The next stage of the spectrum analyzer is the two-channel mixer. It mixes the signals coming from the input LPF and the VCO. The mixer utilizes a transistor-based circuit to amplify the signal from the LPF and then inserts the VCO signal in the emitter region. The VCO signal then mixes with the first signal by varying the Q-point, and thus the amplification of the first signal, according to its amplitude.

Mixer schematic

Figure 5: Mixer schematic

The mixer’s operative equation is given below:

The term on the right is the output of the mixer. E1 and E2 are the input signal amplitudes, given to the base and emitter of Q1 respectively.

The input from the LPF is a complex signal consisting of a number of frequencies in the audio frequency range. The second input (VCO) is a variable frequency signal sweeping from 40 kHz to 60 kHz. Thus, the output of the mixer is a band of frequencies lying within 40±20 kHz. To select only the 40 kHz frequency, we place a Band-Pass Filter (BPF) tuned to the 40 kHz frequency. This filter is explained in more detail in a later section.