Tektronix 2230 question - Page 1
Forum section: electronics / testgear

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2013, 08:30:55 pm
content: Total O-scope newb, here... I've been playing with my scope and an Arduino Mega board. Thought it might be fun to try and generate a sine wave. Also, it'd give me an idea how fast (or slow) Arduino AND my scope are.Anyway, after building my circuit, I tried testing the output with a 10x probe. I had to set the horizontal sweep to about 50ms. I'm not getting a sine wave, but that could be an error in my code or circuit (the wave clearly resembles a square wave with a repeating anomaly like a couple pins are mixed up.) My question is, how long should it take for the sweep to go from one side of the screen to the other? It's close to half a second or so.

author: grumpydoc
author_rank: Super Contributor
timestamp: April 02, 2013, 08:47:49 pm
content: 50ms is 50ms per division - 10 divisions across the screen = 50x10 = 500ms

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2013, 09:05:46 pm
content: Hi Grumpydoc. Yes, that makes perfect sense. Sorry about that, I didn't think through the question before posting.What I should have asked is, is there any way to delay the phosphor from disappearing so fast? A blip bouncing across the screen without a trail is difficult to read. It doesn't look like a steady line until the sweep is set at about .1 ms. Is this normal?

author: nixxon
author_rank: Frequent Contributor
timestamp: April 02, 2013, 09:12:16 pm
content: Quote from: chipwitch on April 02, 2013, 09:05:46 pmCheck that the Scope is set to chop mode (not alternate mode) to make smoother looking traces at slow sweep speeds.IIRC, 1 mS/div horizontal speed should look pretty smooth in chop mode.Info found here:http://www.timcoengr.com/for_sale/Tektronix%202230.pdfP31 green phosphor in the tek 2230 should persist for 10 uS to 1 mS according tohttp://www.jmargolin.com/xy/xymon.htm** Persistence to 10% decay level:VS (Very Short) = less than 1 usS (Short) = 1 us to 10 usMS (Medium-Short) = 10 us to 1 msM (Medium) = 1 ms to 100 msL (Long) = 100ms to 1s

author: nixxon
author_rank: Frequent Contributor
timestamp: April 02, 2013, 09:48:39 pm
content: Here is a link to the 1985 Version of Tektronix'THE XYZs OF USING A SCOPEWith emphasis on the early (analog) 2200-series scopes:http://njarc.org/books/XYZs%20of%20Using%20a%20Scope/XYZs_of_Using_a_Scope.pdfAnd the rest of the interesting manuals are found here:http://w140.com/tekwiki/wiki/2230

author: grumpydoc
author_rank: Super Contributor
timestamp: April 02, 2013, 10:17:33 pm
content: Good links aboveQuoteIt's normal(ish) with the 'scope in analogue mode. You won't see a steady line until the repetition rate is above the flicker fusion threshold - that said not seeing a steady line until you're at 100us per division is a little odd. Generally 5ms per division should be fast enough to perceive a steady trace.Do you have a signal hooked up and do you have the triggering set correctly?The manual is also on theBAMAsite and on ko4bb. Probably worth sitting down and reading it, then having another play with the 'scope. I think it can do rolling and scan display modes at slow timebase speeds when in storage mode.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2013, 10:33:51 pm
content: Okay, that paints me a clearer picture, so to speak. I double checked the output using the adjust signal. There was something off a bit. With the adjust signal's higher frequency, I was able to smooth it out and got an acceptable trace at 1 ms. I think the problem before was that I was trying to read too faint a signal at too low a frequency. All that flashing was about to send me into convulsions!This is my first scope. I have nothing to compare it to. An eevblog member helped me get my first trace just a couple days ago. I bought this thing used on ebay, so basically, the situation is a dummy trying to learn how to use a scope on a machine not know to be in previously good working order. Either I or the machine is to blame. I have to say, I'm a little disappointed that the fault always comes rests on MY shoulders. lolAnyway, you've all been a big help! Thank you.

author: Deckert
author_rank: Regular Contributor
timestamp: April 02, 2013, 10:40:57 pm
content: Quote from: chipwitch on April 02, 2013, 08:30:55 pmJust looking at the Arduino Mega specs I can see it does not have any true analogue outputs, only PWM outputs. Are you filtering the PWM output with an RC low pass filter or measuring directly on the PWM output pin? If the latter, the quare wave you're seeing makes sense.If you're not doing it already, here's a good article on converting PWN output to a (more or less) stable analogue output:http://provideyourown.com/2011/analogwrite-convert-pwm-to-voltage/--deckert

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 03, 2013, 12:30:15 am
content: Neither. Using an 8-bit DAC and 8 digital pins on the Mega board. I'm also simply switching the pins pragmatically in series. I was expecting the serial nature of the pin switching would induce an error; I just didn't know how much. I don't think that's it though. It's very likely the pins could be mis-wired, resulting in bit mismatch between Arduino and DAC. What's more disturbing is the frequency. It's very low. I didn't take particular note of it because it was so far from practical, but I'm guessing less than 1 kHz? I'll check tomorrow and take a picture of the waveform if I don't forget.

author: Deckert
author_rank: Regular Contributor
timestamp: April 03, 2013, 07:25:38 am
content: Quote from: chipwitch on April 03, 2013, 12:30:15 amAh, okay. You can also try to check the output of the two least significant bits directly on the Mega to see if you're at least getting the right binary output on those pins, assuming you can use both channels of the 'scope. Does your DAC not need some sort of clock or latch signal to carry the input over to the internal DAC conversion ladder?I do recall that the MC1408's I used back in the day did not require anything like that, but I did add a 74HC374 just before it to latch the data into place while the next register was being written out from the micro.--deckert

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 03, 2013, 08:15:14 pm
content: Well, I was in the middle of writing a rather lengthy post when an idea hit me... I've been analyzing my project, gathering confidence in the code and circuitry. Still room for improvement. One notable problem is that I'm only getting an amplitude of about 6v. I have 8-bit input ranging from 0 to 255 and 12Vref.It's a moot point, however... back to the "idea". I decided to flip-flop a pin at the beginning of each cycle. I added the code. I got a whopping 2500 Hz. That's not the worst of it... I then discovered I'd made a mistake in my code and that instead of "flipping" the pin at each cycle, I was flipping it at each DATUM POINT!I was taking 255 samplings per cycle. So, thank you Arduino... I managed a whole 10 Hz!I did some reading about the instructions per second... The microprocessor can be programmed in Assembly where a simple flip flop of a digital pin will produce a square wave in excess of 2 MHz. However, the same program written in the Arduino IDE will only generate a little over 1 KHz. Lesson learned! I just received a shipment from Texas Instrument for their version of the Arduino board. I'm going to conduct a test to see how it performs given the same task. It's 32-bit MP compared to Arduino's 8, though I understand they now have a 16 or 32-bit board available. Clock speed on the TI I think is in the 60 Mhz range, three times that of the Arduino.Taking bets... will I get 12 times faster speeds with the TI? 120 Hz? Anyone?