Double balanced mixer
- Goals: Understand the operation of a double balanced mixer
- The above figure shows the mixer designed in the previous experiment. Ideally a mixer fed with VRF and VLO should have only product of these two components at the output. i.e the output should be zero if either of VRF or VLO is zero. But, as seen earlier, with VRF=0, the above mixer still generates an output proportional to VLO(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 +vRF and -vRF and taking the difference between them. It can be seen by inspection that the sum of currents through Q1 and Q2a is always constant as is the sum of currents through Q2 and Q1a. 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 RE, 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 fRF+fLO OR fRF-fLO at the output to the amplitude of the input sinusoid) of 2. Take Voutp or Voutm as the output.
- Drive the input with a low frequency vRF(~ 1kHz) and a high frequency vLO(~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 vRF and a high frequency vLO and observe the output.
- Drive the mixer with a vRF and vLO at close by, but not identical frequencies and observe the low frequency(fRF-fLO) output. For filtering out the high frequency component, use a capacitor of appropriate value across RL(both of them) which will short it out at high frequencies. Filtering will be a lot easier if you choose a higher fLO, 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.