Question about Electronics textbook - diode switching circuits - Analog switch example

Discussion in 'Homework Help' started by noweare, Jul 14, 2017.

  1. noweare

    Thread Starter Member

    Jun 30, 2017
    64
    9
    The example talks about an AC source which is not shown. Would it be connected to the top of the circuit diagram, where the anodes of the diodes are? And capacitively coupled ?
     
  2. WBahn

    Moderator

    Mar 31, 2012
    23,093
    6,946
    And of course there's only one Electronics textbook and of course everyone has it and of course there's only one Analog switch example, so of course everyone knows exactly what circuit diagram you are referring to and so of course everyone knows what this AC source is that isn't shown.

    Sheesh. :rolleyes:
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    4,959
    1,510
    Translation: Welcome to All About Circuits. Please show us the example about which you write.
     
  4. noweare

    Thread Starter Member

    Jun 30, 2017
    64
    9
    I am talking about the Electronics text book under the "Education" tab on this website
    Volume III Semiconductors --> Chapter 3 Diodes & Rectifiers -->Diode switching circuit page
    Analog switch example.

    Question: There is no AC source in the example, but it is discussed. Would it be connected to the anode of the diodes?

    Thank you
     
  5. bertus

    Administrator

    Apr 5, 2008
    18,596
    3,605
    Hello,

    Do you mean this picture:

    [​IMG]

    When the diode is in reverse mode, the resistance is high.
    When the diode is in conducting mode, the resistance is low.

    Bertus
     
  6. noweare

    Thread Starter Member

    Jun 30, 2017
    64
    9
    Yes, that is the circuit I am trying to talk about : )
    Where would the AC source be connected ?
    Thanks
     
  7. bertus

    Administrator

    Apr 5, 2008
    18,596
    3,605
    Hello.

    There is no AC source used.
    The diodes are switched using a DC voltage to have them isolating or conducting.
    The anodes of the diodes are connected to the +5 Volts on the right of the picture.
    When the "digital" voltage is also +5 Volts, the diode is isolating the capacitor from the circuit.
    When the "digital" voltage is 0 Volts, the diode is conducting and placing the capacitor in the circuit.

    Bertus
     
  8. noweare

    Thread Starter Member

    Jun 30, 2017
    64
    9
    Here is text from the example.

    begin copied text
    " The only problem is isolating the AC signal being switched from the DC control signal. The circuit is a parallel resonant network: resonant tuning inductor paralleled by one (or more) of the switched resonator capacitors. This parallel LC resonant circuit could be a preselector filter for a radio receiver. It could be the frequency determining network of an oscillator (not shown). The digital control lines may be driven by a microprocessor interface. "
    end copied text

    So the micro selects which capacitor forms the resonant circuit with the inductor but no AC source is shown. This circuit, I would
    think, have an AC circuit coupled to it in order to work. Or am I confused?
     
  9. bertus

    Administrator

    Apr 5, 2008
    18,596
    3,605
    Hello,

    The blue parts are the resonant circuit for the high frequency filter/oscilator.
    The RFC's are isolating the high frequency AC from the DC.
    The high frequency AC only sees the switched capacitor, wich is switched to the resonance circuit, changing the frequency of it.

    Bertus
     
  10. noweare

    Thread Starter Member

    Jun 30, 2017
    64
    9
    Where is the high frequency AC coming from ? Is it just not shown in the schematic ?
     
  11. bertus

    Administrator

    Apr 5, 2008
    18,596
    3,605
    Hello,

    The circuit shows only the way how to switch capacitors to a resonant circuit.
    It does not show how the high frequency AC is made.

    Bertus
     
  12. noweare

    Thread Starter Member

    Jun 30, 2017
    64
    9
    Ok, thank you
     
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