• Simulating the BJT Mixer

    agp.cooper • 01/04/2017 at 04:04 • 2 comments

    Simulating the BJT Mixer

    After searching the Internet I did find a few circuits with component values and local oscillator power requirements that allowed me to digitise a schematic in order to look at the simulation:

    (source: http://users.belgacom.net/hamradio/schemas/mixer1.gif)


    Here is my initial simulation:

    Okay it is pretty clear that it is a Class B amplifier (an important observation!).

    Next lets mix in the signal:

    And what did I get:

    Now this is not right! Even with a uV signal?

    Okay reduce the coupling capacitor:

    And now it works (all that is required is a low pass filter to extract the 100 kHz signal).:

    Need to look at the range of couple capacitors values that work.

    The above local oscillator (LO) is 1 MHz and 2v pp and the signal is 1.1 MHz and 200 mv pp. Getting a 50% duty (i.e. Class B operation) for the mixer requires adjustment of the LO amplitude or the bias resistors.

    Conclusion

    Well I am happy I now know how this mixer works. It is a switching mixer) configured as a Class B amplifier. It is now easy for me to design and to tweak.

    Recovering Audio

    The modulated RF is 2 mv pp and the recovered audio is 1 mv pp. The line thickness is due to left over RF. Pretty happy with that.

    Port Isolation

    How much is the LO isolated from the input? Practically nothing gets through:

    That is 20 mv pp from a LO of 1.4 v pp.

    Regards AlanX