Mixers are basically electronics circuit that performs multiplication of two signals to produce output signal. Mixers are also referred as frequency translation circuit because mixers output contains signals with sum and difference frequencies of the input signal frequencies as well as other harmonic product signals. When mixers are used in RF transmitter and receivers, they are called modulators and demodulators. Mixers electronics circuits are build with non-linear electronics components like diodes and transistors(BJT or FET see AM modulator using transistor). RF mixer circuit designed using diodes are called passive mixers and RF mixer designed using transistors are called active mixers. RF mixers designed using diodes can be single diode mixer, two diode mixers or diode ring mixers. Diode ring mixers uses four diodes. Diode ring mixers are also called double balanced diode mixers. Here design of double balanced diode ring mixer is illustrated and explained.
The circuit diagram of a double balanced diode ring mixer is shown below.
In the above circuit wiring diagram, the four 1N4148 diodes are placed in a ring topology to form the diode ring mixer. The two input signals are the modulating signal Vm and the carrier signal Vc(also called local oscillator). These signals can be written as follows,
Modulating signal,
\( V_{m} = A_{m} Sin(2 \pi f_{m} t) = A_{m} Sin(w_{m} t) \)
Carrier signal or local oscillator(LO) signal,
\( V_{c} = A_{c} Sin(2 \pi f_{c} t) = A_{c} Sin(w_{c} t)\)
The modulation signal which is usually the message signal or the information signal is applied to the diode ring mixer via one side of the center tapped transformer TR1. The center tap terminal of the other side of the transformer TR1 is the output from the mixer. Similarly, the carrier signal or the local oscillator signal is applied via the center tapped transformer TR2. The center tap terminal of the other side of the transformer TR2 is grounded while the other terminals are connected to the diode ring mixer.
Below is animation that shows how double balanced diode ring mixer works.
In the above animation of double balanced diode ring mixer the signal flow of the modulation, carrier signal and the output signal is demonstrated.
Now we explain how the diode ring modulator works. The carrier signal is used to alternatively turn on and off the diodes.
During the positive half cycle of the carrier signal, the carrier signal enters the upper half of the transformer TR2. This signal flows through and also turns on the diode D2 and D3. The amplitude of the carrier signal has to large enough to turn the diodes. During this stage, the modulating signal is also present in the diode mixer circuit. As illustrated in the double balanced diode mixer circuit picture below, the modulating signal and the carrier signal are mixed up at diode D3 and the output is amplitude modulated signal which enters the load resistor RL.
Similarly, during the negative half cycle of the carrier signal(or local oscillator signal), the carrier signal enters into the diode ring mixer through lower half of the center tapped transformer TR2. This carrier signals turn on and flows through the diode D4 and D1. At the same time the message signal coming from the transformer TR1 gets mixed up with the carrier signal and the two signals are modulated by the diode D1 as illustrated in the double balanced diode mixer circuit picture below. This modulated signal is amplitude modulated signal which then appears in the load resistor RL.
Thus alternating positive and negative half cycle of the carrier frequency the diodes are turned on and off and produces an amplitude modulated(AM) wave at the load resistor RL.
Frequency Spectrum of the output AM modulated signal using modulation signal of frequency 1KHz and carrier signal of frequency 10KHz is shown below.
From the above spectrum, we can see that signal contains two peaks at the sum(11KHz) and difference(9KHz) frequencies for the modulating and carrier signal. We can also observe that there is no carrier or modulating signal spectra at 10KHz and 1KHz. This is why this mixer or modulator is called double balanced because neither the carrier or the modulating signal appears at the output. This is one of the double balanced mixer advantages.
The following shows signal waveform of modulating signal, carrier signal and the AM signal on oscilloscope.
The following video demonstrates how double balanced diode ring mixer works.
The double-balanced mixer design above can be tested in real time using PC oscilloscope and function generator at home. To learn how one can test the designed circuit above see the following tutorials.
- How to use Matlab Simulink as Oscilloscope
- Real Time Simulation of BJT amplifier using Proteus & PC Soundcard
