Frequency Mixer

Definition: Frequency Mixer is a non-linear device. It generates a number of frequencies on the application of two different frequencies at the input of the mixer. It is the core process in RF technology and design.

Here, the input frequency gets converted into different frequency components so as to process the signals more effectively.

It is clearly shown below that out of the two provided frequencies one is the input signal while the other is the local oscillator signal.

Let’s have a look at the Frequency Mixer shown schematically in the figure-

As we can see, two inputs are applied to the frequency mixer. One is an incoming RF signal voltage of frequency f_S and the other is a local oscillator voltage having frequency f_O.

Frequency Mixer is said to be a non-linear device as it has non-linear dynamic characteristics. This is the reason why the two input voltages after heterodyning within the mixer produce an output current. This output contains frequency component f_S, f_O, mf_O ± nf_S where m and n are integers.

A tuned circuit outside the mixer is employed that produces difference frequency component. This difference frequency is known as intermediate frequency.

It is given as –

f_i = f_O – f_S

So, we can say that every RF signal voltage is reduced to an intermediate frequency. Its standard value is 455 KHz for AM.

In order to have, intermediate frequency, the frequency of local oscillator must be greater than that of the signal frequency. It should vary in such a way so as to maintain difference frequency at the output.

To have a detailed explanation about intermediate frequency refer to our previous article Superheterodyne receiver.

Types of Frequency Mixer

1. Self-excited mixer – The mixer circuits that uses FET or MOSFET are known as the self-excited mixer.

• BJT Mixer – Let’s have look at the circuit diagram of BJT mixer:-

Here, the input signal is V_i = V_m1 sin ω_L t  and

the local oscillator signal is V_OSC = V_m2 sin ω_H t

Here, V_i is at a lower frequency as compared to the local oscillator signal. The low-frequency input signal is applied to the base and the oscillator signal is given at the unbypassed emitter of the transistor.

Thus, we can say, the output has two frequency component.

A sum frequency (ω_H + ω_L) and difference frequency (ω_H – ω_L). This difference in frequency is known as Intermediate frequency. A frequency selection network employed at the output of mixer selects intermediate frequency.

• FET Mixer- The active device in case of FET mixer is an N-channel JFET. The figure below shows the circuit diagram of JFET mixer –
  
  As we can see the low-frequency input signal Vi is applied at the gate terminal and the oscillator signal is applied to the source. The drain current I_D is related to gate-source voltage V_GS. This gate to source voltage depends on the difference between input and oscillator voltage: I_DSS = maximum drain current
  V_GS = gate to source voltage
  V_P = pinch off voltage

But,

V_GS = V_G – V_S

V_GS = V_i – V_OSC = V₁ sin ω₁t – V₂ sin ω₂t

Advantages of FET Mixer

• FET mixers are less noisy as compared to BJT.
• In case of high-frequency applications, JFET mixers are used. As JFET is a fast device.

2. Separately excited mixer– The mixer circuits including diodes are known as the separately excited mixers.

• Balanced Mixer – Let’s us consider the figure of the balanced mixer using diodes shown below-Here, the signal V_OSC from the oscillator is applied to anodes of both the diodes. This is done via two halves of secondary windings of transformer T₁. The polarity of input signal voltage decides the polarity of the voltage that is induced in the secondary winding of T₁.

The input voltage to the two diodes D₁ and D₂ in case of the positive half cycle of Vi is given as:-

V_D1 = V_OSC + V_i

V_D2 = V_OSC – V_i

So, the input voltage through D₁ will be higher as compared to D₂.

Resultantly the current I_D1 through diode D₁ will be more as compared to D₂.

I_D1 > I_D2

The voltage at transformer T₂ i.e., the output voltage is proportional to the net primary current. This net primary current is given as ( I_D1 – I_D2 )

Hence, V_O∝ I_D1 – I_D2

Similarly, in case of the negative half cycle, the current through D₂ will be more as compared to D₁. This is so because the input to D₂ will now be higher as compared to D₁.Due to this, the net current of T₂ is negative and the induced voltage will also be negative.

• Diode ring Mixer- The figure below shows the circuit diagram of a diode ring mixer-
  As we have already discussed earlier that oscillator signal voltage is much higher than that of the input signal.

So, here also conduction of diode will depend upon the polarity of the oscillator signal.

In case of positive half of oscillator voltage, diode D₁ and D₂ will be forward biased and in case of a negative half cycle, diodes D₃ and D₄ will become forward biased.

So, the conduction of diodes at a given instant, the secondary induced voltage of T₁ will be connected in the similar manner as shown or in an inverted manner across primary of T₂.

Applications of Frequency Mixer

• It is used for frequency translation. One major application is in Superheterodyne receiver.
• It is used in phase-locked loop as it helps to detect the phase difference between two signals.

During mixing of different frequency signals, the incoming RF signals suffer a reduction and get reduced to a standard frequency value known as intermediate frequency.