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RF circuit , mixer help

Discussion in 'Radio and Wireless' started by minh2211, Oct 14, 2015.

  1. minh2211

    minh2211

    9
    0
    Sep 29, 2015
    Hi, i need help with this circuit attached. Firstly, I havent done impedance matching. I just tried to wire the oscillator with the mixer to see if signals are mixed . But it seems like it doesnt work well. The inputs Vlo+ and Vlo_ when wiring to inputs of differential amp become different form each other. Any helo would be appriciated
     

    Attached Files:

  2. duke37

    duke37

    5,253
    726
    Jan 9, 2011
    My word, that is complicated.

    What is the frequency you are using 1μF seems a lot for radio frequency?
    M5 seems to do nothing, it is turned hard on.
    Using fets (M1 and M2) in the mixer means that you will need a large voltage to turn then on and off, do you have this?
    There is no indication of the output connection. Use the drains of M1 and M2 tied together as output, Only one resistor is needed.

    A ring diode mixer would be much simpler.
     
  3. minh2211

    minh2211

    9
    0
    Sep 29, 2015
    Your right. At the input of the LNA, im not sure why before the interstage cap, the signal is 1.6mV peak to peak, but after it, it suddenly drops to 0.0008 V peak to peak. The cap somehow forms an filter. I havent done matching. When i tested each block of the circuit, everything works fine. But when i connect them together, things turn out to be wrong
     
  4. duke37

    duke37

    5,253
    726
    Jan 9, 2011
    Which is the interstage capacitor? Do you mean C6?
    It is being fed from a source of 100k and feeds into under 20k so would expect about a fifth of the voltage when connected which is about what you have.

    What are all the inductances for, they do not seem to be lead inductances since they all differ.
     
  5. minh2211

    minh2211

    9
    0
    Sep 29, 2015
    I mean this part of the circuit when connecting LNA and mixer. The signal from LNA drops. if you disconnect the LNA and mixer, and then measure the output of LNA, it is much bigger

    The inductance of the mixer is used for impedance matching. The uni tutor told me that
     

    Attached Files:

  6. duke37

    duke37

    5,253
    726
    Jan 9, 2011
    "Much bigger".
    That is because the source is 100k and the load is under 20k.

    You still have not shown the output of the mixer. This would be the two sources of M1 and M2 connected together. The oscillator drives M1 and M2 in opposition so the output is zero until the current is varied by M3.
    This makes it a single balanced mixer.
     
  7. minh2211

    minh2211

    9
    0
    Sep 29, 2015
    thanks for your help. i just got it working. Theres one more question. how can i shift a signal 90 degree without affecting its amplitude
     
  8. duke37

    duke37

    5,253
    726
    Jan 9, 2011
    It depends on its frequency and whether the frequency is constant. The normal way for a constant RF frequency is to split the signal into +45 and -45deg.
    This is done by feeding through a resistor to a capacitor and through another resistor to an inductor.

    For a band of audio frequencies, op-amps set up as all pass filters can be used, Two chains of op-amps can provide a difference of 90deg over about 10:1 frequency ratio.

    I suggest you look up single sideband generation.
     
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