Tuning/Calibrating PropScope
BADHABIT
Posts: 138
While the scope is a magnificent piece of work it appears to need some slight adjustment to get it to return precise frequency measurements. Either that or the Propeller/PPDB frequency testing setup I rigged is way off.
To ascertain wether or not I was getting an accurate reading I loaded a PPDB Prop w/Beau's frequency synth obj and connected·the scope to it.
When taking measurements of set Propeller frequencies the PropScope would display #'s that are not quite exactly what the programmed freq should be. The values are not ridiculously off, but sometimes a couple hundred Hz in a Mhz measurement. If finding or tuning an exact freq wasn't necessary, then having a discrepancy would not matter.
When it is important to hit the number the ability to do precision·tuning to a known value would be immensely beneficial.
I'm sorry if this comes across as condescending to the PropScope, it is certainly not meant to be, yet I feel the need to make this device live up to its full potential. The scope is a marvelous device that works wonderfully.
I would also like to know what the fastest signal is that can be accurately shown. The docs say it takes 25Msps and I've read that 10% of this is usually the max speed of a scope. I've measured faster than 2.5Mhz, much faster, but things start to degrade after about 5 - 7Mhz. Without any way of benchmarking a faster signal it is hard to know where the limits are and where errors comer from.
Also would square wave measurement be more accurately measured at higher·freq's than other waveforms at same freq's·
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BH
To ascertain wether or not I was getting an accurate reading I loaded a PPDB Prop w/Beau's frequency synth obj and connected·the scope to it.
When taking measurements of set Propeller frequencies the PropScope would display #'s that are not quite exactly what the programmed freq should be. The values are not ridiculously off, but sometimes a couple hundred Hz in a Mhz measurement. If finding or tuning an exact freq wasn't necessary, then having a discrepancy would not matter.
When it is important to hit the number the ability to do precision·tuning to a known value would be immensely beneficial.
I'm sorry if this comes across as condescending to the PropScope, it is certainly not meant to be, yet I feel the need to make this device live up to its full potential. The scope is a marvelous device that works wonderfully.
I would also like to know what the fastest signal is that can be accurately shown. The docs say it takes 25Msps and I've read that 10% of this is usually the max speed of a scope. I've measured faster than 2.5Mhz, much faster, but things start to degrade after about 5 - 7Mhz. Without any way of benchmarking a faster signal it is hard to know where the limits are and where errors comer from.
Also would square wave measurement be more accurately measured at higher·freq's than other waveforms at same freq's·
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BH
Comments
Post Edited (Mike Green) : 1/3/2010 4:40:58 AM GMT
I duty cycled my prop chip at 50% and my DMM is reading 1.65 vDC as expected, my prop scope is however reading 2.33 and when I disconnect the probe it reads .68v. If you are wondering if the math works out, it does, 2.33-.68 = 1.65v
Obviously I'm doing something wrong, or there is a calibration necessary, any ideas?
3.3v * 0.707 = 2.33v
Propscope is showing RMS value not Ave value.
(At this point it should be reading a variation of 3.3 and 0 at 50% duty, my DMM reads exactly 1.65v)
When I put an LM358 voltage follower circuit on it, my DMM reads exactly 1.65v
Once again, a crazy reading on the oscope?
The PropScope's are individually calibrated with known voltages for each of the input dividers at the factory- so voltages should be very exact. There was a problem with the gain that was fixed in v1.03- do you have that?
Hanno
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Co-author of the official Propeller Guide- available at Amazon
Developer of ViewPort, the premier visual debugger for the Propeller (read the review here, thread here),
12Blocks, the block-based programming environment (thread here)
and PropScope, the multi-function USB oscilloscope/function generator/logic analyzer
I have v 1.0.2, I'll download 1.0.3 tonight and see if it differs