Tag Archives: Troubleshooting

The Poor Man’s RF Probes

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.

Figure 1: Semi-Rigid SMA Cable Cut to Length

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…

Figure 2: Scraped GND Plane Solder Mask Near Probe Site

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.

Figure 3: Tack Soldered Center Conductor

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.

Figure 4: Soldered Ground Connection

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.

Figure 5: Final Probe Placement for Filter Testing

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The Value of 0 Ω

I learned the value of 0 Ω resistors while on co-op during the summer and fall of 2008.  My official title was NPI (New Product Introduction) Electrical Intern and I worked in the department in charge of transitioning new products from the R&D stage to production.  My responsibilities included building and documenting the electrical test fixtures that were used out on the production floor to test products at various stages of completion. Being at the end of my second of five years in school, I knew very little about anything at the time, and this being my first real world engineering experience, I was not the one who designed these fixtures.

However, being able to work with the man who did design the circuitry for these fixtures was, I can honestly say, awesome.  Over the six months I worked with him I came to think of his as the Yoda of circuit design. He was an old-time analog guru and I have no idea how long he’s been designing circuits but it seemed like he knew everything. Whenever there was a complex board to test or debug Yoda and I would sit together in his cube,measuring various voltages and what not and he would always patiently explain to me how something worked or why a certain portion of the layout had to be a certain way. I learned a lot from Yoda about design and troubleshooting, but my favorite trick he used was to stick 0 Ω resistors and unpopulated passive component footprints in key areas all over the board.

Figure 1: Example Circuit of Yoda's Techniques

Figure 1 above shows an example circuit containing the techniques I learned from Yoda.  Capacitor C1 and Resistor R3 would be marked NP, for Non-Populated, on the schematic by Yoda indicating to myself that the components were not to be soldered down when building the board.  If, when checking output of the op-amp stage, the output looked a little noisy, placing a capacitor in the feedback path to filter out noise above a certain frequency wouldn’t require a complex rework. The component was simply soldered down on the empty pads and testing could continue.

As testing would go on for the op-amp stage, the empty pads where R3 should be were convenient places to place a multi-meter or oscilloscope probe.  Only after the op-amp was working correctly would I be told to solder a 0 Ω resistor in place for R3 and testing would then continue on down the signal chain.

When it came time to finalize the schematic, if C1 was needed, its value would be inserted into the schematic and the NP marker removed, otherwise it would be deleted.  R3 was typically left in place because if a problem arose with U1 in the future the resistor could simply be tombstoned, thus isolating the op-amp from the rest of the circuit for testing.  This in and of itself was a useful debugging technique for if the problem disappeared after removing R3 then we knew something down the line was affecting the loading on U1 and the op-amp was probably fine.

Another thing I admired Yoda for was that he always came up with multiple circuit blocks to implement a desired function.  The two or three most feasible of these blocks would be placed on the test fixture PCB in parallel with each other and 0 Ω resistors were put in series with their inputs and outputs as shown below in Figure 2.  His most likely choice for that given feature was soldered down first and tested.  Should that block not perform to spec, time was saved because the PCB did not need to be re-spun, just move the 0 Ω’s to an auxiliary block and keep going.

Figure 2: Another Example of Yoda's Techniques

Yoda got away with this because most of the time there wasn’t a tight area requirement for the test fixtures. Placing an extra op-amp or two on the board could be done with only a minimal cost increase.  Typically, electrical components went into a standard sized project box that was machined with the proper holes for connectors and any other mechanical parts and that was about it.  So long as the PCB fit in the box all was good.  This freedom to experiment with new and/or different options for various circuits not only made testing and debugging easier, it also made Yoda a better designer because when it came time to design a new product and its required test fixtures, he already had a sense on what type of circuit might work for a given application.

While these techniques may seem like common sense, I for one am glad I got to learn about them while out in the field. The use of 0 Ω resistors and empty pads in key places are not the type of topics that get covered in core EE courses and I probably would not have learned them otherwise while in school. Currently, I am making great use of these techniques while designing and testing the circuit for my thesis. Having these tricks up my sleeve has been quite handy during the whole process.  Thanks Yoda.