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 — courses:ec330_2013:doublebalancedmixer [2013/01/17 02:47] (current) Line 1: Line 1: + ====== 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​ and V<​sub>​LO​ should have only product of these two components at the output. i.e the output should be zero if either of V<​sub>​RF​ or V<​sub>​LO​ is zero. But, as seen earlier, with V<​sub>​RF​=0,​ the above mixer still generates an output proportional to V<​sub>​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<​sub>​RF​ and -v<​sub>​RF​ and taking the difference between them. It can be seen by inspection that, when v<​sub>​RF​=0,​ the sum of currents through Q<​sub>​1​ and Q<​sub>​2a​ is a constant as is the sum of currents through Q<​sub>​2​ and Q<​sub>​1a​. 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,​ 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}} + + ===== To be done before the lab session ===== + * Go through the [[http://​www.ee.iitm.ac.in/​~nagendra/​EC330/​200901/​lectures/​ec330-mixer2/​ec330-mixer2.swf|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<​sub>​RF​+f<​sub>​LO​ OR f<​sub>​RF​-f<​sub>​LO​ at the differential output to the amplitude of the //input sinusoid//) of 4. + + ===== To be done in the lab session ===== + Verify the circuit designed above: + * Take V<​sub>​outp​ or V<​sub>​outm​ as the output. ​ + * Drive the input with a low frequency v<​sub>​RF​(~ 1kHz) and a high frequency v<​sub>​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. + + {{diff2se.png?​500}} + + * 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<​sub>​RF​ and a high frequency v<​sub>​LO​ and observe the  output. ​ + * Drive the mixer with a v<​sub>​RF​ and v<​sub>​LO​ at close by, but not identical frequencies and observe the low frequency(f<​sub>​RF​-f<​sub>​LO​) output. For filtering out the high frequency component, use a capacitor of appropriate value across R<​sub>​L​(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,​ 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. ​