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New oscilloscope, does this look right?

Discussion in 'General CB Services Discussion' started by 9Lives, Feb 4, 2013.

  1. 9Lives

    9Lives
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    Hi everyone, just got my first scope at 29 years old, thks to it I was able to wide band my 148 for direct injection. But I'm still figuring t the scope out.
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    Does this look close to 100%?


     

  2. 9Lives

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    The carrier it's set low of amplification, the peaks are not distorted or over modulating? It is also wide banded. It is shown using rd sampler I built into dummy load I also built.
     
  3. Robb

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    #3 Robb, Feb 4, 2013
    Last edited: Feb 4, 2013
  4. 9Lives

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    Lol! My bad, I got everything hanging from everything, I already put everything up but ill get to it. I'm like hanging from one leg holding a radio in one hand lol, as for just my voice does it look decent?
     
  5. RADIOOMAN

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    Looks about right to me.
    Little bit of negative peak over modulation, but looks pretty typical to me for am carrier cb radio.
    That last picture looks like a bit of a triggering problem.
     
  6. 9Lives

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    Can you help explain that to me? Is that a setting inthe scope or radio problem? I was twisting knobs trying to get best view
     
  7. 9Lives

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    And how can I get it sounding perfect? Back off modulation or more amc?
     
  8. Robb

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  9. 9Lives

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    Alc sorry
     
  10. RADIOOMAN

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    Actually for being new to the scope you actually got it pretty good.
    You might want to play with the slope/level/filter settings on the scope to get a more consistent stable triggering of the waveform.
    Maybe you could tell me what model scope you have and what is the 'rd sampler' built into the dummy load that you speak of.
    Also, Robbs' advice of using a constant tone is really good.
    I use a 'tone box' as I call it.
    Just a box with a speaker and a tone generator with enough power to drive the speaker to a moderate to low level. (something you can fabricate from misc electronic devices or assemblies)
    And then just hold the microphone near the speaker on the box and press the PTT and then your waveform on the scope will be stable (once you get the triggering settings adjusted to suit you) and easier to interpret (and more meaningful).
    But, utimately, you can only tell so much from looking at the waveform on the scope; Tonal quality, sound quality are not embedded into the waveform on the scope; Ultimately the sound quality of your transmission will have to be judged by other people listening to their radios while you are transmitting.
     
  11. Robb

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    I think he is doing a fine job too. Taking the initiative to learn something new and useful will have many benefits. There are LOTS of tutorials on how to use a scope on YouTube. There are a lot of things I don't know how to do on a scope yet; just enough to work on a radio's needs. However necessity is also the mother of learning. It really isn't that hard once you use it for awhile. Be patient and continue.

    Using a reference audio signal/1khz into your mic, you can see if if you modulation is more or less 100%. You can also see if the top of the sine wave is squaring off at the top. This is an indication of positive peak distortion. Keep those peaks curved!

    If it pinches off between the peaks near the middle of the signal; then that is negative peak distortion. Keep those negative peaks as well formed/shaped as the positive peak are! Just so long as it doesn't pinch off altogether, it will be at/very near 100% modulation when you do this.

    If you took the time to watch that video I posted, you can get a fair idea why the Bell's Radio tech said what he did. It also provides a very sturdy and clear idea why not to remove the AM Limiter transistor. More than that, you can see the effects on the scope and why any tech or non-tech advises against doing that silly mod. The proof is on the scope. If more than 100% modulation is the goal; then clipping out the limiter is definitely NOT the way to do it right.

    The NPC/RC mod accomplishes both positive peak increase in modulation % while keeping the negative stroke of the cycle from pinching off - so long as the modulation levels are kept to what was adjusted and observed while using the scope. You can even keep the AM Limiter in so long as you make it ineffective with a 1.5k ohm resistor that feeds that transistor's collector trace.
     
    #11 Robb, Feb 4, 2013
    Last edited: Feb 4, 2013
  12. 9Lives

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    Thanks for the compliments, this is all feeding my addiction do well :). The scope I'm using is a bk precision 2120b I think. It's the one with out the counter. The reason I wanted this is because I work in south Georgia in the ports pulling boxes out for road drivers in this port you will find the most clipped limiters, swing mods and echo boards in the world. I get so tired of hearing those guys talking smack about how much they know about radios. They only know how to make the meter swing! Little do they know there is actually a trim pot in the radio to do that hehehe.. aside from that they're radio's ate loud for half a mile and then disappear.. that's not what I'm going for.

    So where the line in the center gets brighter is the neg peaks crossing? Will alc help this at all? I know we're talking ocd small things but I just want to know ways to correct problems.

    I've look into npc mods a lot and have heard mixed things. Motor mouth says it pinches carrier. But I'm not going for pro.. yet, just want to stay with in my know how. Which version on the npc will work for the cobra 148, diode resistor? I'm guessing the diode clips the wave and resistor smoothes out the curve?
     
  13. Captain Kilowatt

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    Somewhere on your scope panel there should be a knob labeled "TRIGGER LEVEL". Adjust it for the best display. What you have pictured looks like the same trace scanned twice and overlaid. Most scopes have a switch also labeled INTERNAL (INT) EXTERNAL (EXT) and LINE. Usually one of those positions provides the easiest triggering. I usually use INTERNAL mode. I suggest you carefully read the manual for your model scope and also find some good info online about how to use a scope in general as they can be a very handy piece of gear but you must know how to use it first.
     
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  14. Robalo

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    1 person likes this.
  15. 9Lives

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    [​IMG]


    [​IMG]
     

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