I’ve been looking at http://www.bitscope.com/pi/blog/ for some time now, it was one of the reasons that goaded me into buying a Pi 2 in the first place. It’s a scope, logic analyser and signal generator in one. Not something I’ve determined I even need, so for now I’m holding off, plus a little on the pricey side, £100, and of limited bandwidth (10 MHz).
Ah ha! That is the blasted link that caused all this in the first place. However, it is no good for me, a pathetically narrow bandwidth. As I’ve said, it’s about 10 MHz. I was using 20 MHz ‘scopes in Tech College and University, 30 odd years ago, and at the time I had always hankered after a 50 MHz oscilloscope. So 100 MHz would be exceeding previous aspirations. Ah well, I’ll probably never use it anyway. In Thailand, I bought a 100 quid multimeter that does everything, a UNI-T UT-71D, even heats up Baked Beans for you… yet I’ve only ever done continuity tests with it! A total waste of money, when a $10 DMM would have sufficed.
So anyway, I ended up buying a rudy oscilloscope on eBay this morning, half asleep, for 60 quid!!! I didn’t want it really, but had to go to Wokingham to pick it up. However, I also ended up seeing an old friend, who I hadn’t seen for a long while, so not a totally wasted trip.
“What do I need a silly scope for?”, I hear you say… well, I’ve got heavily into Arduinos. To date, I have 3 x Nanos, 2 x Unos, a Mega, a 3v3 Mini pro and a 3v3 Micro pro (these two all linked to the Pi and Motorola Lapdock), and a Leonardo. I’ve done some stuff, written a library for a dual seven segment display and a graphic equaliser display. Totally pointless waste of two months obsession. Now I am building a spectrum analyser. I found a great oscilloscope actually this morning, but only good for up to 300Hz. I got into the Arduinos via Stack Exchange (SE). Basically, by Xmas I was so bored of Thailand and the girlfriend and drinking excessively that I decided to do something productive and educational, instead of picking fights with the local Yakuza. So, I started answering questions on SE Super User. Then I saw SE Raspberry Pi so joined that, then heard about Arduino, looked into it, opened eBay and went wild. I literally had five packages a day for two months arriving cheap from China. So, now I have a suitcase full of multimeters, Arduinos, shields and modules but no clothes.
The item description is a little confused, saying that it is a C6100M, when the photo shows a C6100A, but hey. For £59 it’s not too bad, plus the £30 in petrol for the motorbike. I got to the house, at 4pm, and had to woke the chap up. I had a quick look at it. It looked in good working order, and, after plugging in, a trace came up on the screen. Not the most thorough of tests but good enough for me. The gentleman selling it was a nice old chap, who had worked in Bangkok for a cigarette company and the Royal Thai Navy, back in the 80s and early 90s. So, it was interesting talking to him. He seemed to know Sukhumvit quite well. Anyway, getting back to the ‘scope, it came in a nice carrying bag, that had seen better days, and had obviously been in a shed for years. The front pocket contained a bunch of leads and probes. I stuck it of the back of the bike and rode off.
Unfortunately later, I realised that none of the 5 probes that came with it had a clamp hook, which was a bit peeving. Got it home, ran a few exasperating tests, watched a few tutorial videos (more about them below) and went to sleep wondering if I had done the right thing or had wasted my money.
Finally chuffed. Woke up chuffed. Made the right decision. Initially, I took some given advice and tried to get a DC signal up. Lost the plot for an hour cursing and bemoaning my luck, just got wavey signals or blurs on screen. Turned out that, out of the five probes that came with the ‘scope, the two that I picked had no GND and no Probe line, respectively – they were kaput, wires must be broken inside the sheath. It blew my mind for a while because together, with both connected I could get a signal on one, but not the other, but then testing one at a time gave me nothing. The air was blue for a while, until I realised that I had a floating GND on one and dead probe on t’other. Ended up getting my expensive, but underused, multimeter out (UT71D) and performed continuity tests. However, the impedance of the probes confused me for a while until I turned off continuity and switched to straight forward resistance. The other probes seem to be fine, especially the switchable x1/x10 probe. Lovely. However, these three have no GND lines so I have to use the broken probe with the working GND. The other broken probe, with the dead GND lead also happens to have to end of the probe snapped off, so that doesn’t help matters, but it will pick up a signal however.
Trying to fix the two broken probes
Both of the broken probes are RS probes, not sure if that is significant or not, I wouldn’t like to say. The probe with the broken probe tip and broken earth line is a 489-740 M15 x100, with a handsome mud brown collar and attenuator. The other is an RS x10, which has no other markings, looks rather cheap to be honest, in a straight forward, unflattering, black.
Just as an aside the working (non earthed) probes are:
a HP 10433A 10:1 probe;
a Greenpar switchable x1/x10 and;
a Coline M12x1 (whose bandwidth is a rather limiting 30MHz, see spec below)
Dismantling the RS M15 was relatively straightforward. Unscrew the two rubber cable protectors at either end of the cable (one into the probe and one into the BNC connector). Now, dealing with the probe itself, twist of the earthing collar, twist off the black probe sleeve, hammer the metal probe out of the brown probe handle and that is it really.
I have tested the GND/Earth collar and crocodile clip with the DMM, for continuity, but no joy. I’ve even tried wiggling the lead about to locate the break, without success.
Looking for spares, I ventured on to the RS website, which has plenty to offer.
Now one could buy the Testec 20100 Test Probe Accessory Kit, For Use With Oscilloscope Probe but at nearly £10 it is cheaper to get a second hand (or new maybe if lucky) probe set on eBay, at about £2.50, or a new pair from China for £5.80. Or, in the extreme, buy another oscilloscope (for part or not working) for £5, and just keep the probes and resell the scope.
I could also fix the broken tipped probe with the PMK SET-TSS-5 0,8 Test Probe Tip,For Use With Oscilloscope Probe but I’m having a bit of a job dismantling that one. The broken earth probe came apart relatively easily, but not so the broken tip probe. Oh, tell a lie! it’s just come to pieces in my hand. Well that was easy. Nevertheless, the reasoning above for the accessory kit still stands – it is still cheaper to get a new set on eBay as each tip comes out at £2 each. Also, after further inspection, they aren’t the right ones.
After realising that having not used an oscilloscope for about 25 years had dulled my dexterity with them, I decided to follow some video tutorials on youtube. Looked up my preferred tutor, Martin J. Lorton (mjlorton), of tester.co.uk (I’d watched his teardowns of digital multimeters (DMMs) extensively, before finally plumping (see the decision making here) for my UNI-T UT71D, £90 on eBay, so I had full faith in the chap). There’s also a video from Dave of EEVblog.
How to use an oscilloscope / What is an oscilloscope / Oscilloscope tutorial
Now, having made the purchase, as I mentioned above, I was questioned as to why did I ned a 100MHz ‘scope, and apart from saying big is better and that at Tech/Uni I had used 20MHz ‘scopes, but had always hankered after a 50MHz one (as had my student peers (Harvey Roft, that means you!), I couldn’t remember or say why *exactly* I ned one, just to do the odd test on Arduino based circuits. After all, a PCD8544 clock signal is only about 20KHz, and should I really need to see the Xtal signal then that is only 8MHz or 16MHz at best on an Arduino.
However, after watching the videos listed above, my correct reasoning was confirmed. For standard analogue signals you require a scope with a bandwidth of twice the frequency of the signal that you are measuring, due to the -3dB attenuation at the bandwidth limit. However, for a square wave, as it is composed of many harmonics of higher frequency, as a rule, you require a scope with a bandwidth of five times the frequency of the signal you are trying to measure. For further coverage of these two points, see Intro, Review and Tutorial of Analog Oscilloscopes Pt 2 – GW Instek GOS-630FC @ 10:47.
So, I feel justified in the excessive purchase now.
On top of that after getting hold of the COS6100A manual from Kikusui, I realised that I had quite a blinding scope. 5 input channels, 14 traces, only single beam though, mind, not dual beam.
The scope came with 5 probes, two of which were knackered and the other three were lacking the hook clamps, making things a bit of a mare. I was going to buy 2 probes from china for 5.80, but, on the off chance, I messaged the chap, who sold me the scope, to see if he had the clamps. He reckoned they may have fallen out on the bike (possible I guess). Later he gets back to me saying he has found a couple of packets of new probes in the garage and will send them on to me, for free.