any grounded pad of any part working in high frequencies should be directly connected by avia to the underlying ground plane. And this via must be as close as possible to the pad, not some millimeters away.

Just yesterday I did a design review of a customer’s RF PCB. A small 0402 inductance was grounded through a via that was 3 mm away. It was a bad idea because the inductance was as low as 1 nH. Those 3 mm changed its value completely.—Robert Lacoste, “Mixed-Signal Designs,” CC25:25th Anniversary Issue, 2013. 

Prototyping for Engineers (EE Tip #111)

Prototyping is an essential part of engineering. Whether you’re working on a complicated embedded system or a simple blinking LED project, building a prototype can save you a lot of time, money, and hassle in the long run. You can choose one of three basic styles of prototyping: solderless breadboard, perfboard, and manufactured PCB. Your project goals, your schedule, and your circuit’s complexity are variables that will influence your choice. (I am not including styles like flying leads and wire-wrapping.)PrototypeTable

Table 1 details the pros and cons associated with each of the three prototyping options. Imagine a nifty circuit caught your eye and you want to explore it. If it’s a simple circuit, you can use the solderless breadboard (“white blob”) approach. White blobs come in a variety of sizes and patterns. By “pattern” I mean the number of the solderless connectors and their layout. Each connector is a group (usually five) of tie points placed on 0.1″ centers. Photo 1 shows how these small strips are typically arranged beneath the surface.Prototype p1-4

Following the schematic, you use the tie points to connect up to five components’ leads together. Each tie point is a tiny metal pincer that grips (almost) any lead plugged into it. You can use small wires to connect multiple tie points together or to connect larger external parts (see Photo 2).

If you want something a bit more permanent, you might choose to use the perfboard (“Swiss cheese”) approach. Like the solderless breadboards, perfboards are available in many sizes and patterns; however, I prefer the one-hole/ pad variety (see Photo 3). You can often find perfboards from enclosure manufacturers that are sized to fit the enclosures (see Photo 4).

There is nothing worse than wiring a prototype PCB and finding there isn’t enough room for all your parts. So, it pays to draw a part layout before you get started just to make sure everything fits. While I’m at it, I’ll add my 2¢ about schematic and layout programs.

The staff at Circuit Cellar uses CadSoft EAGLE design software for drawing schematics. (A free version is available for limited size boards.) I use the software for creating PCB layouts, drawing schematics, and popping parts onto PCB layouts using the proper board dimensions. Then I can use the drawing for a prototype using perfboard.

The final option is to have real prototypes manufactured. This is where the CAD software becomes a necessity. If you’ve already done a layout for your hand-wired prototype, most of the work is already done (sans routing). Some engineers will hand-wire a project first to test its performance. Others will go straight to manufactured prototypes. Many prototype PCB manufacturers offer a bare-bones special—without any solder masking or silkscreen—that can save you a few dollars. However, prices have become pretty competitive. (You can get a few copies of your design manufactured for around $100.)

There are two alternatives to having a PCB house manufacture your PCBs: do-it-yourself (DIY) and routing. If you choose DIY approach, you’ll have to work with ferric chloride (or another acid) to remove unwanted copper (see Photo 5). You’ll be able to produce some PCBs quickly, but it will likely be messy (and dangerous).Prototype p5-6

Routing involves using an x-y-z table to route between copper traces to isolate them from one another (see Photo 6). You’ll need access to an x-y-z table, which can be expensive.—CC25, Jeff Bachiochi, “Electrical Engineering: Tricks and Tools for Project Success,” 2013.

This piece originally appeared in CC25 2013