Need Help!- Choosing the Right Oscilloscope

[joprinz]

Hi,

I am planning to buy an oscilloscope so that i can have a better understanding of how my circuits work. My projects are mainly related to cnc machines and related electronics. However i find it hard to pick the right oscilloscope since my only(very little) experience using them was back in college n that was a few years back n we din have the DSO type there. I have come across one model which suits my budget. The Specs is as follows:

25MHz 100MS/s Colour Digital Storage Oscilloscope

SPECIFICATION:
Bandwidth : 25MHz
Channels : 2 + External
Display : 7.8 inch Colour for STN panel ( 640X480 resolution)
Acquisition
Mode : Sample, Peak detect, Averaging
Sample rate (real time): 100MS/s
Input
Input coupling : DC,AC
Input impedance : 1MΩ ± 2% in parallel with 20pF
Probe attenuation factors: 1X, 10X, 100X, 1000X
Max. input voltage : 300V(PK-PK) CAT II
Horizontal system
Sampling range :10S/s ~ 100MS/s
Record length : Max. 5K points
Time base range : 5ns/div ~ 5s/div
: (step as 1 ~ 2.5 ~ 5)
Time base accuracy: 100 ppm
Vertical system
Vertical resolution : 8 bits
Vertical sensitivity : 5mV/div ~ 5V/div (Input to BNC)
Position range: ± 10 div (5mV/div ~5V/div)
Single bandwidth : Full bandwidth
LF Respond (AC, -3dB): ≥5Hz (to BNC)
Rising time (typical on BNC): ≤ 14 ns
DC Gain accuracy : ± 5%
Trigger
Trigger mode : Edge,Video
Trigger slope(Edge) : Rising,Falling
Trigger mode(Edge): Auto,Normal,Single
Trigger sensitivity(Edge):
DC coupling : CH1 and CH2: 1 div
: (DC ~ 25M)
EXT : 100mV(DC ~ 20M)
EXT/5 : 500mV(DC ~ 20M)
Trigger level range(Edge):
AC coupling: CH1 and CH2: 1 div (50Hz ~ Full bandwidth) Internal: ±6 divisions from screen center
EXT : ±600mV
EXT/5 : ±3V
Trigger level accuracy (Edge)
Internal : ±0.3 divisions
EXT : ±40mV ± 6% setting value
EXT/5 : ± 200mV ± 6% setting value
Trigger sync(Video) : Field, Line
Trigger Sensitivity (Video) Internal : 2 divisions EXT : 400mV EXT/5 : 2V
Line/field frequency(Video) : Supports NTSC, PAL and SECAM
Measurement system
Automatic measurement, PK-PK, Averaging, RMS, Frequency, Cycle
Waveform math +, −, INVERTED
Waveform storage : 4 waveform, 4 settings
Lissajou's figure : Available
Probe compensation output
Output voltage : Approx 5V,PK-PK ≥ 1M loading
Frequency : 1KHz square wave
Power supply
Voltage : 100 ~ 240 VAC, 50Hz, CAT II
Power consumption: ≤ 15W
Fuse : 1A, T class, 250V
ACCESSORIES : 1 pair of 1:1 (10:1) passive probe, USB cable, Power cable, Instruction manual , Driver CD.

I was wondering if someone can help me decide if this model is sufficient for debugging circuits related to stepper drivers and interfaces.

Please Advice!

Thank You...

Joe!

[Al_The_Man]

If you want something that is not highly sophisticated in terms of features, have you looked at a storage 'scope? Plug into your USB port.
http://www.syscompdesign.com/oscilloscope.htm
There are pro's and con's for each, but something like this may suit your application.
It is double beam and it is nice to store the trace for future reference.
http://www.syscompdesign.com/scope-specs.pdf
Al.

[bharbour]

That scope will be usable for looking at most CNC type stuff. It won't give you much margin for getting into more advanced stuff though. It probably won't be fast enough for serious micro-controller debug anymore.

Digital scopes are funny critters to get used to (for an old analog scope guy). The most important spec to me is the sample rate. Some vendors play games with sample rates with things like "repetative sample rate" which is only useful for repetative signals. The thing to look for is usually called "one shot" sample rate. Mathematically, you must have a sample rate at least 2x the highest frequency that you want to look at. In reality, 5x to 10 is a nice place to start. I would call that scope a nice 10MHz scope. A 10MHz scope will cover most things that you will run into on machine control and many other applications. You should be able to see motor step signals, most PWM signals and all the sensors you will encounter.

Where digital scopes get wierd is when the sample rate is not high enough for the signal you are looking at. This situation causes aliasing, which can confuse you no end. If a display on a DSO looks very different from what you were expecting, turn up the sample rate/sweep speed and make sure that there is not something much faster in the signal corrupting it.

Good Luck,
BobH

[Santa Fe Al]

HI joprinz,

Most probably, you will be needing a 100Mhz dual trace scope. Such as Tektronic 465/466.

If I'm correct, don't shoot me if I'm not , Mariss recently stated in one of his posts that 100 Mhz dual trace is all that is needed. It is an analog type scope and prices are very reasonable if found on eBay or similar place.

I purchased mine on eBay and got lucky. Mine works well for me. However, I am in agreement with Al-The-Man about being able to store trace data on the computer.

Anyway, a google should pop up several places that has them.

Al

[davesaudio]

Don't you guys recommend an isolated scope?
I would think trying to use differential modes
tricky for the less familiar user?

[bharbour]

I agree with Sante Fe Al that the 465 is a really good scope for many uses. I have one and still use it a good bit. I have not used a 466 analog storage scope in a long time and was never very happy with them.
The digital storage feature is really nice for looking at stuff that does not occur very often and especially for fast stuff that does not occur very often. The newer lunchbox sized digital storage scope are REALLY nice for service type work because of their portability.
Dave was asking about an isolated scope. I don't do differential measurements very often, usually that is for doing power line stuff like working on VFD's or switching power supplies. High speed current measurements would be another case. There are times that an isolated scope would be nice, but I don't think that I would buy a scope just for that feature.

BobH

[joprinz]

Thank You Al, Bob, Santa Fe Al, Dave for your advice...

The scope specs states it has USB connectivity for transferring data to a laptop. Is it similar to the Digital storage feature??? Also are there any other new special features I should be looking for?

Joe!

[bharbour]

Hi Joe,
The title of the spec sheet says Digital Storage Oscilloscope. That refers to the basic way the scope works of digitizing the Y input channels at a selectable rate and then displaying them as a plot of the digital values. An analog scope sweeps the beam horizontally accross the CRT at the rate you select and the Y input provides the Y deflection as the beam goes across. The digital scope sample rate may or may not provide you with a choice of the sample rate, often it is selected automagicaly based on the time/division setting.

A convenient side effect of the digitizing is that you can store the values to some medium that you can get into a PC and do stuff with them later. Some scopes allow the USB flash drives, some allow direct USB connection, some have a floppy drive that will write a disk readable on a PC and some have nework connections.

The scope data sheet you showed has the basic features, but it's sample rate is a little slow in my opinion. If I were spending serious money, I would look for at least 500 MHz sample rate. That would give you a very usable scope for looking at stuff up to 100 MHz or so (a lot like a Tek 465 analog scope). If you are doing digital design, that gives you enough resolution to see timing relationships pretty well for reasonably fast projects. If all you want to do is look at step pulses, gate drive signals, switch input type stuff, the 100 MHz sample rate will be adequate.

Good Luck,
BobH