Improved Intermodulation Rejection in Mixers

Intermodulation is one of the most pernicious forms of spurious response in superheterodyne receivers. This type of interference cannot be completely eliminated by narrowband filters. Improvements in receiver performance can be made only by improving and making more effective use of the mixing element. Intermodulation results from terms of higher order than two in the v-i power series expansion about the dc operating point of the mixing element. The magnitude of these higher order terms must be reduced in order to improve intermodulation rejection. In the work described in this paper, it has been observed that such a reduction in higher order terms can be obtained by proper design of the mixing element and by a proper choice of dc operating point. More than 80 dB intermodulation rejection was obtained with a single ended hot carrier diode mixer. Best performance is obtained by operating two hot carrier diodes at different operating points in a balanced mixer arrangement. Intermodulation ratios greater than 100 dB have been measured for this operating mode. Optimization of performance for IM rejection has little effect on sensitivity or rejection of other spurious responses. A new mixing element, the space-charge-limited resistor (SCLR), was designed to minimize the higher order terms in the current function. A balanced mixer provided the best performance with these mixing elements also. Intermodulation from +5 dBm inputs can be rejected below the mixer noise level. Early models of this device are not as sensitive as hot carrier mixers, but improvement appears to be possible.     

Intermodulation analysis of crystal mixer

An expression for the amplitude of the intermodulation products and harmonics produced in a crystal mixer is derived using the coefficients of the power series expansion of the device. Using this expression and an exponential approximation of the current through a diode [i = i0(ε^αv- 1)], the amplitude of intermodulation produced in a crystal mixer is found to be 2i0ε^αV0R0Is(αV1)Ib(αV2). In(x) is an nth order modified Bessel function of the first kind. The quantities s and b are the signal harmonic and the oscillator harmonic, R0is the output resistance, and V0, V1, and V2are the bias, signal, and oscillator voltages, respectively. The quantities i0, α, R0, and V2are found from the dc E-I diode characteristics, the mixer bias current, and the loss in the desired signal. Experimental tests on a mixer operating from 450 Mc to 850 Mc show that the signal input power necessary to produce a given intermodulation output power can be predicted within 6 db.     

Response Coefficients of a Double-Balanced Diode Mixer

This paper presents the results of a computer-assisted analysis of a commonly used double-balanced diode mixer circuit. It is shown that the magnitudes of some of the responses to one or more input radio frequency signals are sharply dependent on the degree to which the diodes comprising the mixer are balanced. The average rejections imparted to a particular undesired intermodulation (or spurious) response by a random sample ofsuch mixers are plotted as a function ofthe tightness ofthe control of diode characteristics of the population from wl lch the four diodes comprising a single mixer are selected. A computer program based on a Fourier-series expansion of the time-dependent coefficients of a Taylor-series representation of the four diode currents was employed to compute the response coefficients of the mixer [1H3] The response coefficients are defined as the magnitudes of the intermodulations of sinusoidal-input radio-frequency signals and the local-oscillator-frequency signal. The method treats the switching action characteristic of the large local-oscillator power levels, rather than relying on the more usual and invalid [1] assumption of " mild" nonlinearity. Orders of nonlinearity up to the tenth are treated.     

Zero-Bias Mixer Based on AlGaN/GaN Lateral Field-Effect Diodes for High-Temperature Wireless Sensor and RFID Applications

In this paper, a zero-bias mixer using a lateral field-effect diode fabricated on standard GaN-on-Si AlGaN/GaN high-electron-mobility-transistor wafers is demonstrated. The diode features strong nonlinearity near zero bias, enabled by a threshold-voltage modulation using a fluorine-plasma-treatment technique. The maximum change in conductance was adjusted to $sim$0 V, leading to optimal conversion loss (CL) of the mixer at zero bias and eliminating the need for any dc supplies. The mixer is characterized from room temperature (RT) to 250 $^{circ}hbox{C}$ . At 2.5 GHz and at RT, the CL and third-order intermodulation intercept point are 12.9 dB and 17.64 dBm, respectively. The operation of the proposed diode is modeled by a physical equivalent circuit, with the element values extracted from the measured $S$-parameters. The voltage-biasing dependence of the CL can be explained by the model. The high-temperature operation of the mixer shows that the proposed mixer can perform well in high-temperature and ultralow-power applications.      

Spurious Frequency Rejection

A major problem of superheterodyne receivers is receiver response to undesired frequencies. The magnitude of the spurious response, or the spurious rejection, is computed by assuming that the mixer characteristic is the theoretical exponential form. It is shown that the spurious rejection increases with increased local oscillator power, decreases with increased desired signal power and increases with larger, harmonically derived, spurious responses.     

The Stabilization of Mixer Diode Performance Against L.O. Power Changes with Optimum DC Bias

The advantages of using a particular dc forward bias in maintaining receiver sensitivity for fairly large reductions in local oscillator power are experimentally verified. Definite improvement of performance is obtained over broad band operation where L.O. power level may vary considerably. For each of the mixer diodes investigated there is a particular optimum bias for each diode type where both the input signal level and local oscillator RF impedances as well as the IF impedance are practically stationary with respect to very large changes in local oscillator power level. At optimum bias, signal frequency RF and IF impedances were found to vary by only about ten per cent for L.O. power reduced from 1 mw to 1 /spl mu/w. This results in considerably less reduction in tangential signal sensitivity over that obtained in the unbiased or overbiased condition.     

基于GaAs肖特基二极管的330GHz接收前端技术研究

基于GaAs肖特基二极管,设计实现了310~330 GHz的接收机前端.接收机采用330 GHz分谐波混频器作为第一级电路,为降低混频器变频损耗,提高接收机灵敏度,分析讨论了反向并联混频二极管空气桥寄生电感和互感,采用去嵌入阻抗计算方法,提取了二极管的射频、本振和中频端口阻抗,实现了混频器的优化设计,提高了变频损耗仿真精度.接收机的165 GHz本振源由×6×2倍频链实现,其中六倍频采用商用有源器件,二倍频则采用GaAs肖特基二极管实现,其被反向串联安装于悬置线上,实现了偶次平衡式倍频,所设计的倍频链在165 GHz处输出约10 dBm的功率,用以驱动330 GHz接收前端混频器.接收机第二级电路采用中频低噪声放大器,以降低系统总的噪声系数.在310~330 GHz范围内,测得接收机噪声系数小于10.5 dB,在325 GHz处测得最小噪声系数为8.5 dB,系统增益为(31±1)dB.     

A Simplified Method for Deriving Additional Spurious Response Information from Receiver Measurements

The majority of the work on the measurement of spurious responses in receivers done up to now is based upon the measurement of the sensitivity of all spurious responses for each of a few (usually three) tuned frequencies within the tuning range of any one receiver or major radio frequency head. These responses have been identified as the products of the nonlinear mixing of the incoming and local oscillator signals, most often in the mixer. Very little has been said about how the sensitivity of these responses varies with tuned frequency except to assume that the worst response of a kind found applies for all such responses. This paper examines the mechanisms involved in their generation and defines the factors upon which their sensitivity depends. These are basically the coefficients of the nonlinear equation representing the characteristics of the mixer and the radio frequency selectivity preceding the mixer. By use of a selective testing method, these two factors can be separated. Once this has been done, the total spurious response characteristic of the receiver is implied and the whole tuned frequency-spurious response frequency matrix can be filled in with reasonable accuracy. This selective testing method is certainly more productive of information per unit test than the method which is the present standard for such tests. However, the proposed method has not been adequately tested as yet, and it is hoped that this paper will result in such tests.     

Large-Signal Equivalent Circuits of Avalanche Transit-Time Devices

A large-signal analysis for IMPATT diodes is derived, which allows carrier multiplication by impact ionization to occur at every point in the diode. Therefore, the operating characteristics of IMPATT diodes with a wide range of realistic doping profiles can be investigated. For a given operating frequency, RF voltage, dc bias current, and doping profile, the admittance, power output, efficiency, bias voltage of a diode can be obtained. An equivalent circuit the diode package, microwave circuit mount and diode, is obtained experimentally. Using this circuit, the admittance of the diode is measured by a reflection-type circuit and an oscillator circuit as a function of the RF voltage, dc bias current, and frequency.     

Linearity Improvement of a Power Amplifier Using a Series LC Resonant Circuit

A radio frequency power amplifier microwave monolithic integrated circuit with a series LC resonant circuit as well as a bias control circuit for wide-band code division multiple access application is presented. The linearizer that consists of a series LC resonant circuit and base-emitter junction of a bias transistor operates as a diode rectifier circuit. A comparison between the circuits with and without the linearizer has been demonstrated. The power amplifier (PA) with the series LC resonant linearizer exhibits adjacent channel leakage ratio-1 (ACLR1) of -37.2 dBc at output power of 27 dBm, a 5.6 dB improvement compared to the circuit without the linearizer. The bias control circuit reduces consumed average dc current from 83 mA to 57 mA for efficiency improvement. The linearized PA exhibits 1-dB compression point (P₁dB) of 29.3 dBm, power-added efficiency of 45.7%, and power gain of 20.6 dB at low quiescent current of 37 mA with a 3.4 V single supply.     

A GaAs FET-IC mixer achieving high receiver sensitivity for coherent optical heterodyne detection at 2.5 Gb/s

A FET-IC mixer that uses a Gilbert-cell multiplier with a single-to-balance conversion input buffer is proposed. The key point in the FET-IC mixer design is that a single-to-balance conversion input buffer is used to improve the amplitude and phase balance of signals output from the input buffer stage. Excellent squaring characteristics were achieved in the range from 0 to -25 dBm. The FET-IC mixer was successfully tested in a 2.5 Gb/s CPFSK demodulator for optical heterodyne reception. Compared to the best diode passive double-balanced diode mixers, receiver sensitivity was improved by 1 dB at +10 dBm electrical input power. Very small degradation in receiver sensitivity, less than 3 dB, was achieved at electrical input power levels ranging from +10 to -10 dBm.<>     

Nonlinear analysis of the avalanche transit-time oscillator

The nonlinear operating characteristics of the avalanche transit-time oscillator are studied by means of Fourier-series representation. For optimum operation, the oscillator must be designed such that start-oscillation conditions are satisfied simultaneously at the first and the second harmonic of the desired oscillation frequency. Under those conditions the oscillation frequency does not depend on the dc bias current; the signal level increases smoothly with bias current. For large signals, the diode exhibits negative resistance for frequencies substantially below the avalanche frequency; the oscillation frequency therefore may be below the avalanche frequency corresponding to the dc bias current required for large-signal operation. A condition for attaining large-signal operation is that the product of drift-zone capacitance and total load resistance must be small compared to the oscillation period; this condition also yields small starting currents. The output power at the oscillation frequency is obtained explicitly in terms of diode and external circuit parameters. The maximum attainable output power is limited by parasitic series resistance and by permissible RF voltage swing as compared to dc bias voltage. The best power-impedance product is obtained by choosing the transit angle equal to 0.74 π. In practice, it may be advantageous to choose a smaller value for the transit angle, in order that the tuning condition for the second harmonic may be more easily satisfied. The dc-to-RF conversion efficiency in principle is linearly proportional to the dc current density; the maximum efficiency again is limited by parasitic series resistance and by permissible RF voltage swing.     

5.2--Infrared 10.6-micron heterodyne detection with gigahertz IF capability

Analyses and experiments have been carried out on heterodyne detection at 10.6 microns with the objective of obtaining IF bandwidth capability into the microwave region. UHF and microwave measurements on the quantum-noise-limited generation-recombination (G-R) noise spectrum of compensated copper-doped germanium photoconductive mixer elements, measured under operational conditions at 10.6 microns, have shown response to beyond 2 GHz. The experiments were carried out directly at 10.6 microns using a mixer geometry and circuit arrangement intended to yield large mixer conversion gain and IF bandwidth. Engineering design equations are given for noise equivalent power (NEP) and mixer conversion gain (G) in terms of such parameters as IF noise factor, carrier transit time, carrier lifetime, mixer resistance, local oscillator power, dc bias power, etc. An expression for quantum-noise factor (QF) is defined. Graphs are also presented showing the effect on NEP,G, and QF of various parameters, and the tradeoffs possible to achieve high-frequency IF capability. An alternative approach is presented in which mixer conversion gain is calculated directly from the mixerI-Vcharacteristic in a manner analogous to microwave mixers.     

A Novel Circuit Architecture for High Performance of High-order Subharmonic Mixers

A novel circuit architecture for high performance of high-order subharmonic (SH) mixers is proposed in this paper. According to the specified harmonic mixing order, one or more mixer diodes sub-arrays and corresponding power divider as well as phase shift network for RF and LO signals are arranged in the circuit. This proposed SH mixer circuit has improved conversion loss, wide dynamic range and high port isolation for high-order SH mixers. By phase cancellation of idle frequencies, the proposed SH mixer circuit can eliminate complicated design procedure of idle frequency circuits; by phase cancellation of leakage LO power to RF and IF port, and leakage RF power to LO port, the mixer circuit can get high port isolation in LO-IF/RF and RF-LO. The increased antiparallel diode pairs in each sub-array will also lead to well performance by lowering effective series resistance. The proposed SH mixer circuit can be easily realized with power divider and phase shift network for RF and LO signals.     

0.67 THz宽带谐波混频器设计

为研制太赫兹多频段高灵敏度探测仪,依靠太赫兹砷化镓平面肖特基二极管的非线性特性,结合石英薄膜工艺,设计了宽带0.67 THz谐波混频器,并分析了砷化镓平面肖特基二极管性能表征参数指标对太赫兹混频器性能的影响。0.67 THz谐波混频器采用整体综合的设计方法,结合电气仿真软件ADS和电磁仿真软件HFSS,优化电路中不连续性微带与波导之间的电磁空间耦合效率,以混频器的变频损耗为优化目标,最终实现0.67 THz谐波混频器仿真设计。0.62~ 0.72 THz射频范围内,混频器单边带最低变频损耗小于8 dB,本振功率小于4 mW,本振端口与中频端口、射频端口与中频端口之间隔离度大于-30 dB。     

Fabrication and analysis of GaAs Schottky barrier diodes fabricated on thin membranes for terahertz applications

The GaAs Schottky diode is predominantly used as the critical mixer element in heterodyne receivers in the frequency range from 300 GHz to several THz[1]. At operating frequencies above one THz the skin effect adds significant parasitic resistance to the diode which degrades the receiver sensitivity. A novel diode structure called the Schottky barrier membrane diode is proposed to decrease the skin effect resistance by reducing the current path between the Schottky and ohmic contacts. This is accomplished by fabricating the diode on a very thin membrane of GaAs (about 1 μm thickness). A theoretical analysis has shown that this will reduce the substrate resistance by 60% at 3 THz. This reduction in resistance corresponds to a better frequency response which will improve the device's performance as a mixer element.     

Down-converters using Schottky-barrier diodes

The analysis of noise in down-converters, in which signal and image ports are equally terminated, is extended to include the effect of a nonlinear junction capacitance. The theory is applied to Schottky-barrier diodes, which are represented by a constant resistance in series with a parallel combination of a nonlinear conductance and nonlinear capacitance pumped in phase. Using numerical optimization procedures, the minimum overall receiver noise figure, the associated conversion loss, and the required optimum external terminations are calculated and compared with experimental results. It is shown that in a resistive mixer with equal termination of signal and image ports, the addition of a nonlinear capacitance can lower the conversion loss substantially; but the overall noise figure remains nearly invariant. A conversion loss as low as 3.5 dB was measured (3.3 dB calculated) for a silicon Schottky-barrier diode operated at 3 GHz, driven by 0.1 mW LO power, and without dc bias. The overall noise figure, including an IF amplifier noise figure of 1.5 dB, however, stays at 5.7 dB (5.5 dB calculated).     

A Novel Self-Pinching Gate Biasing Scheme for Safe Operation and Characterization of GaN HEMTs

This letter presents a novel gate bias configuration for GaN HEMTs that ensures a safe operation of this kind of device by protecting the gate from forward turn-on. The bias circuit includes a simple series diode in the dc path that blocks any positive current from the gate, in other words it restricts the gate diode of the device to operate in forward bias. The new bias circuit ensures a safe operating condition of FET/HEMT transistors during forward turn-on while not hampering or degrading performance under normal operating condition.      

一种低闪烁噪声高线性度WLAN混频器的设计

提出一种适用于零中频接收的WLAN混频器,采用折叠结构降低开关对的偏置电流,以得到良好的闪烁噪声性能;通过在混频器的驱动级引入辅助管,并优化其衬底电压和尺寸来抵消跨导管的非线性,进而提升电路的线性度.利用Volterra级数辅助分析制约线性度特性的限制因素.基于Chartered O.18 μm CMOS工艺的Spectre-RF仿真表明,电路在WLAN的2.4 GHz频段具有良好的电学性能,噪声系数为8.6 dB,闪烁噪声角为105 kHz,IIP3为5.8 dBm,芯片整体功耗为10 mW,核心电路占用面积为0.09 mm2.     

Performance Enhancement of a Sub-Sampling Circuit for Ultra-Wideband Signal Processing

An ultra-wideband (UWB) sampling mixer has been developed based on utilizing the combined advantages of two known circuit topologies: a wideband balun and a balanced-feed mixer. The developed sampler is integrated with a step-recovery diode strobe-step generator to sub-sample UWB signals. The fabricated sub-sampler demonstrated a 3.5-dB radio frequency to intermediate frequency (RF-IF) conversion loss up to 1 GHz (without the IF amplification), and a wide 3 dB bandwidth that exceeded 3.5-GHz. It has a reduced spurious level of better than -38 dBc, a lower sensitivity to the Schottky diode-placement, an excellent input match, and good isolation.