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
    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.