While working on my thesis I often have to measure signals up into 2.45 GHz range be it testing a mixer or determining the transfer function of a filter. At such high frequencies standard banana jack or alligator clip cables turn into antennas which render any measurements done with them pretty much useless. I get around this inconvenience by using what the engineers I worked with last summer called “Poor Man’s RF Probes.” These probes are very easy to use and you can make a pair of your own for $10 assuming you already have shielded BNC to SMA cables and a few SMA billets. Semi-rigid SMA male to male cables are used to make the probes themselves; here’s a link to the ones that I’m using. There may be cheaper cables out there and if you can find them I’d love to hear about it but in order to make the probes the outer jacket needs to be exposed in order to solder it directly to ground.
To make the probes themselves first cut the semi-rigid cable just below each SMA jack leaving a small amount of shielded cable to strip. Figure 1 below shows two of the probes I am using at the moment next to an uncut cable and a Digi-Key label for reference. Each probe is just over an inch long or so giving me enough room to strip the ends and bend the probe as needed while still keeping the overall length short enough to ensure quality measurements.
When stripping the cut cable, I’ve found the easiest way to strip the rigid outer jacket is to use needle nose pliers. Gripping the end of the jacket with the tip of the pliers and slowly bending it back and forth a few times is usually enough to cause the jacket to break all the way around the cable and you can then just slip it off leaving the center conductor insulator exposed. The center conductor insulator can be stripped using standard wire strippers (~ 20 Gauge). The goal is to minimize the amount of exposed center conductor keeping the probe close to the measurement point. I recommend practicing on the unusable middle portion of the cable that will be left over to get the hang of it before stripping the probes themselves.
Once you get the probes themselves made, figure out where you’re actually placing the probes on your board next. Look for a relatively open area of ground plane close to the pad where you will be measuring from. Bending the stripped probe slightly is necessary for both a good ground connection and the probe’s mechanical stability. Note: It’s possible to snap the center conductor from too much bending resulting in a useless probe that will only cause headaches later on so bend with care.
Once you’ve determined where you are going to solder down the probe, use a hobby knife or similar tool and carefully scrape away the solder mask on the ground plane near the measurement site. Be gentle but firm when scrapping because gouging the board too deeply could short the ground plane to any internal layers that might be in your board. Figure 2 shows what the site around the input to my filter where I’m placing the probe. In the picture, I’m placing the conductor of the probe on the unpopulated pad of R2 making use of the 0Ω resistor trick.
My apologies for the so-so photo quality during the remainder of this post; my camera phone only works so well…
The third step of the process is to solder down the center conductor of the stripped SMA cable. Usually I’ll tin the pad with a little solder before placing the probe, blob some solder on the iron and while holding the probe in my fingers, tack solder the center pin down (Figure 3). Note: The probe should be able to stand on its own right now but I wouldn’t move the board or probe too much as you could lift the pad the center conductor is soldered down to.
The next to last step is to solder the exposed ground plane to the outer metal jacket of the probe for a solid ground connection. This will also make the probe mechanically sound so you can now make sure the center conductor is properly soldered down without worry. Note: This step can be done before the previous step if you so choose. Just don’t be like me and hold the probe with your fingers because if you’re not quick you get burned. I find this method easier as it make shorting the center conductor to ground less likely.
Finally before you can consider yourself done, use a multi-meter and make a quick continuity check. Be sure the center conductor is not shorted to ground and that it is actually connecting to the node you would like it to. Note: If you have to desolder the probe because of a short or to remove it altogether desolder the center conductor first to avoid lifting the pad.
If you treat these probes with a little care they should last you long enough to justify their cost. After enough wear and tear though they will become unusable and have to be replaced. I’ve been using the same two probes for over six months now with no lose in measurement quality and they’ve been removed and resoldered quite a few times. Before each use however, I do recommend checking the continuity of the center conductor just to be sure it’s still good. Troubleshooting a bad probe isn’t exactly fun and can make you lose your hair when your circuit mysteriously stops working.