# Design of a Fixed IF Down-Conversion Double-Balanced Mixer for UHF Band Applications
## 5 Conclusions  
This manuscript offers a novel approach to designing a double-balanced mixer without having the need for a bulky transformer at the RF stage, which is suitable for a wide array of applications, including military radar, satellite communication (SATCOM), and cellular base stations. The down-conversion mixer is made up of a single-ended to a differential-balanced RF stage, a dual feedback linearization for the RF stage, a local oscillator balun, local oscillator mixing cores, and a fixed IF tuned circuit connected between two outputs to serve as a load at 145 MHz. The dual feedback linearization technique improves nonlinear transfer characteristics to increase IIP3 and the 1 dB compression point by linearizing voltage and current at RF stages. The conversion gain of the mixer described in this work is 12 \(\pm\) 1 dB at frequencies between 0.4 and 1.8 GHz, and the lowest SSB noise figure is 7 dB at 1 GHz. The measured noise figure is 7 dB \(\pm\) 0.4 dB at frequencies between 0.4 and 1.8 GHz. The noise floor of the circuit is about \(-\)166 dBm, and IP3 and the 1 dB compression point are roughly 5 dBm and \(-\)8 dBm, respectively, so the available dynamic range is calculated as follows: (\(-\)174-1 dB compression point + NF). From this, (\(-\)174 \(-\) (\(-\)8) +7 \(\approx\) 159 dBm/Hz) is the available dynamic range (however, Table 2 shows (\(-\)174 \(-\) (\(-\)13) + 18 \(\approx\) 143 dBm). In the intended band, the mixer achieves a good return loss of over 8 dB for an RF and LO port, and a measured return loss of over 18 dB at 145 MHz and IF frequency. Furthermore, the design achieved RF-to-IF isolation better than 35 dB and LO feedthrough and LO leakage isolation above 50 dB. Good linearity performance is demonstrated by the measured third-order intercept point of +4.7 dBm and the 1 dB compression point of roughly \(-\)8 dBm, respectively. Applications requiring low voltage can benefit from the suggested mixer architecture. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter Metrics** & **Simulated Performance** & **Measured Performance** & **Calculated Performance** \\ \hline RF Frequency & 1 GHz & 1 GHz & 1 GHz \\ \hline LO Frequency & 855 MHz & 855 MHz & 855 MHz \\ \hline \multirow{2}{*}{Mixer Bandwidth} & 0.5 GHz to 2 GHz constant & 0.5 GHz to 1.8 GHz & \multirow{2}{*}{-} \\  & IF of 145 MHz & constant IF of 145 MHz & \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Conversion Gain \\ Bandwidth \\ \end{tabular} } & \multirow{2}{*}{11.4 \(\pm\) 0.5 dB} & \multirow{2}{*}{12 \(\pm\) 1 dB} & 12.338 dB (predicted \\  & & conversion gain) \\ \hline \multirow{2}{*}{Noise Figure} & 8 dB & 7.03 dB & 7.96 dB (predicted) \\ \cline{2-4}  & & Between \(-\)9 dBm and & \\ \cline{2-4}  & & \(-\)8 dBm & - \\ \hline IIP3 Measurement & 6.716 dBm & 4.7 dBm & - \\ \hline OIP3 Measurement & 17.9 dBm & 15 dBm & \\ \hline L–I Isolation & \(-\)65 dB & \(-\)58 dB & - \\ \hline L–R Isolation & \(-\)65 dB & \(-\)70 dB & - \\ \hline RF Port Return Loss & Better than \(-\)12 dB & Better than \(-\)8 dB & - \\ \hline LO Port Return Loss & Better than \(-\)10 dB & Better than \(-\)8 dB & - \\ \hline IF Port Return Loss & \(-\)26 dB at 145 MHz & \(-\)18 dB at 145 MHz & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison between simulated and measured results after optimization. **Author Contributions:** Conceptualization, T.K., U.L.R. and M.R. ; methodology, T.K. and U.L.R. ; validation, T.K. and U.L.R. ; formal analysis, T.K., U.L.R., H.S. and A.S.; investigation, T.K. and U.L.R. ; resources, U.L.R. ; data curation, T.K., U.L.R., H.S. and A.S.; writing--original draft preparation, T.K. ; writing--review and editing, T.K., U.L.R., H.S. and A.S.; supervision, U.L.R. and M.R. ; funding acquisition, U.L.R. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable.