I am wondering if someone could help me choose a oscilloscope to help me debug a project?
Mods if necessary please move this topic to the appropriate place.
Here is a general description of what my project looks like. I have a adjustable duty cycle 555 timer circuit that has a 25% duty cycle signal with a 83Hz frequency at around 12 Volts peak to peak. I am inputting the 555s square wave into a Atmel micro through a 100Mhz ferret bead, 4.7K resistor, 1uF ceramic cap to ground and a 2.4K resistor to ground. The 4.7K resistor and cap form a low pass filter. This setup occasionally causes the output of the micro to go high.
I am wondering if my scope would be fast enough to catch the occasional pulse to the micro input that would cause it to trigger or would I need a digital storage scope to catch the pulse? If I need a digital scope how many samples per second and memory would I need to catch this event?
Everything is well decoupled and was working fine with a older pcb board layout and the same value components but the manufacturer of the cap was different. The new board layout and different manufacturer but same value cap I think are to blame. I cannot easily swap the cap with the old board due to the SMT footprint been different. The 555 timer circuit is just on a breadboard and is independent of the micro pcb. I can hook up the 555 timer to the old board and it does not cause the output to occasionally go high.
The 555 timer circuit is simulating a can-bus tail light signal so it can not be changed. The Atmel micro (a 90usb162) runes at 16Mhz from a external crystal oscillator and I can see a lower frequency micro square wave of it on the input lines as well as the 5 Volt regulated and filtered input. This signal is more pronounced on the new board layout then the old one.
The output signal of the micro only changes in duty cycle not frequency when it goes high (it is driving some LEDs) so I presume that this would be harder to trigger on.
I have been looking at some HP 5400 series scopes and I found one that is 100Mhz with 20Msa/s for $100 no probes and the auto set button is broken and it is not calibrated from 1997. Would this work or should I be looking at one of the $350 china made scopes with 1 to 2Mb of memory 1Gsa/s and 100Mhz instead? I could also get a Tek TDS320 for $300 but there are no pictures of it working nor probes. These are all scopes that I have found on Kijiji or Allsold.ca in Calgary so the price may change.
A DSO would be nice,but you can almost certainly find this fault with your existing Oscilloscope.
You have the best test instrument available--------Another working unit.
Ideally,you would run both units & swap the probe back & forth,but as you only have one 555 generator circuit,you will have to do the following:
Check all the inputs & outputs of your good one,as well as the power supply pins.
Now check all the same points on your faulty one.
If your memory is poor,draw the waveforms on paper,or take screenshots with a digital camera.
(Not near as easy as DSO storage,but life wasn't meant to be easy!)
If there is a fairly major difference in the waveforms,you will be able to find out why,& probably fix the problem.
If there is no discernible difference,it must be something which occurs rarely,rather than a cumulative effect due to some difference in the observed waveforms.
You said the"setup occasionally causes the output of the micro to go high."
How occasionally?------Every 10 seconds or so?,Every half hour?,every few days?
If it is a reasonably short period,I would suggest you hang the 'scope across the supply pins of the micro,with its input "DC coupled",set the horizontal timebase to a convenient setting to give a continuous horizontal line,adjust the line to the centre line of the 'scope graticule,& wait.
If there is a "glitch" on the DC line,it should manifest itself as a deflection up or down on the display.
If "occasionally" is a very long period,you may be better off with a Logic Analyser rather than either an Analog
or Digital 'scope.
It can still be done with a 'scope,but be prepared for a bit of a boring wait,staring at that screen!
P.S: Are you absolutely sure it isn't a programming error in your micro?