Making DSO Nano V2 & V3 Oscilloscope Probes

I have eight working oscilloscopes; and of course all but two use different probe connectors. My DSO Nano V2 and V3 use 3.5 mm mono jack connectors on the probes.

1x Oscilloscope Probes are the most basic form of oscilloscope probe, they are called a 1x probe because this type of scope probe does not attenuate the incoming voltage. It consists of a connector to the oscilloscope, in this case a 3.5 mm mono jack and a length of coaxial cable which is connected to the probe tip. The probe tip can be a mechanical clip like an alligator clip so the probe can be attached to the test point on a circuit board under testing, and a ground clip to be attached to a ground point on the circuit under test. 1x probes are suitable for many low frequency applications, they typically offer the same input impedance of the oscilloscope which for most oscilloscopes is 1 mΩ. However for applications where better accuracy is needed or as frequencies and voltages start to rise, other test probes are needed like a 10x probe. Unless you have special needs stick with 1x and 10X probes. 10X is usually the best compromise of circuit loading and signal attenuation.

The 1x probes that come with the DSO Nano oscilloscopes V2 & V3 are good for many projects, and the oscilloscopes have a built in signal generator. However when you use the more extreme limits of the oscilloscope you start to pick up stray signals from the ambient signals in the air around you. Mid to high frequency signals just get messed up completely with these probes. One way to filter out ambient signals and improve test signals is to use a 1x probes made with coaxial cable. The braided shield of coaxial cable blocks the ambient signals and improves the test signals.

Unfortunately I cannot buy coaxial cable probes with the 3.5 mm mono jack for the DSO Nano V2 and V3 oscilloscopes. To make things even more fun every time I try to order a BNC to 3.5 mono jack adapter on line so I can use the probes I have, they don’t ship to where I live.

Since I cannot use what I have or buy what I need; I am left with only one option, to make my own coaxial cable probes for my DSO Nano V2 and V3 oscilloscope. There are formulas and calculations you can do that will get you close to the desired component values. But these calculations only get you close after that you need to test until you get the right values. How to make the probes and getting the right values through testing is what I am going to demonstrate in this Instructable.

Wire stripper if you have one or bother using it.

Solder & Soldering Iron

Side Cutters

Needle Nose Pliers

Tin Snips or strong Scissors

String to use as a snake if you need it.

Spring Loaded Tweezers, to hold the wires in place while you solder.

Drill

1 small drill bit just large enough to fit the lead wire.

Hole punch 4, 8, and 9 mm, or ¼, 5/16, and 11/32 drill bits will do.

Solder

Resistor 4.6 mΩ ¼ W 1%

Small Fixed Capacitors, an assortment from 5 to 20 pF.

Small Adjustable Capacitor 0 to 50 pF.

Alligator Clips with Black and Red Boots.

Some Wire

Shrink Tubing

Pens

3.5 mm mono jacks.

Sheet Tin

Coaxial cable, soft and flexible is best for ease of use, with 3.5 mm mono jacks already attached if you can get it.

Lead Wire Test Probe Hooks, I got my test probe hooks at Banggood.

http://www.banggood.com/10Pcs-Multimeter-Lead-Wire-Probe-Test-Hook-Kit-Connector-Electronics-Laborator-p-993562.html

Probe PCB Test Needles, I got these at Banggood also.

http://www.banggood.com/10pcs-Ultra-Pointed-Golden-Flexible-Multimeter-Probe-PCB-Test-Needle-p-960840.html

Coaxial Cable I used coaxial cable with the 3.5 mm mono jacks already attached, it saved me a little work.

Most of the coaxial cable I used was 80 cm or 32 inches long and came with the 3.5 mm mono jacks already attached; however I am still going to need to attach a 3.5 mm mono jack for a 10x probe.

Attaching a 3.5 mm Mono Jack to coaxial cable is quite simple when you know how.

Slip the jack boot over the cable first making sure the threaded end of the boot is facing the end of the coaxial cable you plan to attach the jack pin. This is a good habit to get into, you can cut a piece cable, attach the jack end, and then slip the boot over the length of the cable. You just do not want to do that for twenty or more feet of cable by mistake.

Strip off the outer jacket of the coaxial cable about ½ of an inch or 10 mm to expose the braided shield and ground line.

Move the braided shield to a side to expose the dielectric insulator and strip ¼ of an inch or 5 mm of the dielectric insulator exposing the core conductor.

Solder the core conductor to the center tab of the jack.

Solder the braided shield to the outer tab and fold the sides of the outer tab over the cable. Some people like to fold the sides of the outer tab over the braided shield. Others like to fold the sides of the tab over the outer jacket. I find I have fewer problems in the future if I fold the tab sides over the outer jacket.

Then slip the boot over the joints and screw it in place.

To make alligator clip probe tips you start by attaching a lead wire to the alligator clips. Attaching alligator clips to a lead wire is a little easier than attaching the mono jack to the coaxial cable since it only has one conductor. Pick a wire long enough to suite your needs, most people choose a wire the same color as the alligator clip boot.

Another reason for using a lead wire is, the lead wire can be more flexible than the dielectric insulation and core conductor of the coaxial cable.

Just as you did with the mono jack boot, slip the alligator clip boot onto the wire first with the large open end towards the end of the wire you intend to attach the alligator clip.

Strip ¼ of an inch or 5 mm of the insulation off the end of the wire.

Put the striped end of the wire through the hole in the alligator clip tab and solder the wire to the alligator clip.

Then fold the sides of the alligator clips tab over the wire and slip the boot over the alligator clip.

Now you have alligator clip probes leads to attach to your coaxial cable.

Test hook probes are used to clip your probe in places where alligator clips are too large for the test points on the circuit board you want to test. You can clip a probe test hook onto the leads of small resistors, capacitors, and ICs where there is no room for alligator clips to grip the test point.

Test hook probes are a little different to attach to a lead wire, to connect the hook to circuit you press on the top of the test hook to push the hook out of the end of the probe and clip it onto the circuit under test.

Start by pulling the button top off the test hook and passing the lead wire through the hole in the side of the button and out the bottom of the button.

Strip ¼ of an inch or 5 mm of the insulation off the end of the wire and wrap it around the center of the H of the test hook making sure the wire passes between the posts of the bottom half of the test hook.

Remember on many of these test hooks there is a front, a back, and a side; the lead wire must face the right direction before you solder it.

When you solder the lead wire make sure you only get solder on the center of the H or it may interfere with the functions of the test hook or placing the button back on the test hook.

Slip the button back on the top of the test hook and you have test hook probes ready to attach to the coaxial cable.

Whether you are attaching the alligator clip probes or the test hook probes to the coaxial cable the process is much the same as attaching the mono jack to the coaxial cable.

Slip a piece of large shrink tubing about 1 ½ inches long over the coaxial cable.

Slip a piece of small shrink tubing about 1 inch long over the probes lead wires.

Strip ¼ of an inch or 5 mm of the insulation off the probes lead wires.

Strip off the outer jacket of the coaxial cable about ½ of an inch or 10 mm to expose the braided shield and ground line.

Move the braided shield to a side to expose the dielectric insulator and strip ¼ of an inch or 5 mm of the dielectric insulator exposing the core conductor.

Solder the core conductor to the red probe lead wire.

Solder the braided shield to the black probe lead wire.

Slip the small shrink tube on the probe lead wires over the soldered joints first and heat the shrink tubing to shrink the tubing to the joint.

Then slip the large shrink tubing over the two smaller shrink tubs and the end of the outer jacket of the coaxial cable and heat the large shrink tubing to make it shrink and secure the joints.

Now when you plug these probes into your DSO Nano V2 & V3 oscilloscopes and set the voltage to its most sensitive it is not as affected by the ambient signals around you.

Alligator clips and test hook probes will work on a large number of circuits, but circuits with test points like the test points on this circuit board need a touch probe or a needle point probe. Needle point probes go where test hooks and alligator clips can’t go, and they are easier to handle in small places than an alligator clip or test hook. To make a needle point or touch probe, start with a pen.

Choose a pen with a ballpoint the same size as your test needles and an ink cartridge holder.

Remove the ballpoint from the ink cartridge with a pair of pliers.

Clean the ink out of the cartridge with water and cut off the cartridge holder with side cutters.

Now you have a holder for your test point needle.

Take one of the test needles and press it into the ink cartridge holder with a pair of pliers.

Then make the air vent in the tail cap of the pen large enough to fit the coaxial cable.

Last drill a hole in the side of the pen body approximately where the ground lead to the coaxial cable is going to exit the pen body. This should only be about ¼ of an inch 5 mm behind the ballpoint cap and just a little larger than the ground lead wire.

Now that you have the three main parts of the needle test probe, the coaxial cable with a 3.5 mm mono jack, the ground probe lead, and the needle handle and test point, you can start to assemble the probe.

Start by fishing a loop of string through the hole in the pen body and out the end of the pen body.

Strip ¼ of an inch or 5 mm of the insulation off the ground probe lead wire.

Attach the end of the ground probe lead wire to the loop and pull the wire through the hole and out the end of the pen body.

Fish the coaxial cable through the tail cap and pen body.

Strip off the outer jacket of the coaxial cable about ½ of an inch or 10 mm to expose the braided shield and ground line.

Move the braided shield to a side to expose the dielectric insulator and strip ¼ of an inch or 5 mm of the dielectric insulator exposing the core conductor.

Solder the braided shield to the ground probe lead.

Solder the core conductor to the test needle point.

Pull the coaxial cable and the ground lead until the needle point holder is in place and screw the ballpoint cap on and press the tail cap in place.

Now that you have assembled the needle point test probe connect it to your oscilloscope to check that it works as it should blocking ambient signals and pickup nice clean test signals.

Now you have three 1x probes for testing your circuits that are not as susceptible to ambient signals. The probes work well with low frequencies however with high frequencies above 50 kHz there is some distortion. This distortion is caused by the capacitance in the coaxial cable. The one uncertainty of these probes is the coaxial cables capacitance, I used 80 cm, 32 inch cables and most factory cables are 1 to 1.5 meters 40 to 60 inches, and the greater the length of cable the greater the capacitance in the cable. To clean up the distortion at higher frequencies you need a calibrated probe.

To enable better accuracy at higher frequencies and voltages higher levels of impedance is required. To achieve this attenuators are built into the end of the probes that connects with the circuit under test. The most common type of probe with a built in attenuator gives an attenuation of ten, and it is known as a 10X oscilloscope probe. The attenuation enables the impedance presented to the circuit under test to be increased by a factor of ten, and this enables more accurate measurements to be made. In particular the level of capacitance seen by the circuit is reduced and this reduces the high frequency loading of the circuit by the probe.

As the 10x probe attenuates the signal by a factor of ten, this obviously means that the signal entering the scope itself is reduced. This has to be taken into account. Some oscilloscopes automatically adjust the scales according to the probe present, although not all are able to do this. It is worth checking before making a reading.

Most 10x scope probes use a series resistor of 9 mΩ to provide a 10 : 1 attenuation when it is used with the 1 mΩ input impedance of the scope itself. A 1 mΩ impedance is the standard impedance used for oscilloscope inputs and this enables scope probes to be interchanged between most oscilloscopes of different manufacturers.

However the DSO Nano V2 and V3 have 510 kΩ input impedance so most 10x probes with a 9 mΩ resistor in the probe tip won’t read right. Standard 10x probes connected to the DSO Nano V2 and V3 have an attenuation of 18.65 not 10x or 20x.

Since the DSO Nano V2 and V3 have an input impedance of 510 kΩ it is a simple calculation to find the probe tip resistor value. The input impedance of the scope is 1/10th of the attenuation and the probe tip is the other 9/10ths of the attenuation. The probe tip resistor is seen as nine times input impedance.

9 x R2 = R1

9 x 510 kΩ = 4.59 mΩ

R1 = 4.59 mΩ

I used a ¼ watt 4.64 mΩ 1% resistor since it was the closest to 4.59 mΩ.

However if you like to do calculations they are in the above sketch, I just find fractions are easier to work with when using voltage dividers.

10x oscilloscope probes also allow some compensation for frequency variations present. A capacitor network is embodied into the probe as shown. An adjustable capacitor in the probe tip or a fixed capacitor in the probe tip and an adjustable capacitor connected from the core conductor to the ground at the oscilloscope end of the probe, that can then be used to equalize the frequency performance of the probe. I use the ball park usual capacitor value’s and then tweak it on a bread board using my 1x probes.

5 to 20 pF for the fixed capacitor and 0 to 50 pF for the adjustable capacitor.

Start by gathering your parts and preparing a pen as in steps 7 to 9.

Since the coaxial cable is a guessing variable somewhere between 24 to 30 pF I attached the mono 3.5 mm jack and the 4.64 mΩ resistor to the other end of the coaxial cables core conductor.

DSO Nano V2 and V3 oscilloscopes have a small square wave oscillator output.

By attaching the partially assembled oscilloscope probe from the input to the square wave output I checked the signal attenuation.

It was a 4.12 V signal reduced to a 400 mV signal, that was as close as I could get to a 10x attenuation.

Then I added the capacitor to the unfinished probe tip.

Since a square wave requires all the harmonics to be present in the correct proportions to provide a square wave, I adjust the capacitors value accordingly. If the leading edges of square wave, has rounded corners, when viewed on the screen. The high frequency response of the probe is low and I need a larger capacitor in the probe tip. However if the leading edges have spikes and rise too high, falling back to the required level, then the high frequency response has been enhanced and the capacitors value needs to be reduced. Only when the square wave is truly square is the frequency response correct.

A truly square wave with fixed capacitors is almost impossible to achieve and since I didn’t have enough room for an adjustable capacitor in the pen I went with a 15 pF capacitor with just a small rise at the leading edge of the square wave. Then I put the calibrating variable capacitor in a box at the jack end of the coaxial cable.

Now that you have the resistor and capacitor for the probe tip connected to the coaxial cable core conductor, slip a large piece of shrink tube over the coaxial cable.

Then slip a piece of shrink tube over the resistor and capacitor and shrink it to the resistor and capacitor.

Replace the braided shielding over the resistor and capacitor with a little tin foil.

Solder the ground probe lead to a few strands of the braided shielding.

Then slip the large shrink tubing over the works leaving the ground lead and one end of the resistor and capacitor leads sticking out of the end of the shrink tubing and shrink the shrink tubing.

Slip the coaxial cable into the pen body leaving just enough of the resistor and capacitor leads sticking out to solder the needle point to the component leads, and solder them together.

Pull the coaxial cable and the ground lead until the needle point holder is in place and screw the ballpoint cap on and press the tail cap in place.

Now that you have the 10x probe tip made you are going to need a box for the calibrating capacitor.

Remove the 3.5 mm mono jack and attach a coaxial box connector.

Slip the clamp nut, washer, and compression ring, over the cable first making sure the threaded end of the clamp nut is facing the end of the coaxial cable you plan to attach to the box.

Strip off the outer jacket of the coaxial cable about 3/4 of an inch or 15 mm to expose the braided shield and ground line.

Slip the compression ring over the braided shielding and cut the braided shield to about ¼ of an inch or 5mm from the compression ring exposing the dielectric insulator.

Spread the braided shielding over the end of the compression ring and screw on the box connector clamp body.

I put the box connector lock washer and nut on the box connector until I was finished making the calibrating box.

I took my time thinking about how I want to make the box.

To keep the center of balance inside the oscilloscope I decided to make the box with the 3.5 mm mono jack and the coaxial cable box connector at a 90⁰ angle and two sides open for assembly.

First I mapped out the box to scale.

Since I like to salvage and repurpose, for my box I used the tin from a cookie tin.

The cookie tin cuts easy with ordinary scissors, so I cut a blank 16 x 46 mm.

I etched the folds and remainder of the cuts in the tin before cutting the 3.5 mm mono jack and box connector holes.

Then I made the folds in the tin.

Once I made the finale folds and cuts in the main box I soldered the two open seams.

Last I etched, cut, and folded the two sided cap, you may want to save punching the calibration hole until you are ready for the final assembly, just in case the adjustment screw on the variable capacitor is not perfectly centered.

Find or make a nut to fit the threads on the 3.5 mm mono jack and attach the mono jack to the box.

Attach the coaxial cable box connector to the box.

The box will act as the braided shield and ground between the two.

Solder the core conductor of the coaxial cable to the center tab of the 3.5 mm mono jack.

Solder the leads of the variable capacitor to the outer tab of the 3.5 mm mono jack and the core conductor of the coaxial cable.

Attach the partially assembled oscilloscope probe from the oscilloscope input to the square wave output.

Make sure you get a signal and fix any connections if necessary.

Check the signal and adjust the variable capacitor until you get a perfect square wave before you put the double sided cap on the tin box. Replace the variable capacitor if it is necessary to get a perfect square wave.

When you can get a perfect square wave it is time to finish the calibration box. Punch the hole in the double sided cap above the adjustment screw of the variable capacitor and solder the cap on the box.

Attach the assembled 10x probe to the oscilloscope probe from the oscilloscope input to the square wave output.

Check the signal and adjust the variable capacitor if you need to until you get a perfect square wave.

As the square wave requires all the harmonics to be present in the correct proportions to provide a square wave, adjust the capacitors value accordingly. If the leading edges of square wave, has rounded corners, when viewed on the screen. The high frequency response of the probe is low or under calibrated and the value of the variable capacitor needs to be increased. However if the leading edges have spikes and rise too high, falling back to the required level, then the high frequency response is enhanced or over calibrated and the variable capacitors value needs to be reduced. Only when the square wave is truly square is the frequency response correct.

Now you have four probes for the DSO Nano V2 and V3 oscilloscopes ready for testing circuits.

Alligator clip test probe for low frequencies and large test points.

Hook clip test probe for low frequencies and small test points.

Needle point test probe for low frequencies and very small test points you can't clip to.

And a 10x test probe for high frequencies and high voltages.

Just one note the 10x probe works best if you only use it on one oscilloscope, if you keep switching between oscilloscopes you may need to recalibrate the probe and this can wear out the variable capacitor.