Contents

 

 

Mixers

AM Mixers (Modulators)

            Bipolar Switching

            Unipolar Switching

            Square Law Modulator

 

Mixers in Radio Receivers

            Mixer Comparison

Mixer Outputs

 

 

Assignment Questions

 

 

 

Mixers

 

The purpose of this section is to:

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http://www3.ncsu.edu/ECE480/index-map.htm

http://www.minicircuits.com/

 

The following notes from Mini-Circuits offer some insight on mixer operation:

Modern Mixer Terms Defined

Understanding Mixers

How to Select a Mixer

Improve Two-Tone, Third Order Testing

Mixer FAQs

 

Mixing

Mixing is simply time domain multiplication. This has the peculiar property of creating addition and subtraction in the frequency domain.

This means that two new frequencies have been generated, one shifted up and the other shifted down in frequency. Often times a filter is used to select one of these two.

 

AM Mixers (Modulators)

A basic equation describing amplitude modulation is:

From this we notice that AM involves a process of multiplication. There are several ways to perform this function electronically. The simplest method uses a switch.

Switching Modulators

Switching modulators can all be placed into two categories: unipolar and bipolar.

SystemView Bipolar Switching Modulator Model

SystemView Unipolar Switching Modulator

Square Law Modulator

The voltage-current relationship of a diode is nonlinear near the knee and is of the form:     . The coefficients a and b are constants associated with the diode itself.

SystemView Square Law Modulator

Amplitude modulation occurs if the diode is kept in the square law region when signals combine.

Mixers in Radio Receivers

Every super heterodyne radio receiver has a mixer. It’s purpose is to shift the radio station of interest down to the IF frequency.

1 MHz AM Carrier into the mixer

The mixer oscillator input is 455 KHz above the incoming carrier frequency.

An ideal mixer will combine the incoming carrier with the local oscillator to create sum and difference frequencies.

SystemView Mixer Models

Ideal Mixer Output

A real mixer combines two signals and creates a host of new frequencies:

•   A dc level

•   The original two frequencies

•   The sum and difference of the two input frequencies

•   Harmonics of the two input frequencies

•   Sums and differences of all of the harmonics

Non-Ideal Mixer Out

The principle mixer output signals of interest are the sum and difference frequencies, either of which could be used as an IF. However, the IF is generally chosen to be lower than the lowest frequency being received. Consequently, the IF in an AM radio has been standardized to 455 KHz.

Mixer Comparison

 

Mixer Outputs

Assignment Questions

 

Quiz

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Modeling Questions

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