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Mixer

Objectives:
  • The goals of the Mixer lab are:​
    • ​To become familiar with how a mixer works.
    • To analyze the mixer, generating a spectrum using the FFT function.
    • Build and test a mixer for use in the AM heterodyne radio.

Mixing Theory 

  • The primary objective of this lab is to introduce the mixer and demonstrate how it works with the AM heterodyne radio. 

  • The mixer multiplies an RF signal (1000 kHz) with an local oscillator signal (1200 kHz) to produce a number of signals at different frequencies. The output is known as the difference frequency (200 kHz), or the intermediate frequency. 

  • The mixing occurs as a result of the nonlinearity in the mixing element (diode, or transistor). 

 

This lab was completed with a lab partner as it required the use of two function generators to produce both the RF signal and the local oscillator (LO) signal. The first mixing circuit was breadboarded and can the viewed below. 

Using the ocsilloscope, the output could be observed across the load. The frequency spectrum was also accessed by utilizing the Math Menu button and selecting the 'FFT' operation. As the oscillator frequency varied, the difference frequency was noticed on the output spectrum. This was the case for increasing and decreasing the local oscillator frequency. An image of the output spectrum can be seen below.

 

Figure 1: Mixing circuit

Figure 2: Output spectrum

The same procedure was repeated after inserting the RFC (which is used to hide the Rc resistor from the AC circuit). Again, the difference frequency could be realized on the output spectrum as the oscillator frequency varied. 

 

The final step was to include the bandpass filter and use the output spectrum to find the new intermediate frequency. The exact resonance frequency was unknown because the capacitors and inductors were non-ideal. The signal levels were adjusted; the VRFin was adjusted to 20 mV and the VLO was adjusted to 200 mV. The IF was found by sweeping the oscillator frequency such that the output difference frequency occurred at filter resonance. The intermediate frequency was found to be 192 kHz. This IF was slightly lower than the original IF.  

Figure 3: Mixing circuit with bandpass filter included.

The Mixer lab served the purpose of being informative in many ways. This lab taught concepts as well as design. The information on the transistor described in the lab and how it is used to realized the mixer helped utilize knowledge learned in Digital Electronics as it introduces the transistor and diode functionality. The mixer itself is a newly introduced concept. This lab helped build hands-on skill as well as gain useful knowledge in terms of circuitry, function generators, and oscilloscope functionality. 

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