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Characteristics of Mixer

Discussion in 'Electronics Homework Help' started by Mercy Chong, Jul 20, 2014.

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  1. Mercy Chong

    Mercy Chong

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    Jul 20, 2014
    lab3waveform1.jpg AClab3ckt.jpg
    I am doing an experiment about the characteristics of Mixer. 1st diagram is the output waveform from output point A. 2nd diagram is the circuit. I don't understand why the output waveform is look like square wave? can anyone explain it?
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Hello and welcome to Electronics Point :)

    It's not a square wave; it's a very badly clipped sinewave. My guess is that the transistor is being overdriven and/or is incorrectly biased. Try reducing the amplitude of the VX and VY generators. If you still have problems:

    1. What is the frequency, waveform, and amplitude of the VX voltage generator?
    2. What is the frequency, waveform, and amplitude of the VY voltage generator?
    3. Can you show us the top part of the schematic, including the base bias resistor and the collector load resistor, and the supply voltage.
     
  3. Mercy Chong

    Mercy Chong

    3
    0
    Jul 20, 2014
    lab3waveform2&3.jpg
    Vx, f = 101kHz, sine wave, amplitude = 1 Vp-p
    Vy, f = 100kHz, sine wave, amplitude = 0.1 Vp-p
    From the point B output i can get a very thick sine wave with many ripples. After that from the Vout i get a thin sine wave compared from point B output. As above.
    The badly clipped sinewave it is a mixing waveforms?
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    1,271
    Nov 28, 2011
    Those amplitudes are far too high. Try reducing them both to around 0.02V peak-to-peak. Or for a clearer output waveform, try setting them to integer multiples, and different amplitudes. For example, try this:
    VX: f = 40 kHz; amplitude = 0.04V p-p;
    VY: f = 200 kHz; amplitude = 0.01V p-p.

    This should give you an interesting looking waveform at point A, showing the mixing of the two frequencies. This is probably not the aim of this lesson though, which is to show how sum and difference frequencies are produced by a mixer. Try these settings:

    VX: f = 100 kHz; amplitude = 0.03V p-p;
    VY: f = 101 kHz; amplitude = 0.02V p-p.

    This will produce the messy waveform that you describe as a "thick" sinewave with many ripples at point A. At point B, and Vout, you should see a cleaner waveform at 1 kHz - the difference frequency.
     
  5. Mercy Chong

    Mercy Chong

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    0
    Jul 20, 2014
    ok..
    tq~
     
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