Mixers in electronics are electronics circuit that produces multiplicative, additive, subtractive output of two or more inputs. Electronics mixers can be used in different application.When they are employed in radio frequency(RF) circuit they perform modulation and demodulation of signal(s). Modulators and demodulators are used in wireless transmitter and receivers respectfully. Essentially in RF application they perform frequency conversion where a low frequency signal is combined with high frequency signal to produce modulated signal at higher frequency or demodulate a high frequency to a lower frequency signal.
There are different ways of constructing a mixer/modulator but essentially a non-linear device such as diode or BJT or FET transistors are required. Furthermore mixer can be classified depending upon what kind of output the mixer produces by suppressing some signal components and as such there are single balanced mixers or modulators and double balanced mixers or modulators. Thus we can have Single Diode Modulator design, Double Balanced Diode Ring Mixer design, mixer or modulator circuit using transistor and others.Here we show and explain how to design a balanced mixer with two diodes.
The schematic diagram of a simple two diode balanced mixer design is shown below.
As you can see in the above we have used two 1N4148 diodes D1 and D2. These two diodes are alternatively switched on and off by the carrier signal Vc. The modulation signal or the information signal Vm enters the mixer/modulator via the center tapped transformer TR1. The center terminal on other side of the transformer is connected to the carrier signal. The carrier signal which has higher frequency than the modulation signal passes the sum of the modulating and carrier signal through each diode alternatively over half a cycle. The output is taken from the middle of the two diodes. The output is an amplitude modulated signal. The inductor L2 is just an RF choke that prevents leakage of the modulated signal. In this design the output is taken directly from the junction of the two diodes. But an alternative and better way is use another center tapped transformer at the output which is shown in the tutorial How Two Diode Single Balanced Mixer Design Works.
The following shows signal flow in the balanced mixer during the positive half cycle of the carrier signal Vc (also called the local oscillator).
The following shows signal flow through the balanced modulator during the negative half cycle of the carrier signal Vc.
The following is animation showing how signal flows in two diode balanced modulator.
To view which signal components gets through after the modulator and inductor L2 we can plot the frequency spectrum of the output modulated signal. The graph below shows frequency spectrum of the output from the two diode balanced modulator.
Similarly we can view the signal inputs and output modulated waveform using oscilloscope as shown below.
The first waveform is that of the modulating signal, the second is that of the carrier signal and the third is the modulated AM signal.
Using band pass filter
In the above balanced mixer we have placed an RF choke but we can replace it with a LC tuned band pass filter at the output to get better filtering of the harmonics component in the modulated signal. The following circuit diagram shows LC tuned band pass filter following the double diode balanced mixer/modulator.
The working principle of this circuit is the same as explained previously. The LC resonant tank just performs the filtering action so that out of band frequencies are reject. The above LC tank is resonant at the carrier frequency of 10KHz. The inductor L1 and capacitor C1 components value can be calculated using the LC Parallel Resonant Circuit Online Calculator.
The following shows how signal flows in the balanced mixer during the negative half cycle of the carrier signal Vc or the local oscillator(LO).
And the following shows how signal flows in the balanced mixer during the positive half cycle of the carrier signal Vc or the local oscillator(LO).
Below is an animation of the signal flow in the two diode balanced mixer and the LC tuned filter.
We can analyze the performance of using band pass filter after the two diode mixer by plotting the frequency spectrum at the output of the filter. The spectrum is shown below.
Comparing this spectrum with the one above we can see that using band pass filter the harmonic component magnitude is reduced.
The following shows the waveforms of the modulating signal, carrier signal and the output amplitude modulated signal.
In this way balanced mixer can be designed using two diodes. This mixer or modulator can be tested with matlab PC oscilloscope or labview based PC oscilloscope at home.
