====== Double balanced mixer ======

  * **Goals**: Understand the operation of a double balanced mixer

{{mixer.png?600}}

  * The above figure shows the mixer designed in the previous experiment. Ideally a mixer fed with V<sub>RF</sub> and V<sub>LO</sub> should have only product of these two components at the output. i.e the output should be zero if either of V<sub>RF</sub> or V<sub>LO</sub> is zero. But, as seen earlier, with V<sub>RF</sub>=0, the above mixer still generates an output proportional to V<sub>LO</sub>(LO feedthrough). Such a mixer is known as a single balanced mixer. 

  * LO feedthrough can be eliminated as shown in the circuit below by having two mixers driven by +v<sub>RF</sub> and -v<sub>RF</sub> and taking the difference between them. It can be seen by inspection that the sum of currents through Q<sub>1</sub> and Q<sub>2a</sub> is always constant as is the sum of currents through Q<sub>2</sub> and Q<sub>1a</sub>. This is a double balanced mixer.

{{:courses:ec330_2008:mixer1.png?600}}

  * The above circuit is available in the form of an IC-the MC1496 double balanced modulator. Its internal schematic is shown below. Most of the circuitry including the biasing arrangements are inside the IC. Only R<sub>E</sub>, the load resistors, and the bias current setting resistor need to be connected externally. Compared to the circuit above, the bottom two transistors and their degenerating resistors are arranged as a differential pair inside the MC1496 integrated circuit.

{{:courses:ec330_2008:mixer2.png?500}}

  * Design a double balanced mixer around the MC1496 IC. Use a 12V supply and 1mA current through the bias branch. The mixer should be able to take in an input amplitude of 1V and have a conversion gain(ratio of the //sinusoidal component// at f<sub>RF</sub>+f<sub>LO</sub> OR f<sub>RF</sub>-f<sub>LO</sub> at the output to the amplitude of the //input sinusoid//) of 2. Take V<sub>outp</sub> or V<sub>outm</sub> as the output. 
    * Drive the input with a low frequency v<sub>RF</sub>(~ 1kHz) and a high frequency v<sub>LO</sub>(~10kHz) and observe the output. You can use the oscillator designed in the previous experiment as the 10kHz source. 
    * Verify that the outputs are as expected.

{{diff2se.png?500}}

  * Build the differential to single ended converter shown above and drive it from the mixer. 
    * Drive the mixer with a low frequency v<sub>RF</sub> and a high frequency v<sub>LO</sub> and observe the  output. 
    * Drive the mixer with a v<sub>RF</sub> and v<sub>LO</sub> at close by, but not identical frequencies and observe the low frequency(f<sub>RF</sub>-f<sub>LO</sub>) output. For filtering out the high frequency component, use a capacitor of appropriate value across R<sub>L</sub>(both of them) which will short it out at high frequencies. Filtering will be a lot easier if you choose a higher f<sub>LO</sub>, say 25kHz or 50kHz, and a difference frequency around 1kHz. 
    * Remove the RF input and observe the output.
    * Remove the LO input and observe the output.

  * **Applications**: This circuit is very commonly used for frequency translation in radio transmitters and receivers. 

  * **Something to try on your own**: Drive the lower input with audio, say from your computer or digital player. Drive the LO input with a sinusoid in the AM band(0.5-1.5MHz). You should be able to use an AM radio placed close by to receive the transmitted audio. You can use a short wire connected to the output node as an antenna.