20-04-2011, 08:52 PM
One or two forum mebers have mentioned scope calibration, and I've referred to it in my posts about crystal calibrators. Given that most of us first add a scope to our range of test gear, then when we've got it, try to learn how to use it, maybe a few words of explanation about scope calibrators might be of interest, though I'm not setting myself up as an expert. First, I might be stating the obvious as to what a scope does, but basically it draws a graph of voltage - the vertical ('y') axis, over time - the horizontal ('x') axis. If you get x and y mixed up, think 'y' as in sky, which is up, and 'x' as in 'a cross'. These two axis are calibrated in precise units of Voltage and time per centimetre of deflection, with ther graticule marked off in 1 cm squares.
When it comes to checking the calibration, the 'x' timebase can be calibrated by displaying a precise frequency from the output of a crystal calibrator or other frequency standard. (Many scopes have a built in 50Hz square wave derived from the mains frequency). When it comes to the calibration of the 'y' amplifier & attenuator, it requires a square wave of a known amplitude over the range of 1mV to 5V peak-to-peak to match the usual 1-2-5 sequence of the oscilloscope attenuator range.
Of course, the calibration test gear doesn't actually calibrate the scope - it simply checks the calibration, and if it's out, then it's necessary to delve into the scope handbook to see how the claibration may then be adjusted.
That said, my eyes glaze over when people on certain forums bang on about the importance of calibration, or buy or recommend very complex scopes which far exceed their needs or undertstanding. Many such scopes are years old, full of aging, obsolete chips, and the higher the compnnenet count, the greater the probabilty of something failing over time, be it resistors going high, transistors dying, or capacitors developing a high ESR. To the questions 'which is the best scope?' the answer must be 'the simplest cheapest one that meets your likely needs',(unless of course a higher spec one falls into your lap, then all well and good!).
There's nothing that we do as hobbyists which calls for a highly calibrated scope - we're primarily interested in observing waveforms rather than in measuring precise frequencies. We can do that with a frequency counter, which give a more precise and unambiguous display. UIndeed, unless we work with scopes every day, it's unlikely that we'll be able to do the mental arithmetic which tells us that 1kHz = 1 mSec, 100 kHz = 10uSecs etc. A frequency counter will tell us to one Hz what the frequency is. But if, for example, we wanted to see the shape of the waveform and any distortion, a counter can't do that, but we could set the scope to display ten cycles, then check the output waveform to see if only one cycle is displayed. (On a dual beam scope we could check both waveforms at the same time). We could check the actual frequency with a frequency counter of course, but the counter wouldn't show us what shape the waveform is.
I've attached a pic which show a 10kHz frequency from my homebrew waveform generator connected to my homebrew frequency counter, from which the frequency can be seen. (It's actually showing 10.003 because the frequency is set by hand on a single turn pot and to set it precisely to a few Hz is a bit ticklish!). I've also attached pics showing that frequency displayed on my scope with the scope set to 0.1 secs, for three waveforms - sine, square and triangular. The scope is set to 2Volts/cm, and two vertical divisions will be seen, which equates to 4 Volts peak-to-peak, to which I've set the adjustable output, as can be seen on the far RH knob of the waveform genrator.
I mentioned earlier that I intend to design a cheap and simple crystal calibrator to give square outputs at 5 MHz, 1 Mhz, 100 kHz, 10 kHz and 1 kHz, as a homebrew 'forum project'for interested parties. But meanwhile, I thought it wouldn't go amiss to say that if you've got a scope, the likelihood is that its calibration will be as close as it needs to be for any conceivable purpose to which any of us are likely to put it. Whilst we like to htink our test gear is accurate, we're not putting rockets into space of reparing safety critical life support systems - we're hobbysits, mending old radlios and tellies for fun!
About the only use for a scope on radios that I can think of, is setting up the IF stage, which requires a signal generator, a frequency counter, and a wobbulator. Most people are content with just using a signal generator.
So why, I wonder, do so many radio hobbyists lust after 200mHz digital storage scopes and the like? Ask them the question and they don't know the answer. (Big boys toys?).
Just my own thoughts.
Hope it's of interest.
David
When it comes to checking the calibration, the 'x' timebase can be calibrated by displaying a precise frequency from the output of a crystal calibrator or other frequency standard. (Many scopes have a built in 50Hz square wave derived from the mains frequency). When it comes to the calibration of the 'y' amplifier & attenuator, it requires a square wave of a known amplitude over the range of 1mV to 5V peak-to-peak to match the usual 1-2-5 sequence of the oscilloscope attenuator range.
Of course, the calibration test gear doesn't actually calibrate the scope - it simply checks the calibration, and if it's out, then it's necessary to delve into the scope handbook to see how the claibration may then be adjusted.
That said, my eyes glaze over when people on certain forums bang on about the importance of calibration, or buy or recommend very complex scopes which far exceed their needs or undertstanding. Many such scopes are years old, full of aging, obsolete chips, and the higher the compnnenet count, the greater the probabilty of something failing over time, be it resistors going high, transistors dying, or capacitors developing a high ESR. To the questions 'which is the best scope?' the answer must be 'the simplest cheapest one that meets your likely needs',(unless of course a higher spec one falls into your lap, then all well and good!).
There's nothing that we do as hobbyists which calls for a highly calibrated scope - we're primarily interested in observing waveforms rather than in measuring precise frequencies. We can do that with a frequency counter, which give a more precise and unambiguous display. UIndeed, unless we work with scopes every day, it's unlikely that we'll be able to do the mental arithmetic which tells us that 1kHz = 1 mSec, 100 kHz = 10uSecs etc. A frequency counter will tell us to one Hz what the frequency is. But if, for example, we wanted to see the shape of the waveform and any distortion, a counter can't do that, but we could set the scope to display ten cycles, then check the output waveform to see if only one cycle is displayed. (On a dual beam scope we could check both waveforms at the same time). We could check the actual frequency with a frequency counter of course, but the counter wouldn't show us what shape the waveform is.
I've attached a pic which show a 10kHz frequency from my homebrew waveform generator connected to my homebrew frequency counter, from which the frequency can be seen. (It's actually showing 10.003 because the frequency is set by hand on a single turn pot and to set it precisely to a few Hz is a bit ticklish!). I've also attached pics showing that frequency displayed on my scope with the scope set to 0.1 secs, for three waveforms - sine, square and triangular. The scope is set to 2Volts/cm, and two vertical divisions will be seen, which equates to 4 Volts peak-to-peak, to which I've set the adjustable output, as can be seen on the far RH knob of the waveform genrator.
I mentioned earlier that I intend to design a cheap and simple crystal calibrator to give square outputs at 5 MHz, 1 Mhz, 100 kHz, 10 kHz and 1 kHz, as a homebrew 'forum project'for interested parties. But meanwhile, I thought it wouldn't go amiss to say that if you've got a scope, the likelihood is that its calibration will be as close as it needs to be for any conceivable purpose to which any of us are likely to put it. Whilst we like to htink our test gear is accurate, we're not putting rockets into space of reparing safety critical life support systems - we're hobbysits, mending old radlios and tellies for fun!
About the only use for a scope on radios that I can think of, is setting up the IF stage, which requires a signal generator, a frequency counter, and a wobbulator. Most people are content with just using a signal generator.
So why, I wonder, do so many radio hobbyists lust after 200mHz digital storage scopes and the like? Ask them the question and they don't know the answer. (Big boys toys?).
Just my own thoughts.
Hope it's of interest.
David
