Here the working principle of Two Diode Single Balanced Mixer is explained. Mixers are used for operations such as frequency translation, modulation, demodulation and detection and others. There are different technique of designing a mixer such as single diode mixer, single balanced mixer, double balanced mixers which can be designed using diodes, BJT transistors and FET transistors. Here single balanced mixer design with two diode is illustrated.
Below shows the animation of modulating signal flow, carrier signal flow and modulated signal flow in the two diode single balanced mixer.
Mixers in electronics are semiconductor devices with non-linear behavior between the input and output signal. This non-linear behavior is used to produce output(or outputs) which is the product of two input signals. For example, if we have two sinusoidal inputs, the output is a sinusoidal signal at the sum and difference frequencies. Following shows mixing process of RF signal and local oscillator signal producing intermediate frequency(IF) signal.
When mixers are used in wireless transmitter and receiver are called modulator and demodulators. When mixer is used as modulator in the transmitter, a low frequency information signal is converted to higher frequency signal using carrier signal and when used in wireless receiver the high frequency signal is converted back to low frequency information signal.
A basic superheterodyne radio transmitter block diagram with mixers is shown below.
The following shows basic superheterodyne radio receiver block diagram showing mixers.
There are number of electronics components which can be used to construct mixers or modulators such as transistors(BJTs and FETs). Diodes are non-linear devices and therefore can be used for building mixers or modulator/demodulators. Different type of modulators can be designed using diodes, they are single diode mixers, single balanced mixers and double balanced mixers. Single diode mixers was explained the previous tutorial How does Single Diode Modulator Circuit work?. Here two diode single balanced mixer is explained.
The circuit diagram of two diode single balanced mixer is shown below.
In the above circuit diagram, the information signal or the modulating signal(Vm) enters the two diode mixer via the center tapped transformer TR1. The carrier signal Vc enters the mixer circuit via the center terminal wire of the transformer TR1. The two switching diodes D1 and D2 are connected to each end of the transformer TR1 and TR2 as shown. The middle terminal of the center tapped transformer TR2 is connected to the ground. The output signal is taken from the other side of the transformer TR2. R2 resistor is the load resistor.
The two diodes are alternatively turned ON and OFF using the alternating sinusoidal carrier signal. During the positive half cycle of the carrier signal, the diode D1 is turned on while the diode D2 is turned off. During this time modulating signal enters into the mixer circuit via transformer TR1 this modulating signal and the carrier signal are non-linearly mixed by the diode D1. The mixed product signal are exited out of the mixer circuit via the transformer TR2 into the load resistor R2. This is illustrated below.
Similarly during the negative half cycle of the carrier signal, the diode D2 conducts and the diode D1 is switched off. The modulating signal maybe in either the positive half cycle or negative half cycle because the carrier frequency is much higher than the modulating signal frequency. Whatever the modulating signal stage, it is entered into the mixer circuit and the mixed with the carrier signal by the diode D2. The output from the diode D2 which is the mixed product exits the mixer circuit via the transformer TR2 and into the load resistor R2. This is illustrated below.
Consider the modulating signal is a sine wave with frequency of 1KHz and the carrier signal is sine wave with frequency of 10KHz. Mathematically we can write modulating signal and the carrier signal as follows.
Modulating signal,
\( V_{m} = A_{m} Sin(2 \pi f_{m} t) = A_{m} Sin(w_{m} t) \)
Carrier signal,
\( V_{c} = A_{c} Sin(2 \pi f_{c} t) = A_{c} Sin(w_{c} t)\)
The modulation happens because a diode is non-linear device which obeys square law function. That is the output current from the diode is non-linearly related to voltage across. The following is diode equation for positive and negative cycle of the carrier signal.
\( I_{+}= + I_{s} (exp^{\frac{V_{c}+V_{m}}{V_{T}}} -1)\)
\( I_{-} = - I_{s} (exp^{\frac{V_{c}-V_{m}}{V_{T}}} -1)\)
Since, \( exp^{\frac{V_{c}+V_{m}}{V_{T}}} >> 1\) and \( exp^{\frac{V_{c}-V_{m}}{V_{T}}} >> 1\), the currents are,
\( I_{+}= + I_{s} exp^{\frac{V_{c}+V_{m}}{V_{T}}} \)
\( I_{-} = - I_{s} exp^{\frac{V_{c}-V_{m}}{V_{T}}} \)
For this circuit the frequency spectrum is shown below.
The above spectrum of the output signal shows that the carrier signal is suppressed, the sum and difference signal of the modulating and carrier wave are present and the modulating signal is also present. Besides these there are also harmonic intermodulation components whose magnitude is relatively small. Because the above mixer suppresses the carrier signal it is called single balanced mixer.
Band Pass Filter with two diode single balanced mixer
To remove harmonic intermodulation component and suppress the modulating signal we can insert a band pass filter at the output of the two diode single balanced mixer. Below circuit diagram shows a LC resonant circuit tuned to carrier frequency of 10KHz.
The value for the inductor and capacitor can be calculated using the online LC parallel resonant calculator. The signal spectrum after the band pass filter is shown below.
From the above frequency spectrum we can see that the modulating signal at 1KHz is largely suppressed compared to the spectrum of the output signal without the band pass filter.
As can be seen the output from the two diode mixer is an AM signal. Thus the two diode single balanced mixer produces amplitude modulated signal.
Following video illustrates how two diode single balanced mixer works with animation of signal flow.
See other modulation circuit tutorials:
- Simple Amplitude Modulation (AM) circuit using Single Diode Modulator
wrong explanation. the carrier signal is fed via the centre tap and both diodes are simultaneously biased in the same polarity.
it does not matter whether you fed signal x or y into any input channel since they will be mixed(multiplied) and output is just output= x*y = y*x, you can replace x and y, look at the signal flow in the diagram, the polarity is correct, check the signal waveform and the spectrum.