CAD of Microstrip Balanced Mixer

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The mixer is one of the basic building blocks of any communication receiver and transmitters. Mixers translate signals from one frequency band to another. The output of the mixer consists of multiple images of the mixers input signal where each image is shifted up or down by multiples of the local oscillator (LO) frequency. The most important mixer output signals are usually the signals translated up and down by one LO frequency.The purpose ofthe mixer is to down-convert the RF frequency signal to the baseband for digitization and thus further processing. In reality mixer output is distorted due to non-linearity in the mixer. In addition, the mixer components and a non-ideal LO signal introduce more noise to the output. Bad design might also cause leakage effects, complicating the design of the complete system.

Noise performance and rejection of out-of-band interferers are both critical to the receiver system because they both limit the receiver system’s sensitivity. Linearity is important to transmitter performance, where the user wants an error-free output signal. Gain, NF, and non-linearity are the most important parameters in mixer design.

In the present work, a balanced mixer is designed and developed at 1 GHz using microstrip line. Here, 1.0 GHz signal is converted to 100 MHz IF signal using 900 MHz local oscillator The balanced mixer circuit consists of 3 dB branch line coupler and a low pass filter for IF section. Here a circular form of branch line couple has been designed. A maximally flat Butterworth filter is used in this work. For proper function of the balanced mixer additional circuitry on the printed circuit board at the input port of LO has been used. The required design dimension using microstrip line has been calculated using synthesis subroutine.The CAD has been developed to design the 3 dB branch line coupler having input parameters as i) Frequency of operation, ii) dielectric constant, iii) height of substrate , iv) Thickness of Cu strip, v)characteristic impedance. The output parameter are i) width of stripline, ii) effective dielectric constant iii) Guide wavelength. The layout using equivalent length of the microstrip line are generated. The hardware is implemented on a printed circuit board made of a GR10 substrate having dielectric constant 4.8 & thickness 1.6 mm.

The completed PCB is mounted on a rectangular brass plate and whole assembly is measured using a spectrum analyzer.The frequency response of the balanced mixer has been measured by spectrum analyser and two signal generators. Experimental results shows an output power of –39 dBm for an input power of –30 dBm. Since a 3 dB branch line coupler is used the output power should be 3 dB lower than the input power. The 6 dB additional loss is due to manufacturing tolerances and step discontinuities. The linearity of the mixer is also verified from the measurement results where the output power level changed equally with input power level upto 0 dBm.The designed balanced mixer can be implemented in practical applications.


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