For large received signals, high P1dB is required to avoid saturation of the tuner. By adjusting the RF variable-gain amplifier for lower gain, the P1dB is increased significantly. This enables reception of large signals. Since this reduced RF gain increases the tuner noise figure, careful system design tradeoffs are required between P1dB and noise figure. This application note is a tool to calculate these tradeoffs.
Pin 7 (GC1) is the variable gain control for the RF front-end. The gain control line is typically controlled by a filtered PWM signal. A baseband demodulator IC generates the PWM signal. In a closed-loop system the filtered PWM output will force a constant amplitude signal on the MAX2116 IQ output.
Pins 37 and 34 on the MAX2116 are the I/Q outputs. The single-ended output amplifier gains can be set to deliver 800mVP-P at the output.
After the mixer, there is a variable-gain baseband amplifier. This amplifier's gain is set by register GC2, which is adjustable from 0 to 31 (decimal). System P1dB is set by the particular combination of GC1 and GC2.
Figure 1. Typical operating circuit for the MAX2116 direct-conversion tuner.
Figure 2 shows the P1dB of the MAX2116 for different RF and baseband gain control settings. The trends of increasing P1dB as the front-end and back-end gains decrease are evident, with the front-end impact dominating. The P1dB limit of about 3.5dBm is reached for any GC2 setting at a minimum front-end gain setting of 2.6V. The data in Figure 2 is measured with a midband carrier-wave RF input at 1550MHz and a 5MHz IF frequency.
Figure 3. Voltage gain vs. GC2 code.
Figure 3 shows the MAX2116 voltage gain at different gain-control settings that correspond to those shown in Figure 2 above. The maximum gain is 76dB; the GC1 range is 72dB and the GC2 range is 22dB. Each of these values is near the typical data sheet values.
For further data analysis, you download access an Excel spreadsheet containing the raw data shown in Figures 2 and 3.
For more information, please see the Maxim application note #2240: Extended Noise Figure Data for the MAX2118 Satellite Tuner.