Buying an Oscilloscope

[Electro Tunes]

I wish to buy one.

I'm thinking of one of the following:

OWON PDS5022S 25MHz Benchtop Digital Oscilloscope ($349)
Hantek DSO-2090 USB PC Oscilloscope (250$)

I'm don't really understand all the properties, and don't know how to choose...

 

[Sceadwian]

If you don't understand then DO NOT BUY. There have been more than one post about this topic here, use the search function to find them. Also use Google to try to find useful information.

 

[tunedwolf]

Unless there is a particular reason why you would want or require a DSO, or worse, a trinket masquerading as one, I would recommend getting your hands on a good second hand analog Tektronix. Maybe a 2465B with the inbuilt computer interface, or perhaps one from a slightly newer series.

These older scopes, when looked after properly, will last a lifetime. They are accurate and pretty well equipped to handle most of what you will ever need to see in a waveform right out of the box. Personally I wouldn't look past them.

The down side would be the cost of genuine probes for them, especially the high bandwidth ones, though these too can be had second hand at reasonable money as long as you're willing to wait for a bargain.

Scope purchases can be very expensive if you don't have the features needed to cover your particular area of interest, so think very carefully before purchasing any instrument.

Write down a wish list of features, take from that a practical sub set of required features, the more honest you are with yourself the better. Look for an instrument that covers all of your required features well, then tick the wish list for bonuses. If you settle on an instrument that covers all of your essential requirements plus a few extra features you will buy well.

Also, dare I mention it, make sure that whatever you buy can be serviced and calibrated locally. There's no point in owning a scope that takes three months in the post if something fails on it. In fact, your local instrument calibration centre would be a good place to seek advice from and sometimes can they can also point you towards a bargain because they know the larger companies disposal policies etc.

Whatever your choice, make it a wise one...good luck


rgds

 

[kjennejohn]

Hi. One thing to think about is bandwidth. The stated bandwidth is good for sine waves. Think of the fastest squarewave you'll want to study (think processor clock here) and multiply by ten. A 25MHz o-scope will allow you to study a 2.5MHz square wave reliably. The trace degrades for any signal faster than that. If you have processors running at 4MHz and you want to get a proper clock reading your o-scope must do 40MHz or better. When you look at an otherwise good 20MHz clock pulse (squarewave) with a 25MHz 'scope you'll see something like a voltage-reduced sine wave. This is because you actually need a 200MHz 'scope for a proper reading.

The next consideration is sampling rate. As a rule of thumb this should be more than 2 times faster than the given bandwidth. Three to four time is prefered. This goes back to the Nyquist Theorem which states that the rate must be a minimum of twice the frequency plus one. So if you want to reliably read a 4MHz signal, you must sample faster than 8 million samples per second (8 MS/S). This is true for a single event. One way to get around this with a constant signal is to do a large number of samples over a long time period and then build a representative signal from those samples. You'll see quotes like 2.4 GS/S (GigaSamples/Sec) or more. However, if the o-scope offers single-shot signal capture, then you'll need to know what the one-time sample rate is to determine if that model will do the job.

I think automatic measurements are a big plus. And math functions. The more the better. And features like battery operation, if you want to get away from the bench. And multiple triggering methods beyond the usual level and edge detection. A universal power supply is good, i.e., any AC wall supply from 97Vac to 240Vac, 50 to 60Hz.
Deep memory per channel is a must have, say greater than 2500 points per trace. Most hobby 'scopes give you at least 6k of storage per channel. Some models spec one large memory value, but confuse matters by stating how the memory allocated per channel is based on how many channels you are using, and the sweep speed. If you can get more memory assigned to a single channel when using only one, this is a good thing. Be aware that some 'scopes use less memory at the highest sweep speeds, then fudge the values from the reduced data collection. Even my high end Tektronix model does this.
USB connectability is another large plus. Most 'scopes touting USB at least allow you to plug in a USB memory stick and record setups and trace data. Some go further and allow you to talk to a printer or computer, perhaps even use remote control.
Download and read the manual. Most started in another language other than English. The translations vary from atrocious to quite good. If they couldn't bother to produce a serviceable manual, what does that say about their attention to detail and service?

And the list goes on. The first two considerations, bandwidth and sampling rate, are my main two considerations. The more features you get, the better. You can get some really good (not great) hobbyist 'scopes nowadays for less that $400. And being able to store traces and settings is something I wish I had access to when I stared out. As a technician I wouldn't touch anything less than 100MHz. But when I see a color LCD 50 - 60MHz 'scope with battery operation for less than $500 I am _SO_ going to snap it up!

Your mileage may vary. So there.
kenjj

 

[kjennejohn]

The down side would be the cost of genuine probes for them, especially the high bandwidth ones, though these too can be had second hand at reasonable money as long as you're willing to wait for a bargain.

Amen to that one! I bought a 200MHz Philips four-channel scope that had trigger and function buttons built into the probes. I saw the scope on Ebay and went ahead and bought it. I had used one of these in an earlier job and was impressed with all the features. I figured I'd just pop over to some discount probe shop online and pick up four probes for this.

NEHHHH! says Mr. Buzzer. They were only available from European suppliers, at $300 apiece!! Owwww! I started haunting Ebay again for these probes. Four months later I had managed to collect five of the original probes in good working order. And one dud, which the seller stiffed me on when I tried to contact him. This set me back another $300 or so. You really REALLY want to be sure you get decent probes with your scope!

Later.
kenjj

 

[Space Varmint]

I agree with tuned wolf. You get more bang for your buck with a refurbished oscilloscope.