PSG COLLEGE OF TECHNOLOGY

EXP NO.8 FREQUENCY RESPONSE OF A MIXER CIRCUIT

AIM:

To analyze the frequency response of a Mixer Circuit

APPARUTUS REQUIRED:COMPONENTS RANGE QUANTITYBread board 1Transistor BF 194 1IF Transformer 1Resistors 120KΏ

1.5KΏ51

Capacitors 0. 1µF 2

MSO 1Powersupply 1Function generator 2Connecting wires,Probes As required

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WORKING:

In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 - f2 of the original frequencies, called heterodynes. Other frequency components may also be produced in a practical frequency mixer.

Mixers are widely used to shift signals from one frequency range to another, a process known as heterodyning, for convenience in transmission or further signal processing. For example, a key component of a super heterodyne receiver is a mixer used to move received signals to a common intermediate frequency. Frequency mixers are also used to modulate a carrier frequency in radio transmitters. Mixers are used for frequency conversion and are critical components in modern radio frequency (RF) systems. A mixer converts RF power at one frequency into power at another frequency to make signal processing easier and also inexpensive. A fundamental reason for frequency conversion is to allow amplification of the received signal at a frequency other than the RF, or the audio, frequency. A receiver may require as much as 140 decibels (dB) of gain. It might not be possible to put more than 40 dB of gain into the RF section without risking instability and potential oscillations. Likewise the gain of the audio section might be limited to 60 dB because of parasitic feedback paths, and micro phonics. The additional gain needed for a sensitive receiver is normally achieved in an intermediate frequency (IF) section of the receiver.

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Figure 1: Symbol of a Mixer Circuit

A device that has a non-linear characteristic can act as a mixer. Passive mixers use one or more diodes and rely on the non-linear relation between voltage and current to provide the multiplying element. In a passive mixer, the desired output signal is always of lower power than the input signals.Active mixers use an amplifying device (such as a transistor or vacuum tube) to increase the strength of the product signal. Active mixers improve isolation between the ports, but may have higher noise and more power consumption. An active mixer can be less tolerant of overload. Mixers may be built of discrete components, may be part of integrated circuits, or can be delivered as hybrid modules. Mixers may also be classified by their topology. Unbalanced mixers allow some of both input signals to pass through to the output. A single balanced mixer is arranged so that either the local oscillator (LO) or signal input (RF) is suppressed at the output, but not both. A double balanced mixer has symmetrical paths for both inputs, so that neither of the input signals and only the product signal appears at the output. Double balanced mixers

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are more complex and require higher drive levels than unbalanced and single balanced designs. Selection of a mixer type is a tradeoff for a particular application.Another form of mixer operates by switching, with the smaller input signal being passed inverted or uninverted according to the phase of the local oscillator (LO). This would be typical of the normal operating mode of a packaged double balanced mixer, with the local oscillator drive considerably higher than the signal amplitude.The aim of a switching mixer is to achieve linear operation over the signal level, and hard switching driven by the local oscillator. Mathematically the switching mixer is not much different from a multiplying mixer, just because instead of the LO sine wave term we would use the signum function. In the frequency domain the switching mixer operation leads to the usual sum and difference frequencies, but also to further terms e.g. +-3*fLO, +-5*fLO, etc. The advantage of a switching mixer is that it can achieve - with the same effort - a lower noise figure (NF) and larger conversion gain. This come because the switching diodes or transistors act either like a low resistor (switch closed) or large resistor (switch open) and in both cases only minimum noise is added. From the circuit perspective many multiplying mixers can be used as switching mixers, just by increasing the LO amplitude.

Assuming the desired station is transmitting at 1490 KHz, with a high side local oscillator (LO) of 1945 KHz, and an IF of 455 KHz, then a station transmitting at 2400 KHz would also convert to an IF of 455 KHz, interfering with the desired station, if the 2400 KHz signal were not filtered out prior to the mixer input. In a typical AM radio, the front end is tuned simultaneously with the LO at a constant difference of 455 KHz over the entire AM radio band of 550 - 1600 KHz.

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Figure 2: Front End of a AM Superheterodyne receiver.

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CIRCUIT DIAGRAM:

Mixer Circuit:

Model Graph:

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Tabulation:

No AGC:Vi(mV) Vo(V)20 330 4.450 7.270 9.4490 11.65100 13.9200 22.8300 25.4400 26.8500 27.1600 27.6700 28.1800 28.5900 28.61000 28.8

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PRELAB Questions:

1) What is automatic Gain Control?

It is a closed loop regulating circuit, the purpose of which is to provide a controlled signal amplitude at its output despite variation of the amplitude in the input signal. The average or peak value is used to dynamically adjust the input to output gain to a suitable value.

2) Explain the working of an AGC?

The signal to be gain controlled goes to a diode and a capacitor, which produce a peak following DC voltage. This is fed to the gain blocks to alter their bias thus altering their gain.

3) What does a delayed AGC do?

An automatic gain control circuit which is activated only when the input signal exceeds a predetermined magnitude. This allows maximum amplification of weaker signals . This contrasts with instantaneous AGC, in which the circuit is activated immediately when the amplitude of the input signal changes .

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Result:Thus the frequency response of the Mixer is analyzed successfully.

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