Mixers ECE141 PDF

Mixers Principles of Communications Systems ECE141 Paul Emmanuel Empas, ECE, MSEE RF Mixers Mixer is a nonlinear circuit that combines two signals in such a way as to produce the sum and difference of the two input frequencies at the output....

Mixers Principles of Communications Systems ECE141 Paul Emmanuel Empas, ECE, MSEE RF Mixers Mixer is a nonlinear circuit that combines two signals in such a way as to produce the sum and difference of the two input frequencies at the output.  Linear Summing vs RF Mixing Linear Summing Input Frequencies Output Frequencies RF Mixing Input Frequencies Output Frequencies RF Mixers In general, a nonlinear device produces a signal at its output that can be represented by a power series, for example, 𝑣𝑜 = 𝐴𝑣𝑖 + 𝐵𝑣𝑖2 + 𝐶𝑣𝑖3 + ⋯ With a single input frequency, the output will contain all harmonics as well as the fundamental. When the input contains two different frequencies, we get cross products that represent sum and difference frequencies. If two frequencies are 𝑓1 and 𝑓2 , we get 𝑚𝑓1 ± 𝑛𝑓2 where m and n are integers. The most important outputs are the 𝑓1 + 𝑓2 and 𝑓1 − 𝑓2 frequencies. Mixer Performance Parameters 1. Conversion Loss 2. Isolation 3. 1-dB Compression Point 4. Intermodulation Distortion Conversion Loss In passive mixers, conversion loss is defined as the difference in signal level between the amplitude of one the input signals (usually the non-LO signal) and the amplitude of the desired output signal (either the 𝑓1 + 𝑓2 or the 𝑓1 − 𝑓2 signal). Conversion loss is expressed as a positive number in decibels. Typical values of conversion loss can range from approximately 4.5 to 9 dB, depending on the mixer. Isolation Isolation is a measurement of the amount of power that leaks from one port to another. Isolation is defined as the difference in signal level between the amplitude of an input signal and the amplitude of the leaked power from that input signal to another port. 1-dB Compression Point When a mixer’s conversion loss increases by 1 dB, the 1-dB compression point has been reached. The 1-dB compression point of a mixer is defined as the amplitude of the input signal required to increase the conversion loss by 1 dB. A mixer’s 1-dB compression point determines the upper limit of its dynamic range. Intermodulation Distortion This happens in a multi-carrier signal environment. These two signals interact with each other and with the LO signal, which creates distortion. In a receiver, two-tone third-order IMD is a serious problem because it can generate third-order distortion products that fall within the mixer bandwidth. Balanced Mixers A balanced mixer is one in which the input frequencies do not appear at the output. Ideally, the only frequencies produced are the sum and Input Frequencies difference, 𝑓1 + 𝑓2 and 𝑓1 − 𝑓2 , of the input frequencies. Output Frequencies END