The MAX2640 is a low-cost, low-noise amplifier designed for applications in the 400MHz to 2500MHz frequency range. This application note shows the adjustment of the MAX2640 RF matching circuit for 470MHz to 770MHz ISDB-T applications. The circuit is optimized to meet the following specifications over the entire operating frequency band: noise figure < 1.2dB, gain > 15dB, input return loss < -3dB, output return loss < -12dB, IIP > -18dBm, input P1dB > -26dBm.

　　The MAX2640 is a low-cost, low-noise amplifier (LNA) designed for applications in the 400MHz to 2500MHz frequency range. The device operates over a wide +2.7 to +5.5V supply range, consumes only 3.3mA typical, and features low noise figure, high gain, and high input IP3.

　　Integrated Services Digital Broadcasting (ISDB) is a digital television and broadcasting specification proposed by Japan for digital broadcast multimedia services. ISDB-T is the core standard for terrestrial and mobile multimedia applications. In the 470MHz to 770MHz frequency band, ISDB-T divides the 6MHz bandwidth into 13 equal segments. After the channel is segmented, different combinations of segments can be used to send programs with different bandwidth requirements (for example, HDTV, SDTV, digital broadcasting, etc.), thereby increasing the flexibility of program playback, and mobile devices only occupy one of the 13 segments.

　　Maxim introduces the MAX2160, MAX2161/MAX2162 complete integrated tuners for ISDB-T applications. The MAX2640 LNA can be added to the front end of an ISDB-T tuner with proper tuning to improve the system's noise figure and gain. This application note provides the appropriate matching circuits to optimize the MAX2640 design for ISDB-T applications.

　　Optimizing LNA performance

　　The performance of RF amplifiers can be improved by using appropriate matching components for a specific frequency range. The input and output of the amplifier should be connected to appropriate signal source and load impedances to ensure effective signal transmission between system modules and minimize the noise introduced by the amplifier to the signal. Generally, the requirements for signal source and load impedance for the best noise figure are different from those for the maximum gain. In order to optimize the performance of the low noise amplifier, the input and output matching circuits should be adjusted, taking into account the noise figure, return loss and gain indicators.

　　The MAX2640 EV kit helps to quickly evaluate the MAX2640. This application note uses the MAX2640EVKIT to optimize the input and output matching circuits.

　　First, determine the inherent gain and stability of the MAX2640. The VCC pin of the MAX2640 allows for flexible placement of the VCC bypass capacitor, thereby adjusting the series inductance of the VCC pin. The series inductance of the VCC pin has a significant impact on the amplifier, introducing an additional parameter to improve performance. Data from previous performance analysis of the MAX2640 shows that placing the VCC bypass capacitor approximately 4mm to 5mm from the VCC pin can achieve a good compromise between inherent gain and stability.

　　For op amps with poor isolation, tuning the output match will inevitably affect the input match circuit. However, due to the high isolation between the output and input of the MAX2640, the input and output matching networks can be tuned separately. In this application note, the output match is tuned first to optimize gain and output return loss.

　　Secondly, the input matching circuit should be optimized. The circuit operating bandwidth required by this application note increases the complexity of input tuning. To ensure the ideal flat gain and noise figure over the entire frequency range, an appropriate trade-off should be made. In this application note, the goal is to maintain constant gain over the entire frequency band and minimize the noise figure. To achieve this goal, the input return loss performance has to be sacrificed. A T-type matching network is used at the LNA input to provide a broadband match; at the same time, a DC blocking capacitor is required at the LNA input.

　　Figure 1 shows the final circuit, and Table 1 gives the component list.

Figure 1. MAX2640 tuning circuit for 470MHz to 770MHz ISDB-T applications.

Table 1. MAX2640 evaluation component list for 470MHz to 770MHz ISDB-T applications.

　　Using the above circuit, the performance of the MAX2640 was measured at VCC = +2.8V and TA = +25°C. At the center of the band, the LNA has a 1.05dB noise figure, 15.1dB gain, -5dB input return loss, -16.5dB output return loss, -16dBm IIP3, and -26dBm input P1dB. Across the band, the noise figure is less than 1.2dB, the gain flatness is approximately ±0.1dB, the input return loss is less than -3dB, the output return loss is less than -12.3dB, the IIP3 is better than -18dBm, and the input P1dB is greater than -26dBm.

　　To ensure accurate measurements of precision and noise figure in the Faraday cavity, circuit losses and input and output matching component losses are included in all measurements. The performance over the entire frequency band is shown in Figures 2–4.

Figure 2. Optimized noise figure vs. frequency.

Figure 3. Optimized gain vs. frequency

Figure 4. Optimized input/output return loss vs. frequency