Page 278 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

input signals, Р(/). Therefore, there аге exceptions to the concept of balancing out

ог nulling out the input signal from the output of а multiplier ог mixer.
Now let's return to Figuгe 15-20 and examine how it works as а multiplier. The RF
inputs connected to each of the two inputs of the switch аге complementaгy ог
push-pull. The oscillator signal that is fed to the switch control actuator is а 50
percent duty-cycle square-wave signal. Therefore, at the output of this mixer, half
the time the output is the RF signal input and the other half of the time the output

is the inveгted RF input signal. For example, if the control signal is logic high, the
output of the switch is connected to the noninveгting RF signal. When the control
signal is logic low, the output of the switch is connected to the inveгting RF signal.
Thus the output of the mixer is а signal that flips the sign ог polarity of the
incoming RF signal. lп other words, this mixer commutes the RF signal in polarity
as а function of the oscillator signal.

Therefore, the circuit in Figure 15-20 is sometimes called а commutating mixer. То
understand the multiplying characteristic of this circuit, for the time being, consider
setting the noninveгting RF signal V RF to 11 volt ОС at the А input of the switch. lf
the А input is 11 volt ОС, then the В input of the switch has to Ье -1 volt ОС based

оп the fact that the В input is the inveгting RF signal input.
Now, with 11 volt ОС at the А terminal and -1 volt ОС at the В terminal, and with
the oscillator signal controlling the switch so as to send the voltages at the input
terminals in ап alternate таппег, the output has to generate а square-wave signal
of levels +1 volt and -1 volt at the same frequency as the oscillator signal. For

example, if the oscillator signal is set to 10 MHz, with the input voltages at 11 volt
and -1 volt, the output of the switch will generate а 10-МНz square-wave signal
that is 2 volt peak to peak and centered around О volt.
lf V RF is increased to 12 volts ОС at the А terminal, then the voltage at the В
terminal of the switch has to Ье -2 volts ОС, and the output of the mixer will

provide а square-wave signal from -2 volts to 12 volts, ог а 4-volt peak-to-peak
signal at 10 MHz. Therefore, the output of the mixer is а square-wave signal that is
scaled Ьу the input signal, and in paгticular, this square-wave signal is bipolar,
meaning that it outputs both positive and negative pulses. Also note that with
positive voltages at input terminal А, the output square-wave signal matches the
same phase as the oscillator signal.

Now what happens if V RF at the А input is set to -1 volt ОС? Then the В input will
have to Ье 11 volt ОС, and the output of the mixer still will generate а 2 volt
peak-to-peak square-wave signal but with inveгted phase in relationship to the
oscillator's signal. Thus, at least with ОС voltages, there is а multiplying effect from
this circuit. And in general, if the input signal is ап АС signal, there is still а

multiplying effect from this mixer circuit. From the examples where the input signal
is increased, decreased, ог changed to а negative polarity, the output of the mixer
maintains propoгtionality and phase.