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Where are the points of a receiver circuit signals waveform that can be seen by an oscilloscope?

Discussion in 'Radio and Communications' started by BRL6, Apr 16, 2018.

  1. BRL6

    BRL6 New Member

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    Hello,

    In a receiver circuit like as an AM radio receiver from antenna to speaker ;

    1-Where are the points that can be seen their signals waveform by an oscilloscope?

    2-And why we can not see their signal at some points by an oscilloscope?

    Very Thanks,
     
  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Which particular signals do you want to see? Raw RF, IF, demodulated audio, DC levels, ....?
    With a good quality, sensitive oscilloscope you should be able to monitor any point in the circuit.
     
  3. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    This schematic is small and hard to see.
    TP8 at the output will have a signal you can see by an oscilloscope. (output of audio amplifier)
    TP6 is a very small audio signal you can see. (input of audio amplifier)
    Signals on T1, T2, & T3 are very small and are 455khz IF frequency and Audio together. T3 you probably can see but T1 is so small that a oscilloscope probably can not find it.
    Signals at the left most transistor are at RF frequencies, very very small. (micro volts)
    [​IMG]
     
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  4. dave miyares

    Dave New Member

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  5. BRL6

    BRL6 New Member

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    With thanks;
    ronsimpson ;thanks, but I can not open and see the schematic that you placed.
    can I see a waveform at antenna by oscilloscope? and why?
    I want all kinds of signals ,ac or dc from antenna to speaker; RF and IF and ....,etc.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If your 'scope shows a signal at the antenna then the radio station must be very powerful and close by. or it is picking up 50Hz or 60Hz mains hum.
     
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  7. RadioRon

    RadioRon Well-Known Member

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    Radio signals that appear at the terminal of your antenna can vary in strength from zero to as high as a few millivolts, depending on how far away the transmitter is and how much power they transmit. Most radio signals that you receive are well below one millivolt. Oscilloscopes are simply not sensitive enough to show these signals. It is not unusual for the most sensitive range on an oscilloscope to be 1 mV per division, and in this range there is often some noise on the trace even with nothing connected to the probe. It is difficult or impossible to see a radio signal of, for example, 50 microvolts, in this noisy trace.
     
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  8. dave miyares

    Dave New Member

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  9. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    At the antenna there a are millions of radio stations all at the same time. (ok maybe 1000s or 100s)
    It will look like noise. If you can see anything.
    upload_2018-4-16_18-18-13.gif
    Hope you can see this picture. The top signal is audio. The middle signal is what a AM radio station transmits. The bottom signal is what a FM radio station transmits.
     
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  10. BRL6

    BRL6 New Member

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    Very thanks for your useful answers.special tanks to ronsimpson for his great modulations animation.So, in fact,if I want to localize points of a receiver in which we can see a reasonable
    signals , which points are there?
    Could you please get me those points even at a typical receiver block diagram ?
     
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  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Why do you ask? The test points are on the schematic of the simple AM radio in post #3.
     
  12. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Here is a learning kit radio. The top half is FM and the bottom is AM.
    "IF" is intermediate frequency. One of the pins on the metal can should have 455khz with audio. You might see it on the Collector pin of the transistor. (very small signal)
    upload_2018-4-17_7-0-26.png
    Thanks for the Likes!
     
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  13. unclejed613

    unclejed613 Well-Known Member

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    wow, that's a pretty cool kit. i ought to get one for my youngest son. who makes that?



    one way to see what's going on in RF and IF circuits is to feed an AM signal from a signal generator. i think most oscopes will be able to see a 100uV signal. so that would allow you to trace the signal all the way through from RF input to audio output. another often used test point would be the local oscillator output
     
    Last edited: Apr 18, 2018
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  14. Colin

    Colin Active Member

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    The problem you might be having is called TRIGGERING.
    To get a trace across the screen, the cro needs to look at point on a waveform (called an amplitude) and then start to show the incoming signal.
    If you want the TRACE (the waveform on the screen) to be steady, the incoming signal must be exactly the same as the previous cycle or cycles. With most signals in an AM and FM radio circuit, the signal is changing all the time due to voice or music being processed.
    The CRO will have a hard time "latching onto" the signal and it generally results in nothing being shown.
    You have to firstly read the output of an oscillator that has a completely stable and reliable waveform and get to know how to adjust the TRIGGER LEVEL control.
    It has a manual and automatic setting and the difference is normally obtained by pulling the control and thereby clicking the switch that is connected to the shaft.
    Once you know how to get a steady trace you can probe higher and higher oscillator frequencies and then go to audio amplifiers that are connected to a signal generator.
    You will then the able to read very low amplitude signals.
    Once you all this ability you can go to a radio tuned into a TONE and finally you can look at reception signals.
     
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  15. BRL6

    BRL6 New Member

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    With thanks,
    I meant that are there points that theoretically can no be seen ?
     
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Let's say difficult. The one thing that makes troubleshooting easier is the injection of a known periodic signal. This is what an oscilloscope is designed to show. Triggering will be difficult from a station broadcast. Signal levels can also be higher, but with the design limits of the AM radio.
     
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  17. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    upload_2018-4-21_17-9-42.jpeg
     
  18. unclejed613

    unclejed613 Well-Known Member

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    very nice. i get the idea that the jumper wires in various parts of the circuit are for simulating faults? watch out for the speaker frame... it's been my experience that when the speaker has a field coil, the frame is at B+ potential.
     

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