Integrated Circuits and Systems group, IIT Madras


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courses:el3203_2014:doublebalancedmixer [2014/02/06 03:58] (current)
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 +====== 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 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, when v<​sub>​RF</​sub>​=0,​ the sum of currents through Q<​sub>​1</​sub>​ and Q<​sub>​2a</​sub>​ is a constant as is the sum of currents through Q<​sub>​2</​sub>​ and Q<​sub>​1a</​sub>​. 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<​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.
 +===== To be done before the lab session =====
 +  * Go through the [[http://​​~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</​sub>​+f<​sub>​LO</​sub>​ OR f<​sub>​RF</​sub>​-f<​sub>​LO</​sub>​ 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</​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.
 +  * 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</​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. ​