Oscilloscope Buyer’s Guide – Top 4
Multimeters are an excellent way of measuring DC signals and some well-behaved periodic AC signals (a topic I covered previously), but at some point, you’re going to want to use an oscilloscope to visualise a waveform. Whether it’s hunting for jitter on a PWM signal or bringing up a badly-behaved I2C or SPI bus, you can’t beat an oscilloscope for getting into the nitty gritty. As well as allowing you into the time domain for debugging and characterisation, I always find I learn so much more about any circuit I am working on with a scope attached – for want of a better phrase I feel it really brings a project to life and gives it context.
So which scope should you buy? As with any question like this, there are numerous options and many opinions on the matter. This isn’t intended to be an impartial or complete guide – more like a roundup of tips biased heavily by my preferences and experience. Here goes!
Digital Storage Oscilloscopes (commonly referred to as DSOs) are now so widespread and available at such a low cost that getting a second-hand analogue scope is almost pointless these days. When you can easily get a sub-£250 dual channel DSO new and delivered, why would you risk something older without the convenience of built-in measurements and cursors? Well, if you’re really pinched for budget it’s possible to save £100 here and get something that will get you started with scopes and which will likely hold its value well if you intend to resell it.
Go for established brands (Tektronix, Hameg, Philips, Fluke) where the build quality should be good and if you can find an eBay listing with local collection only all the better as you will have fewer bids to compete with. Analogue scopes have another advantage in that they are inherently very fast updating (the screen updates every time the scope trace triggers) and on noisy or jittery signals the brightness of an area of trace gives a measure of the relative frequency of that particular event within the overall signal.
Note that some digital oscilloscopes have this capability too – but ALL analogue scopes have it by their very nature. Personally, I would only recommend getting an analogue scope if you already have a decent DSO and are looking for particularly subtle artifacts in noisy signals or if you really need to save that £100. One other point – if you are going to bother with a second-hand analogue scope, go for something with at least 70MHz bandwidth – anything lower really will be a waste of your time.
This 100MHz, 2-channel DSO was the first scope I ever owned – it was fished out of a skip outside Imperial College and I paid the guy who gave it to me a bottle of wine in thanks. At that price, it was a bargain and I still use it a bit as a “throw it in a bag for site visits” scope or as a secondary display when I run out of channels. However, there’s no way I would buy one now. I’ve seen these listed for well over £1000 on eBay – I can only imagine that this is a replacement for old automated test setups where the cost of rewriting the software makes this an attractive proposition. There’s no way I would pay that much for this old dog now! If you’re given one or offered something similar for £50 go for it – otherwise see above or below.
Rigol are a relative newcomer to the scope market (founded in 1998) but have really opened up the market at the low end (although they also do some impressive high-speed scopes too). This 4-channel, 50MHz DSO is incredible value at a shade under £350 and the 2-channel equivalent DS1052E is about £275. They have all the features you would expect from a basic modern DSO – a wide choice of measurements, mathematical functions, filters, cursors, FFT and some advanced trigger options. You also get a nice big 1Mpts sample depth (useful for zooming in on details or long digital captures) and a USB port for saving screenshots to a USB stick.
The hardware on the DS1054Z is also capable of 100MHz bandwidth, a larger 24Mpts sample depth, and more advanced triggering, along with digital protocol decoders. These are available from Rigol as a paid upgrade – so if you need them in future you just order, enter the license key you receive and off you go – instant upgrade. Here at Bare Conductive, we use the DS1104Z which is a 4-channel 100MHz model with more memory, a larger display, and a built-in signal generator. It’s been a fine match for all the problems we have come up against so far.
Formerly Agilent, which was formerly part of HP, Keysight is the new name for a very established test and measurement company. If the previous few options have been budget-conscious tips that I think will help guide most hobbyists, this is really more of a dream machine for me. There are more expensive scopes (take a look at Dave Jones tearing down the Agilent DSA91304A) but if I could get everything I would really use one package, this would be pretty much it. 4 analogue channels are paired with 16 digital channels (useful for synchronous analysis of multiple serial buses) and a huge 8.5-inch touch screen. 1GHz bandwidth takes it way up to the limit of any signal I am likely to be working with anytime soon, but the real killer for me is the feature list.
The scope boasts complex trigger options and an incredible search facility allows you to delve back into a previously acquired buffer looking for unusual events. A lot of the most difficult to solve problems that I use a scope and/or logic analyser for are intermittent issues where you’re not sure how often a particular event will occur or even the circumstances that precede it. Being able to either trigger a complex condition or go back and review a previous capture with sophisticated tools would speed up a lot of work dramatically. However, at £10,000, it’s out of my budget – although maybe not yours! For the whole of March 2017, however, Keysight are running a Scope Month promotion where they will be giving away 125 oscilloscopes. I’ll be entering for sure.
Dave Jones DSA91304A teardown
So that’s an incomplete guide with a lot of opinion in it. The long and short of it is – if you can afford one of the new generation of low-cost DSOs, go for that. If you can’t, spend some time on eBay and get an old analogue scope as a starting point. If you’ve got money to spare, companies like Keysight, LeCroy, and Tektronix have some very impressive high-end models. But if you’re after one of those, you are probably well beyond seeking the advice in this article!
Before I leave you, I want to add two types of scope that I don’t think most people should buy.
On the face of it, these should be a great idea. You already have a screen on your laptop/lab PC, why not use that and spend more money on the analogue acquisition front-end? Makes perfect sense right? Except that sadly, in most cases it doesn’t. Firstly, most PC-based oscilloscopes have much less overvoltage protection than traditional standalone types – typically around 100V whereas the Rigols I mentioned previously are protected up to 1000V for a transient event and 300V RMS continuous.
If you think I am connecting something with protection only up to 100V to my precious laptop (which I use for lots of other things besides signal measurement) you can think again. Additionally, I might reasonably want to use my laptop for any number of other things in the lab – to view a specification sheet for a component, to send control data to the device I am working on or even just to listen to the Amp Hour podcast.
This is not to mention the generally rubbish quality of the GUI software and the fact that display update rates are usually much slower than equivalently priced scopes. Don’t get me wrong, there is good gear out there (Pico Technology have been going for 25 years and Keysight make some excellent DAQs) and if you want to write your own custom interface or are involved in automated test engineering there are a lot more positives to these devices. You still won’t catch me with one anytime soon!
If you’re into electronics as a hobby, you’ve probably made a few kits in your time. If you need a new oscilloscope and like making kits, an oscilloscope kit would be a great project right? Maybe. It’s certainly interesting to study the schematic and code (if they are both available) and try to understand the theory behind quantisation, discrete-time sampling, and signal recovery. But would I ever use a scope like this to seriously analyse a signal? Never.
While their low price-point may be very attractive (I have seen direct-ship items from China for under £10 on eBay) I need to be able to trust my test equipment more than anything else. Your test equipment is your reference and if it’s possible that it is misleading you then you have no basis for any further deduction. Add to that a generally pitiful user interface, analogue bandwidths as low as 10kHz (the highest I have seen is 3MHz) and no support of any kind and I can only recommend these to people who want to learn about the design challenges faced by test equipment designers. Even then, buying a cheap old analogue scope that has a higher level of functionality and opening it up to see what’s inside is likely to be more enlightening.
Totally agree? Couldn’t disagree more? Let us know via Twitter or Facebook. Peace out people and happy probing!