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, when v_RF=0, the sum of currents through Q₁ and Q_2a is a 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.

To be done before the lab session

Go through the lecture on double balanced mixers.
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 RF input peak of 1V and have a conversion gain(ratio of the sinusoidal component at f_RF+f_LO OR f_RF-f_LO at the differential output to the amplitude of the input sinusoid) of 4.

Verify the circuit designed above:

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. Choose appropriate supply voltages for the opamp.
- 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.