Upgrading the Probes on the DSO Nano
- Published: Sunday, 08 November 2009 23:54
- Written by Justblair
The DSO Nano from SeeedStudio is a cheap but useful, portable colour digital storage oscilloscope. As you can tell from my previous review, I was pretty impressed with this budget scope. For the price that Seed studio are selling it for, it offers excellent value and a highly practical device. However, costs have to be cut somewhere to reach that $89 price point that Seed have achieved and one look at the standard probes tell you that Seeed did not burst the budget on these items.
So what can you do when you inevitably end up breaking the flimsy 1x probes that are supplied with your Nano? Luckily it is a pretty easy job to modify a 10x probe readily available on the internet. Not only that their are additional benefits a plenty to be had from this modification.
It pains me to even look at the paltry probes included with the DSO Nano. The probes themselves connect to the DSO Nano with a stereo mini-jack connector, the sort of connector that attaches your headphones your Mp3 player. To connect probes to most oscilloscopes a BNC connector is usually used.
The finish of the probes is flimsy and the clipping action is basic, of restricted use and mechanically poor. Touch these when in place and they ping of the measurement point all too easily.
It's clear that you just can’t buy a set of probes for he DSO Nano. What I hoped was that it would be relatively easy to adapt a standard set of probes for use with the Nano.
A quick root round with my multi-meter (and a little common sense) told me that not all of the contacts of the mini-jack were being used to connect the probes. Oscilloscope probes have two connections to an oscilloscope, one attaches the ground and the other the measuring tip of the probes and supplies the measured signal.
On the mini-jack connector the Signal from the probe (The red connector on the supplied probes) is connected to the DSO Nano via the tip of the jack. The ground by the contact area closest to the cable just as ground would be if this was a set of headphones.
Now finding a set of replacement probes was not a difficult thing. I used E-bay which has various vendors selling probes. I was looking for X10 probes to use with my scope taking a bit of a gamble that the DSO Nano would be able to make use of the X10 setting on the probe. The DSO Nano has a X10 range which can be selected from its user interface.
In the end I settled on a set of probes from a vendor by the name of UKshowcase. The probes I purchased were £19.99 for a pair though cheaper probes were available. Obviously the DSO Nano only has inputs for a single probe, but the probes that were supplied with my analogue scope have seen better days and were missing their attachments when I bought it (it was a second hand scope again purchased via E-bay).
I am sure that most probes will give good results. Obviously though I can only report results for the probes that I have used. Your mileage may vary!
The other component required obviously was a mini-jack for the probe set. For this I used an inexpensive mini-jack from Maplins (£2.99). I chose to use a Metal 3.5mm Stereo Plug for this as it should offer more shielding of the signal than the cheaper plastic barrelled connectors
Other than that a couple of sizes of heat-shrink insulation (Again from Maplins) were also used. I would recommend using heat shrink though it is not strictly necessary.
Modifying the X10 standard probes to work with the DSO Nano is not a difficult modification, but I will detail it anyhow. Tool wise all that is required is a soldering iron though a set of decent wire strippers will help things along as well.
Before you prepare the cable of the probe to add the new 3.5mm jack, I would recommend that you unscrew the barrel of the jack and disassemble it. It is useful to have the inside of the 3.5mm mini-jack available to gauge how much insulation to strip back
Next strip the outer insulation from the cable. This will reveal the outer layer of wire and hopefully some foil shielding (The manufacturer really cheaped out on cable if the foil is not there).
The outer wire shielding can be un-knitted and rolled up into a cable, trim off any exposed foil. This is your ground connection and is going to be soldered to the largest of the metal tabs on the mini-jack.
Trim back the next layer of insulation very carefully. The signal cable is in the middle of this and its pretty thin (and so of course easy to cut through by accident.) By now your cable should look like the picture to the left.
The next bit is the important one. Remember to thread all of the components that make up the 3.5mm mini-jack plus some heat-shrink. The components need to be threaded in the correct order which in my case was heat-shrink, outer barrel (the gripped one), the strain relief spring and finally the the clear insulating tube.
You can see on the picture how this is wired (click on the image for a close up view). The signal cable is attached to the smallest inner tab. Check with a multi-meter that this corresponds to the tip of the 3.5mm mini-jack.
The ground cable solders to the longest of the tabs which also will need to be crimped around the outer insulation to provide strain relief to the cable. Trim off any loose wire after soldering.
Before screwing the 3.5mm Mini-Jack assembly back together I tidied things up with some heat shrink. I also used some wider heat shrink to cover the strain relief spring on the cable. The X10 Probes came with thinner cable than the 3.5mm jack is designed for, the heat-shrink provides additional strain relief and looks pretty good I think.
That's it! modification done and its time then to get to test out our new probes..
Attaching the probes to the Nano I had a number of tests to perform. You see there was a bit of a gamble involved in using unknown probes with the DSO Nano. Oscilloscope probes are fairly standard items. But their design is based on a couple of assumptions:
- The input impedance of the oscilloscope will be high (usually 1Mohm)
- The input capacitance of the oscilloscope will be small, in 10x operation this will require a small adjustment to compensate for.
The first thing to check was that the new modified probes worked correctly in X1 mode.
The probes that I purchased had various fittings, a clip (as seen in the above image) an insulated tip and a BNC converter. I attached the clip to the DSO Nano’s square wave generator and switched the probe to X1 mode. Success. The probes operate just fine in X1 mode. This is not too much of a surprise as in this mode, the probes are acting as a straight through connection from the test circuit to the oscilloscope.
2.00 v avg
On the 2v/Div scale both probes gave an anomaly but other than that the new probes are running well on the X1 setting.
The X10 setting was next. This was always going to be more of an unknown. To explain how the X10 works. When the switch on the probe is set to X10 a 9M resistor is placed in series with the signal. Combined with the 1M input impedance of a normal scope this reduces the signal by a factor of 10. This is useful for reading larger signals but also serves another purpose. A downside of course is that the signal entering the scope will be smaller. The DSO Nano has a X10 scale to take this into account. Some finer voltage variation will inevitably be lost due to the attenuation.
The higher attenuation enables the impedance seen by the circuit being tested to be increased by a factor of ten. This means that the circuit under test is driving less current through the oscilloscope, and hence more accurate measurements are made, i.e. the oscilloscope itself has less influence on the workings of the circuit it measures. Also the level of capacitance seen by the circuit is reduced and this reduces the high frequency loading of the circuit by the probe.
There is however a little capacitance designed into the probes to compensate for the remaining capacitance across the oscilloscopes terminals. The probes have an adjusting screw that will vary this capacitance so that the capacitance of both probe and oscilloscope can be matched to each other.
The first test was then to see if the probes that I had modified could be adjusted to the oscilloscope. As the Nano’s inputs are a little different to a standard scope I had my doubts as to whether this would be possible. Changing over to X10 operation (both by switching the probe and changing the scope to X10 operation) the news was good. I managed to adjust the scope to produce a pretty close square wave.
The next thing to do was check the readings.
Using a variety of power sources I compared DC readings at different voltages. Up to 24V (I ran out of sources) I was getting accurate DC readings. This was also a relief because normally the input impedance required to give X10 readings is made up from both the probes internal resistance added to that of the oscilloscope’s own input impedance. The DSO Nano’s input impedance is quite a bit lower than 1 Mega Ohm, in fact it is almost half that.
Getting the accurate DC reading means that the DSO has been calibrated on its X10 scales with standard oscilloscope probes. My gamble then had paid off…
The Fly in the Ointment
Well almost a perfect result. There has been one problem with the X10 probes. For some reason, several of the scales do not give readings at all with the X10 probe. On the lowest voltage scales this is to be expected but other scales should be OK.
The 2,5 and 10V/Div scales don't work at all, but those either side i.e 1v/Div and 20V/Div give accurate readings. Its frustrating. to say the least.
I have tried to get some assistance on the Seeed Studio Forums on this issue, but have not had much in the way of answers. Possibly it will require a few more people to try out better probes to see if this is an isolated hardware issue, a design problem or something that can be addressed with improved firmware.
Despite the slightly irregular results when operating in X10 mode, I would thoroughly recommend that DSO owners upgrade their probes. Even if you are never going to us the probes in anything other than X1 mode, the modified probes are far more durable than the flimsy default items.
The clips that are supplied on the default probes are pretty fragile also and the slightest wrong move leaves them pinging off your test points. With the improved probes this is not possible thanks to their sprung hook design.
To top it all the X10 give increased function. Their is a Ref setting that when switched to gives you a 0V reading for setting your Y position easily. This can be done by attaching the two probes together on the cheap probes, but with the X10 probes you need not remove them from the circuit being tested to do this. A minor improvement but an improvement none the same.
And although I got mixed results I now have X10 functionality thanks to my modified probes. Granted several of the scales are at the moment missing, but X10 can give more accurate measurements in circuits, especially since the normal input impedance of the DSO Nano is below that of a standard (i.e. expensive) oscilloscope. I do hope that the missing scales are not gone for good. If they were functioning correctly I think the DSO Nano would be far closer to perfection than it’s price tag would deem fair!