Mixer
A mixer circuit is one that 'multiplies' two signals and outputs a signal that has different frequencies, the frequencies of the output are at the sum and difference of the two input signals. Mixers usually work with a local oscillator, which will be discussed later. The mixing occurs because the circuit has a nonlinear behavior, which means diodes and transistors can be sued to build a mixer, there are some differences between using a diode and transistor that is beyond the scope of the discussion here.
Part 1: Basic Mixer
The mixer discussed here uses a transistor and is shown in figure 1. The mixer takes in the RF signal, at 1 MHz and multiplies it with the local oscillator signal (LO), the output signal should be the difference and sum. However, the only concern here is the difference signal, which comes out at 200 kHz. It can be seen that there indeed is a signal at the difference frequency that has the highest magnitude, as demonstrated by figures 2-5, there are other peaks at some other different frequencies (which can be seen more clearly in figures 4 and 5) these are the harmonics, which can be ignored for now.
Figure 1: Basic mixer
Part 2: Mixer with RFC
Adding a radio frequency choke (RFC) to the circuit will make the collector resistor appear as a short for the AC signal, by eliminating the DC signal from the output, the interference it causes to the AC signal is eliminated which results in some gain to the output. The difference is more visible when comparing figure 9 with figure 4, the signal at 200 kHz has a magnitude of ~50dB in figure 4 (without RFC), while in figure 9 the magnitude is ~65dB.
Figure 6: Mixer with RFC
Part 2: Mixer with RFC and BPF
The mixer adds more frequencies into the signal, which results in more noise in the signal, the previously discussed bandpass filter (BPF) is very helpful in getting rid of the unwarted frequencies as it sets a range for the frequencies that pass through the circuit. The only drawback of this approach is that difference frequency is not where it is supposed to be, it gets shifted slightly, as demonstrated by figures 12 and 13, the output voltage is much lower than it supposed to be.
Figure 11: Mixer with RFC and BPF
The shift in frequency happens because the capacitors and inductors are not ideal, the new maximum frequency can be found easily by moving the cursor on the oscilloscope. As shown in the figure below, the maximum output was shifted to 215 kHz.
Figure 16: Value of the shifted frequency