Double balanced mixer

The above figure shows the mixer designed in the previous experiment. Ideally a mixer fed with V_RF and V_LO should have only product of these two components at the output. i.e the output should be zero if either of V_RF or V_LO is zero. But, as seen earlier, with V_RF=0, the above mixer still generates an output proportional to V_LO(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_RF and -v_RF and taking the difference between them. It can be seen by inspection that the sum of currents through Q₁ and Q_2a is always constant as is the sum of currents through Q₂ and Q_1a. This is a double balanced mixer.

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_E, 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.

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_RF+f_LO OR f_RF-f_LO at the output to the amplitude of the input sinusoid) of 2. Take V_outp or V_outm as the output.
- Drive the input with a low frequency v_RF(~ 1kHz) and a high frequency v_LO(~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.

Build the differential to single ended converter shown above and drive it from the mixer.
- Drive the mixer with a low frequency v_RF and a high frequency v_LO and observe the output.
- Drive the mixer with a v_RF and v_LO at close by, but not identical frequencies and observe the low frequency(f_RF-f_LO) output. For filtering out the high frequency component, use a capacitor of appropriate value across R_L(both of them) which will short it out at high frequencies. Filtering will be a lot easier if you choose a higher f_LO, 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.