Page 223 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

4. Kenneth K. Clarke and Donald T. Hess, Communication Circuits: Analysis and

Design. Reading: Addison-Wesley, 1971.
Chapter 14

Mixer Circuits and Harmonic Mixers

An idealized mixer is a multiplier circuit that takes two signals and literally multiplies
the values of each signal to provide an output. In one sense, this concept is easier
to understand with multiplication of numbers. For example, there are two analog
signals, each connected to an analog-to-digital converter (ADC). The digital outputs

from the two ADCs then provide numbers from each signal, and these numbers are
updated over time. If the outputs of the ADCs are fed to a digital multiplier, then
the output of the digital multiplier will provide a stream of numbers that are the
product of the two streams of numbers from the outputs of the ADCs. The output
of the digital multiplier then is connected to a digital-to-analog converter (DAC) to
provide an analog signal that represents the product of the two analog signals.

But back in the days of the superheterodyne radio in the late teens of the twentieth
century, multiplication, or an approximation of it, had to be done all in the analog
domain. Therefore, vacuum tubes such as triodes and pentodes were used
commonly to provide an analog multiplication effect of two signals such as the
radio-frequency (RF) signal and the local oscillator signal. For the triode, which is a
very linear amplification device, a very large local oscillator voltage was supplied to

its cathode or grid to essentially turn the triode on and off to cause a chopping
effect on the RF signal such that the output signal at the plate of the triode
provided an intermediate-frequency (IF) signal.
Pentodes were handled similarly to triodes in terms of RF mixing, with the oscillator
signal and RF signal combined in the control grid. The pentode had a screen grid,
and transconductance depended on the voltage supplied to the screen grid.

Usually, the screen grid was connected to a direct-current (DC) source to set a
fixed transconductance. However, some mixer designs connected the RF signal to
the control grid, whereas the local oscillator's signal was connected to the screen
grid to modulate the transconductance of the pentode. The modulated
transconductance then provided a multiplying effect of the RF signal.

But other analog multiplying methods and circuits also were used in the early years
of radio. For example, diodes could be used as mixers or as part of a more complex
mixer circuit such as a double-balanced modulator. And in particular, in the late
1920s or by 1930, switch-mode mixers using solid-state copper oxide diodes were
used in commutating switch-mode mixers or balanced modulators.
Mixers can be viewed in two ways:

1. Two (or more) signals are applied to a nonlinear device to generate distortion
and thus provide an IF signal.