AM transmitter with diode ring mixer HELP!

Discussion in 'The Projects Forum' started by Incrinage, Dec 8, 2014.

  1. Incrinage

    Thread Starter New Member

    Dec 8, 2014
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    Hey guys I've been having a lot of trouble with a project for school For my Capstone design course I'm required to simulate and build an AM transmitter and receiver with a diode ring mixer to produce a 27MHz signal. I'm using Multisim 11 to simulate everything.

    I have the mixer amplifier and filter working separately. The problem I'm having is not getting enough gain when I put the circuit together. If I were to demodulate the demodulate the signal using a diode and low pass filter i would not be able to. For the LO I am using a 15MHz signal with 1.5V pk-pk and for my test input signal I am using a 12MHz at 300mV pk-pk. I am using a 4:1 audio transformer which gives better performance than the other transformers in Multisim.

    From the output of the mixer I have a 1k Ohm resistor followed by a capacitor of 1uF to block DC from effecting the common emitter amplifier. Without the resistor and the capacitor left alone I get better amplification but my signal is distorted and the mixer output looks nothing like it should.

    The transistor I'm using is a 2N3904. I have a 14k Ohm resistor in series with a 10k Ohm resistor to bias the transistor with about 2V using a 5V as Vcc. I have 800 Ohm resistors in the collector and emitter branches with the emitter resistances shunted by a 1uF capacitor so it doesn't effect the AC gain.

    From the collector branch I attached the filter and have a 1k resistor at the end for a load. I'm sure the filter has some lost but I don't think that's where the problem lies since without the filter I still get a poor signal at the collector branch.

    When I use a multimeter during the simulation I get about 10mV on that load . Which is too low to do anything with . I'm not sure if there is an impedance matching problem or poor amplifier design. I think it could also be because of the LO I chose or the the gain bandwidth of the transistor when dealing with signals close to 30MHz. Any advice will be greatly appreciated.

    Any advice will be greatly appreciated. I attached a picture of the circuit.
     
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