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Richard Erlacher
06/09/12 20:58
Read: 776 times
Denver, Co

#187649 - Doesn't it depend on what THEY mean by bandwidth?
Responding to: Mahmood Elnasser's previous message
It's possible (hopefully not!) that the manufacturer of this instrument has assigned a different meaning to the term bandwidth. The old TEKTRONIX scope is considered to have a 200 MHz bandwidth when a 200 MHz sinusoid is reduced 3 db in amplitude.

What you describe suggests that this instrument does, indeed, have a wider bandwidth than the 3 MHz that's been mentioned here and there. Do you have a signal source that can produce a sine wave at frequencies in the 5-15 MHz range? What would be interesting, too, would be one such generator for each of the four channels, just to see how it displays different sgnals at precisely the same frequency, but in different phase, as well as how it displays four signals at slightly different frequencies.

None of these investigations are intended to malign your new instrument, but, rather, to point out the abilities and limitations, particularly in the context of what you might see, as an indication that it's time to get out the old analog 'scope.

Commercially available signal sources seem to be getting harder and harder to buy, these days, first because they are often limited in both frequency and amplitude, and because they seldom have the features to which I, as an old-timer, have become accustomed. You can obtain a pretty good DDS generator today, but the really complete and really "good" ones are quite expensive. They should allow you to drive at least a 50-ohm load, as that's standard, and swing 15-20 volts to either side of common when terminated into 50 ohms. I have had generators of that class for decades, and certainly rely on them. Arbitrary waveform generators are pretty useful, too, as you then don't have to build application-specific circuitry. Back in the '70's, I became enamored of the TEKTRONIX TM5000-series of instruments. These are like the older TM500-series but are most often MCU controlled and GPIB-capable. As a consequence, I often, incorrectly, assume that anyone working in a serious lab facility has such equipment at his disposal. Both the TM500 and TM5000 series are mainframe-based, i.e. they rely on a power chassis into which the various instruments are plugged as modules. Even the old FG504 signal generator was capable of wide voltage swing over the range from DC to 40 MHz with sweep capability, phase-locked loop, amplitude and frequency modulation, external trigger, and numerous other features that I don't use frequently. I do use the adjustable rise and fall time, duty cycle, and other adjustments frequently, however, and they'd be really handy in "checking out" your new 'scope.

These things were once VERY expensive, but are now not so costly, and, with a little study, can be locally adjusted and maintained. Both series had counters, generators, power supplies, 'scopes, DVM's, arbitrary waveform generators, and lots of other functions. The only problem, and I'd caution you about that, is that it's very hard to find anyone smart enough to repair one if it is broken in some way. The circuits often use matched transistors or diodes, and those are not so easy to replace. My main hassle has been that the 40-year-old electrolytic cap's are gradually in need of replacement.

You might find that exercising your 'scope with 20 ns rather than 200 ns variation in frequency will show you things that later prove valuable, as the 'scope may not show the 20 ns difference reliably. You will learn to recognize the symptom, though, and then will simply switch to the analog instrument.

Good luck!


List of 74 messages in thread
DSO for cheap!      Joseph Hebert      05/11/12 13:35      
   very good      Mahmood Elnasser      05/11/12 14:14      
      Limitations      Joseph Hebert      05/12/12 09:49      
         test results      Mahmood Elnasser      05/12/12 12:46      
            That's valuable information      Joseph Hebert      05/12/12 13:30      
         well, the fact is ..      Richard Erlacher      06/03/12 11:43      
            tektronix 475 comparison      Mahmood Elnasser      06/07/12 10:37      
               Really? Why?      Richard Erlacher      06/07/12 14:51      
               another double post ... sorry      Richard Erlacher      06/07/12 14:51      
   I think ...      Erik Malund      05/11/12 14:34      
      Application dependant      Mahmood Elnasser      05/12/12 05:55      
         Indeed      Andy Neil      05/12/12 09:10      
            It's still too low a bandwidth, wouldn't you say?      Richard Erlacher      06/03/12 11:50      
   just curious, Joe could you try      Erik Malund      05/11/12 14:36      
      Unusual with short pulses for hobbyist projects      Per Westermark      05/11/12 14:53      
         is the clock running?      Erik Malund      05/11/12 16:01      
      Sorry, but no I can't.      Joseph Hebert      05/11/12 16:42      
      10nSec Pulse      Mahmood Elnasser      05/12/12 12:59      
         "at random" means what exactly ?      Jim Granville      05/13/12 18:41      
            Answers      Mahmood Elnasser      05/14/12 07:41      
               more observations      Mahmood Elnasser      06/03/12 10:51      
                  It may not be the sample rate ...      Richard Erlacher      06/07/12 15:07      
                     digital vs analog      Mahmood Elnasser      06/08/12 15:34      
                        Don't get me wrong, here ...      Richard Erlacher      06/08/12 21:34      
                           poor mans's scope      Mahmood Elnasser      06/09/12 02:01      
                              Doesn't it depend on what THEY mean by bandwidth?      Richard Erlacher      06/09/12 20:58      
                                 20 nsec steps      Mahmood Elnasser      06/09/12 23:08      
                                    Well, one thing at a time ...      Richard Erlacher      06/10/12 11:43      
                                    Impulse Tests are better      Jim Granville      06/10/12 16:49      
                                       more capable scope      Mahmood Elnasser      06/11/12 14:03      
                                          I must have missed a step ...      Richard Erlacher      06/12/12 08:07      
                                             TEK475      Mahmood Elnasser      06/12/12 16:16      
                                                here's another alternative ...      Richard Erlacher      06/12/12 22:16      
                                                   DAC and pot in parallel?      Per Westermark      06/13/12 01:07      
                                                      Logically, not physically      Richard Erlacher      06/13/12 07:57      
                                                   new theory      Mahmood Elnasser      06/14/12 00:57      
   3MHz is rather poor Bandwidth ?      Jim Granville      05/11/12 16:51      
      ordinary      Erik Malund      05/11/12 17:20      
         Ther are many params needed      Jim Granville      05/11/12 18:21      
   "only" $200 ?      Andy Neil      05/11/12 23:59      
      Low cost oscilloscopes      Andy Neil      05/12/12 09:18      
         291 > 2*($199)      Joseph Hebert      05/12/12 09:45      
            It doesn't always work that way      Andy Neil      05/12/12 10:48      
               Once upon a time ...      Joseph Hebert      05/12/12 13:28      
   SDS8202      Mahmood Elnasser      06/14/12 13:58      
      Read all the "fine print' first ... and ask any questions      Richard Erlacher      06/14/12 20:31      
         bought it      Mahmood Elnasser      06/16/12 03:07      
            Please let us know how this works out for you      Richard Erlacher      06/16/12 04:19      
               datasheet      Mahmood Elnasser      06/16/12 05:26      
                  2mV/div - but min trig level?      Per Westermark      06/16/12 07:01      
                     Tests      Mahmood Elnasser      06/16/12 09:55      
                        24ns      Per Westermark      06/16/12 11:09      
                           and that's the crux      Erik Malund      06/16/12 11:41      
                              FPGA pulse Expander (edited)      Mahmood Elnasser      06/16/12 12:06      
                                 some love the mother, some love the daughter        Erik Malund      06/16/12 12:13      
                                 Here's omne fairly simple thing ...      Richard Erlacher      06/16/12 15:22      
                                 self calibtrate?      Jim Granville      06/16/12 20:42      
                                    interesting      Mahmood Elnasser      06/16/12 23:01      
                                       yes      Jim Granville      06/16/12 23:15      
                                          glurk      Jez Smith      06/17/12 13:33      
                                             DCMs      Mahmood Elnasser      06/17/12 14:49      
                                                Well....      Jez Smith      06/17/12 15:48      
                                                   I don't know where you're going with this ...      Richard Erlacher      06/17/12 21:25      
                                             xxxxx      Mahmood Elnasser      06/20/12 12:53      
                           333ps      Mahmood Elnasser      06/18/12 00:59      
      SDS9302      Mahmood Elnasser      07/05/12 10:32      
         So, how are you exercising this new 'scope?      Richard Erlacher      07/05/12 10:53      
            so far so good      Mahmood Elnasser      07/08/12 09:45      
               So far so good ...      Richard Erlacher      07/08/12 10:49      
                  rise time      Mahmood Elnasser      08/15/12 13:28      
                     The rise time indicates that high frequencies gets dampened      Per Westermark      08/15/12 16:26      
                     What does "bandwidth" mean?      Richard Erlacher      08/15/12 17:45      
                        good test      Mahmood Elnasser      08/30/12 15:09      
                           Perhaps you should repeat the test ...       Richard Erlacher      08/30/12 23:08      

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