Advances in Electrical Engineering

Volume 2016, Article ID 8039679, 17 pages

http://dx.doi.org/10.1155/2016/8039679

## Comparison of 6 Diode and 6 Transistor Mixers Based on Analysis and Measurement

^{1}Ericsson Telecom Hungary Ltd., Irinyi József Utca 4-20, Budapest 1117, Hungary^{2}Department of Broadband Infocommunications and Electromagnetic Theory, Budapest University of Technology and Economics, Egry József Utca 18, Budapest 1111, Hungary

Received 7 March 2016; Accepted 6 April 2016

Academic Editor: Mamun B. Ibne Reaz

Copyright © 2016 J. Ladvánszky and K. M. Osbáth. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Our goal is to overview semiconductor mixers designed for good large signal performance. Twelve different mixers were compared utilizing pn diodes, bipolar transistors, and/or junction field effect transistors. The main aspect of comparison is the third-order intercept point (IP3), and both circuit analysis and measurement results have been considered. IP3 has been analyzed by the program AWR (NI AWR Design Environment) and measured by two-tone test (Keysight Technologies). We provide three ways of improvement of large signal performance: application of a diplexer at the RF port, reduction of DC currents, and exploiting a region of RF input power with infinite IP3. In addition to that, our contributions are several modifications of existing mixers and a new mixer circuit (as illustrated in the figures). It is widely believed that the slope of the third-order intermodulation product versus input power is always greater than that of the first-order product. However, measurement and analysis revealed (as illustrated in the figures) that the two lines may be parallel over a broad range of input power, thus resulting in infinite IP3. Mixer knowledge may be useful for a wide range of readers because almost every radio contains at least one mixer.

#### 1. Introduction

During the last decade, mixer research reached a quiescent point. It is time to look back and make an overview. This is the main goal of our paper. A mixer is a three-port device having two inputs (RF and LO, radio frequency and local oscillator, resp.) and one output (IF, intermediate frequency). It is used, for example, to convert the RF signal (that may be of variable frequency) to a fixed IF, because filtration is easier at a fixed frequency. In this paper we use fixed RF. At the RF and LO ports, periodic excitations are applied, with fundamental angular frequencies and , respectively, and we are interested in the IF signals at . These are the useful IF signals or first-order products. Higher order intermodulation products are also present at the IF port, and among them the third-order products are the most disturbing ones because if and are near to each other, then the third-order intermodulation products at and are also near to the useful signals.

The mixing effect (time domain multiplication of the RF signal by the LO signal) is a result of nonlinearity and/or time variance. It can be modeled by time-invariant nonlinear and/or time-varying linear circuit elements, depending on whether the LO signal is a sinusoid or a square. Both can be treated by Fourier analysis [1]. The big difference between them is that a time-invariant nonlinear circuit element always produces intermodulation, while a time-varying linear circuit element never does it. For this reason, to reach low intermodulation, it is advantageous to apply square LO signal or sinusoid of high amplitude. The reason is that, for a square LO signal, semiconductor will contribute (almost) as time-varying element and does not produce intermodulation, and a similar thing occurs in case of large sinusoidal LO. The simplest time-varying linear circuit element is an ideal switch. The practical situation usually is that both nonlinear and time-varying effects are present and are difficult to distinguish.

Third-order intercept point IP3 is a power level where the straight lines fit to the first- and third-order IF output power versus RF input power curves intersect each other. With the condition that the slope of the third-order IF product versus RF input power is three times as much as that of the first-order product, the higher the IP3 is, the smaller the disturbance caused by the third-order intermodulation is. IIP3 and OIP3 are the input and output third-order intercept points, respectively. Their difference is the conversion loss.

Another important mixer characteristic is the 1 dB compression point . It is the RF input power when the first-order IF versus RF curve saturates by 1 dB.

In this paper, we compare twelve different mixer configurations: a two-diode mixer [2], a diode ring mixer [3], a triple balanced mixer with diodes [4], a rectifier-like mixer [5], a half H mixer with diodes (our contribution), a full H mixer with diodes [6], a four-quadrant multiplier [7] modified by us, a half H mixer [8], Russian version [6] of the half H mixer modified by us, the original full H mixer [6], a full H mixer simplified by us, and an FET ring mixer [9]. We excluded from the comparison the transmission line transformer version of the triple balanced mixer [10] because, in the analysis, it did not behave like triple balanced. Main aspect of comparison is IP3, but we include conversion loss and LO to RF and LO to IF isolations as well.

In Section 2, the qualitative theory of operation using switches is obtained. In Section 3, large signal analysis results are compared. For large signal analysis, we use the harmonic balance method [12]. Measurements are included in Section 4. We apply here the two-tone test [13]. In Section 5, three ways for improvement of large signal performance have been obtained; however, this knowledge has not been published yet: application of a diplexer at RF port, reduction of DC currents, and exploiting an RF range with infinite IP3 [14].

Recently, many fine papers about mixers have appeared (e.g., [15, 16]). The difference between those papers and this one is that our intention is to provide an overview based on agreement of analysis and measurement results. Therefore, all mixers are analyzed, built up, and measured under the same conditions; see please Sections 3 and 4.

This is a preparatory work to design microwave mixers. Our concept is to check large signal performance first at a few MHz, as it has been done here, and then to turn to microwave mixer design, in a later publication. With this step, investigations of large signal performance and frequency dependence have been somewhat separated.

For understanding the schematics, basic knowledge of the analysis program AWR [17] is necessary.

This paper introduces a unified terminology in mixer names, has tutorial value by overviewing such mixers that are rarely used or not so widely known, and includes novelties such as our modifications to well-known mixer topologies and a new mixer circuit, which contains a proof by analysis and measurement in agreement that IP3 can be infinite, and a method how to generate an RF input power region with infinite IP3 [14].

#### 2. Qualitative Theory of Operation

In this section we idealize circuit operation by replacing semiconductors by switches whenever possible. From this point of view, operation of all mixers is based on the fact that RF signal appears at the IF port with polarity changed in the pace of the LO signal. The first mixer is an exception as we observe. Note that, for the qualitative theory of operation, RF signal is always assumed to be negligible as compared to the LO signal. Numbering of ports is the same for all mixers: Ports 1, 2, and 3 are the RF, LO, and IF ports, respectively.

Schematics of the first mixer are shown in Figure 1.