# Design of a Fixed IF Down-Conversion Double-Balanced Mixer for UHF Band Applications
## 4 Simulation and Experimental Results
### Experimental Validation of Down-Conversion Double-Balanced Mixer Prototype
#### 4.5.3 Linearity Measurement: IIP3 and 1 dB Compression Point  
Both the Rohde & Schwarz FSWP spectrum analyzer and the SMA100A (with a frequency range of 9 KHz to 6 GHz) signal generator were used to measure the 1 dB compression point and IP3 measurement tests. Figure 27 illustrates the compression characteristic of the down-conversion double-balanced mixer by showing the IF output as a function of RF frequency. The parameters used for the measurements are \(f_{LO}=855\) MHz, and \(f_{RF}=1000\) MHz with the RF power being incrementally increased for each data point. When the RF power reaches the value of \(-8\) dBm, the initial gain, which is roughly 10.79 dB without compression, drops by 1 dB. The input IP3 point of the mixer is measured by applying two sinusoidal tones to the input at frequencies of 1000.05 MHz and 999.95 MHz. Every tone has a peak amplitude of \(-30\) dBm. The two-tone measurement setup used to determine the IIP3 point is presented in Figure 28. A 3 dB power divider coupler separated RF1 and RF2 tones, and two tones were spaced 100 KHz apart, generating two IF peaks at a frequency of 144.95 MHz and 145.05 MHz, while their third-order intermodulation products were generated at 144.85 MHz and 145.15 MHz, as shown in Figure 29. This shows the mixer's down-converted spectrum, revealing a difference of \(\Delta P=68.18\) dB between the fundamental and \(IM_{3}\) tones, yielding \(IIP_{3}=4.7\) dBm measured in a 50-ohm system. The third-order intercept point can be calculated as follows [41; 42]:  
\[IIP_{3}=P_{in}+\frac{\Delta P}{2} \tag{41}\]  
where \(P_{in}=-30\) dBm and \(\Delta P=P_{out}-P_{IM3}\). Now, for a lower sideband tone, \(\Delta P=P_{out}-P_{IM3}=-21.93-(-90.11)=68.18\) and thus, \(IIP_{3\_LSB}=4.09\) dBm. For an upper sideband tone, \(\Delta P=P_{out}-P_{IM3}=-21.73-(-91.13)=68.18\) and thus, \(IIP_{3\_USB}=4.7\) dBm. The \(OIP_{3}\) point is simply \(IIP_{3}+\text{Gain}\), so the calculated \(OIP_{3}\) point was about 15 dBm. Figure 27: Measured compression characteristic of the down-conversion double-balanced mixer prototype. Figure 30 compares the noise figure and conversion gain between the measured and simulated values as a function of RF frequency. The IF is down converted to 145 MHz. Therefore, the LO frequency is 145 MHz less in comparison to the RF frequency. The frequency response of the LO balun determines the local oscillator bandwidth. The LO and RF signals are AC coupled through a coupling capacitor (68 pF), which sets the lower frequency limit. There is no accurate model for the parasitics (mainly substrate capacitance) that existed from the board layout; these effects are dominant at higher frequencies. This is the main reason that the measured gain and noise figure bandwidth are less in comparison to the simulated ones. The noise floor of the circuit is about \(-166\) dBm, and IP3 and the 1 dB compression points are roughly 5 dBm and \(-8\) dBm, respectively, and so the available dynamic range is calculated as follows: (\(-174\) dB-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\(/\)Hz, Ref. [33]). Figure 28: Two-tone test measurement setup, showing DUT is measuring the third-order intermodulation product. Figure 29: Measured two-tone test of the 1 GHz down-conversion double-balanced mixer. Table 2 displays a comparison of several performance measures of the proposed double-balanced mixer, integrating the performance improvement approaches in this study with existing ones in the literature (only the most relevant papers are presented in the table; the complete list is [33; 34; 35; 36; 37; 38; 39; 40]). In Table 3, the parametric performance of the down-conversion double-balanced mixer is compared between simulated and measured results.