AWR eBooks

RF Electronics Chapter 5: Frequency Mixers Page 110 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. A mixer produces sum and difference frequencies. An ideal multiplier is a perfect mixer since mathematically it produces sum and difference frequencies. The aim in frequency mixer design is thus to make the frequency mixer behave as close to an ideal multiplier as possible. This chapter describes different circuits to achieve this. For a receiver the RF signal is down-converted. A receiver has a Local Oscillator (LO), the frequency of which is determined by the radio frequency (RF) of the TV, WiFi, GPS or Cellular Phone signal that is to be received. Figure 5.2 shows the frequencies for a receiver or down-converter. F LO is the frequency of the LO and F RF is the frequency of the RF signal. The RF signal is mixed with a LO signal to produce sum and difference frequencies. The sum frequency F LO + F RF is outside the operating frequency range of the system and the difference frequency F LO - F RF is the required Intermediate Frequency (IF) signal, which is filtered and amplified using an IF filter and its associated amplifiers. The IF frequency is F IF = F LO - F RF . It should be noted that a signal at the image frequency results in a signal at the same IF frequency since for the image frequency F IF = F IM - F LO , where F IM is the image frequency. The RF filter should be sufficiently narrow so that the image frequency, shown in figure 5.2, is not passed through the RF filter, since the difference frequency of the image frequency and the local oscillator is at exactly the same frequency as the required IF signal. For an up-converter, the LO signal is multiplied by an IF signal and a double sideband suppressed carrier RF signal results. The resulting RF frequencies are F RF = F LO ± F IF . Generally only one of these RF signals are required, so that an RF filter, like the one shown in figure 5.2 is required to remove the unwanted signal. There are two types of frequency mixers: 1) Passive mixers, using diodes, FETs or other nonlinear devices. In passive mixers the power in the LO signal provides the power for producing the sum and difference frequencies. 2) Active mixers, where transistors or FETs supplied with DC power provide the frequency mixing action. Definition of Terms Conversion Loss For a down-converter, the conversion loss [1] is the ratio of the RF input signal to the wanted IF output signal. Most frequency mixers are used in receivers, for which this definition is applicable. For up-conversion, the conversion loss is the ratio of one of the wanted RF output signal spectral components to the IF input signal. For an ideal frequency mixer, half the input power is frequency shifted to the difference frequency and half the power is shifted to the sum frequency. The conversion loss is the ratio of the input signal to either the sum or the difference component. An ideal passive mixer will thus have a conversion loss of 3 dB. Practical balanced or double balanced mixers typically have a conversion loss of less than 6 dB. The conversion loss does depend on the amount of LO signal power applied to the LO port as can be seen in figures 5.17, 5.26 and 5.34. The frequency mixer is normally operated at a LO power close to that giving the lowest conversion loss. Active mixers can have a conversion gain. The conversion loss must be taken into account in noise figure calculations of a receiver. A frequency mixer with a 6 dB conversion loss typically has a 6.5 dB noise figure. For high quality receivers, an amplifier with a gain much greater than the conversion loss is normally used before the mixer, to ensure that the mixer does not dominate the noise performance of the receiver. RF Electronics: Design and Simulation 110 www.cadence.com/go/awr

• Cover