My Velleman Hps10se Won't Calibrate A Perfect Square Wave

[moh.vze.com]

Thanks for the confirmation. I guess I've been spoiled by those 10,000$ techtronics.net scopes at my school ~_~

I feel better now to know that my scope is fine.

I guess I better start figuring out wtf is wrong with my high quality eD nine.1 amp now.

[moh.vze.com]

Blah. Just to be safe, can someone confirm this? Is what you see in my image what you get when you calibrate?

[Boon]

I use expensive scopes so I've never seen anything like that but yeah, that image shows correct calibration, as was mentioned it's the top of the wave that matters.

[moh.vze.com]

I use expensive scopes so I've never seen anything like that but yeah, that image shows correct calibration, as was mentioned it's the top of the wave that matters.

You said it's the top of the wave that matters?

Well, this is what I see when I zoom in

[moh.vze.com]

Nevermind. I'm just going to call it good. I guess I'm just confused about how to properly calibrate these cheaper oscopes.

[cliffyk]

Nevermind. I'm just going to call it good. I guess I'm just confused about how to properly calibrate these cheaper oscopes.

In the first, I'd like to establish that in performing the exercise we have been discussing one is not calibrating the instrument, but rather adjusting the impedance match between a particular probe and the 'scope's input. The process should not even be called "calibration", as nothing is being calibrated.

I realise that many do refer to the process as "calibration", nonetheless engineers and manufacturers refer to it as probe compensation, I.e. impedance matching of the probe and the oscilloscope input.

While there is little to no difference in probe compensation between inexpensive and expensive oscilloscopes, one way to make a cheap 'scope (and mostly what separates cheap 'scopes from expensive 'scopes) is to cut corners on the analog signal "front-end". This almost always results in differing input impedances (and very often inaccurate vertical accuracy) between ranges. The impedance changes at different ranges is what affects probe compensation as you change ranges.

This recent photo shows the probe/scope compensation to be peaked at higher frequencies, however you indicated that you "zoomed in" (changed the vertical range?) to capture the trace. I suspect that what happened was that when you changed ranges the input impedance of the 'scope also change slightly, upsetting the impedance match between the probe and the instrument's input. In precision applications (with any oscilloscope) one would adjust probe compensation at the selected range before making measurements.

However, this all needs to be put in perspective. What we have here is a 10MS/s, 2MHz bandwidth oscilloscope, with a 128 x 64 pixel display--I.e. (and forgive me for saying this, I do not mean to be provocative) a toy oscilloscope. It will probably be fine for the sorts of applications I have seen mentioned in this thread and forum (although I believe an amplifier could be well into clipping before it would be readily observed on the small display).

Beyond this, at the audio and low range supersonic frequencies (DC to 200kHz) that seem to be focus of the work being performed by most here, and given the 2MHz bandwidth of the unit is question, HF probe compensation is very nearly an irrelevant issue. The complete range of HF adjustment reflected in your photo series probably amounts to less than a +/- 1dB error at 2MHz (well under the 'scopes overall performance). At 20kHz the error becomes less than negligible.

One last comment, in the interest of full disclosure, I do have a Velleman PCSU1000 'scope which was given to me by Velleman for some software development work I did for their PCGU1000 function generator. I mention this because while the function generator is a very competent instrument and a tremendous value, the PCSU1000 scope is only a bit better than "ho-hum"--yet another toy FPGA based oscilloscope with a PC interface. The vertical accuracy and input impedance bounce around as you change ranges, and with signals of greater than 1MHz or so the jitter makes it of marginal value. Dual channel measurements at higher frequencies (it has a 60MHz analog BW) are flawed by the instrument's use of a single FPGA operating at 50MHz for all 'scope functions (A/D conversion for both channels, triggering, timebase, and digital output). This causes a significant timing offset between channels.

This is NOT because Velleman has produced an inferior or defective product, but rather manifestations of the compromises required to make an inexpensive "toy" oscilloscope.

Edited May 14, 2009 by cliffyk