# Design of a Fixed IF Down-Conversion Double-Balanced Mixer for UHF Band Applications
## 1 Introduction  
A mixer combines a radiofrequency (RF) input signal at frequency \(f_{RF}\) and a local oscillator (LO) signal at frequency \(f_{LO}\) to produce an intermediate frequency (IF) output signal containing the sum and difference frequencies, \(f_{RF}\pm f_{LO}\). Either the sum (\(f_{RF}+f_{LO}\)) or the difference (\(f_{RF}-f_{LO}\)) frequency is selected for further processing using a bandpass filter [1, 2, 3]. Mixers are frequently classified as passive or active. Passive mixers can be employed to generate an IF output for an LO driving signal by using passive switches. In contrast to active mixers, passive mixers do not provide a positive conversion gain (CG) [2, 3]. However, passive mixers are straightforward. They require a high LO drive level but are highly linear and have a good noise figure (NF), which is typically the same as conversionloss and consumes zero power. Unlike passive mixers, active mixers offer exceptional CGs, effective port isolation, minimal NF, and require less LO drive. Therefore, active mixers are favored over passive mixers when all of these factors are taken into account. In the past years, literature has presented several types of mixers [4; 5; 6; 7; 8; 9; 10; 11]. For instance, Ref. [5] proposes a broadband double-balanced mixer that operates at 1-6 GHz. Two techniques are proposed to enhance the mixer's performance. First, at high frequencies, loading capacitors improves the amplitude and phase imbalance of the balun. Second, by cascading a first-order LC filter at the intermediate-frequency port, RF and LO signal leakage at the IF port is decreased by around 8 dB and 7 dB, respectively. With a 1.13 mm \(\times\) 0.95 mm chip size and 34 dB of isolation between the IF and RF ports, the device produces 13.5 dBm of input power at the 1 dB compression point. In order to build the chip, the GaAs pHEMT technique is employed [5]. Based on reformed particle swarm optimization, the authors of [8] suggest an up-conversion mixer circuit for radar. The suggested approach assists in resolving the issues of premature convergence and local optima in conventional particle swarm optimization. Additionally, the paper [8] presents new circuit designs based on the two-fold transconductance route, a linearity-enhancing approach. The suggested optimal mixer obtains a measured conversion gain of 2.5 dB. The mixer proposed in the aforementioned study has a good linearity and a 1 dB compression point of 4.2 dBm. The noise figure of the suggested mixer is roughly 3.1 dB [8]. In paper [9], a novel linearization method for active mixers is proposed, which takes advantage of the \"reverse uplift\" phenomena to significantly increase the input and output 1 dB compression points. This technique is used to design and build a 130 nm SiGe BiCMOS ultra-high-linearity double-balanced down-conversion mixer with a broad IF bandwidth. A Gilbert cell, two phase-adjusting inductors, and an output network based on a Marchand balun are all components of the suggested mixer. According to the measurement results, the mixer has the best linearity performance in mixers under silicon-based processes, whether active or passive, with an IP1 dB of +7.2\(\rightarrow\)+10.1 dBm and an average OP1 dB of +5.4 dBm. In addition, a wide 8 GHz IF bandwidth between 3 and 11 GHz was achieved [9]. Finally, a reconfigurable, image-rejected down-conversion mixer, based on the SiGe 8 HP technology, has been presented in [10]. The suggested mixer is appropriate for software-defined radio applications and operates within the 0.9-13.5 GHz range. The conversion mixer is made up of a tunable filter section, a resistive biased radiofrequency section, and double-balanced Gilbert-cell mixer core sections. The mixer achieves a good return loss \(\ket{S111}\) of \(<\)-10 dB for the whole operating band. Additionally, at maximum frequency, the design achieves an outstanding CG of 22 dB, an NF of 2.5 dB, and an image-rejection ratio of 30.2 dB.