The purpose of this section is to:
The following notes from Mini-Circuits offer some insight on mixer operation:
Mixing is simply time domain multiplication. This has the peculiar property of creating addition and subtraction in the frequency domain.
This means that two new frequencies have been generated, one shifted up and the other shifted down in frequency. Often times a filter is used to select one of these two.
A basic equation describing amplitude modulation is:
From this we notice that AM involves a process of multiplication. There are several ways to perform this function electronically. The simplest method uses a switch.
Switching modulators can all be placed into two categories: unipolar and bipolar.
The voltage-current relationship of a diode is nonlinear near the knee and is of the form: . The coefficients a and b are constants associated with the diode itself.
SystemView Square Law Modulator
Amplitude modulation occurs if the diode is kept in the square law region when signals combine.
Every super heterodyne radio receiver has a mixer. Its purpose is to shift the radio station of interest down to the IF frequency.
1 MHz AM Carrier into the mixer
The mixer oscillator input is 455 KHz above the incoming carrier frequency.
An ideal mixer will combine the incoming carrier with the local oscillator to create sum and difference frequencies.
SystemView Mixer Models
Ideal Mixer Output
A real mixer combines two signals and creates a host of new frequencies:
A dc level
The original two frequencies
The sum and difference of the two input frequencies
Harmonics of the two input frequencies
Sums and differences of all of the harmonics
Non-Ideal Mixer Out
The principle mixer output signals of interest are the sum and difference frequencies, either of which could be used as an IF. However, the IF is generally chosen to be lower than the lowest frequency being received. Consequently, the IF in an AM radio has been standardized to 455 KHz.