In this weeks lab activity, I studied the mixer circuit, a device which multiplies an RF signal in tandem with an oscillator signal to produce multiple signals at different frequencies. The intermediate frequency, the desired frequency to which to tune the radio, is defined as the difference between the carrier frequency and the oscillator frequency. Through the following exercises, I studied the behavior of the mixer circuit.
First, I built the default mixer circuit shown in Figure 1. Utilizing the oscilloscope, I analyzed the output Vout to make sure that the circuit was functioning properly before using the oscilloscope's spectrum analyzer function to view the circuit's frequency spectrum. In Figure 2, the oscilloscope's markers indicate the carrier frequency (1 MHz) and the oscillator frequency (1.2 MHz). In Figure 3, the carrier frequency (the peak in between markers a and b), the oscillator frequency (marker b), and the difference frequency (marker a) can be seen. As evident by the spectrum in Figure 3, the difference frequency is at 200 kHz, which is the difference between the carrier and oscillator frequencies.
In the next step, I added the RF choke (RFC) to the circuit as shown in Figure 4. Then, following the previous steps, I viewed the frequency spectrum again, varying the carrier frequency as shown in the video below. Finally, adding the bandpass filter (from the previous lab activity) to the mixer, I constructed the circuit shown in Figure 5. Adjusting the function generator's settings to output a voltage of 20 mV at Vin and 200 mV at the input to the circuit from the oscillator, I analyzed the spectrum, measuring the intermediate frequency (difference frequency) to be 200 kHz, which is calculated from the circuit's resonant frequency (not known because of the nonideal circuit components) and the oscillator frequency.
Comments