Extended Noise Figure Data for the MAX2118 Satellite Tuner

This application note provides expanded noise figure data for the MAX2118 satellite tuner. This data is a supplement to the MAX2118 data sheet. It provides more details on the noise figure and gain data for this device under typical operating conditions.

Overview

The MAX2116/MAX2118 family of low-cost direct-conversion tuner ICs is designed for digital direct broadcast satellite (DBS) television systems, professional VSAT (very small aperture terminal) systems, and two-way "satellite Internet" applications. The devices convert L-band signals directly to baseband using a broadband I/Q downconverter. The operating frequency range is from 925MHz to 2175MHz.

Each IC in this family includes an LNA with gain control, I/Q downconversion mixer, baseband lowpass filter with adjustable cutoff frequency, and variable gain amplifier. The features of all these units add up to an RF and baseband variable gain amplifier with a gain control range of more than 79dB.

These devices also include a completely monolithic VCO and full-frequency synthesizer, which is controlled and configured through a 2-wire serial interface. The MAX2116/MAX2118 family of products is the most fully featured DBS product currently available. They have both single-ended and differential baseband outputs and are compatible with almost all QPSK/8-PSK demodulators. This series of products is packaged in a very small 40-pin QFN package.


Figure 1. Typical operating circuit

Work Overview

Figure 1 shows a typical operating circuit for the MAX2118. Pins 4 and 5 are the differential RF inputs. The RF front end will determine the noise figure of the system. For small input signals, the system noise figure will limit the carrier-to-noise ratio (C/N) of the tuner. For large input signals, the C/N is limited by the linearity of the output amplifier.

Pin 7 (GC1) is the variable gain control of the RF front end. The gain control line is typically controlled by a filtered PWM signal. The PWM signal is generated by a baseband demodulator IC. In a closed-loop system, the filtered PWM output controls the gain to produce a constant amplitude signal at the I/Q output of the MAX2118.

Pins 36, 37 and pins 33, 34 of the MAX2118 are differential I/Q outputs. The gain of the differential output amplifier can be set to achieve a differential output of 590mV _PP or 1V _PP . The "DL" bit in the control register controls the gain setting of the output amplifier. When DL=1, the output amplifier gain is increased by an additional 4.58dB. Setting DL to 1 is typically used to improve system linearity. In the closed loop system, the front-end gain is reduced by 4.58dB while the output amplifier gain is increased by an additional 4.58dB.

The mixer is followed by a variable gain baseband amplifier whose gain is controlled by register GC2. GC2 can take values ​​from 0 to 31 (decimal). The gain variation range of the variable gain baseband amplifier is 24dB.

The noise figure of the system is almost exclusively determined by the RF front-end gain. The GC1 voltage is used to set the RF front-end gain, so the noise figure is also set by the GC1 voltage. Figure 2 shows this relationship.


Figure 2. Relationship between noise figure and gain control voltage

In Figure 2, the change in back-end gain is controlled by adjusting GC2. Observing Figure 2, it can be seen that the noise figure is almost constant at a given GC1 voltage.

Extended Noise Figure Data

We have obtained extended noise figure data for the MAX2118. This data can be used to configure the MAX2118 for optimal operation.


Figure 3. Noise Figure vs. Gain and GC2 Setting (DL=0)

Figure 3 shows the relationship between noise figure and gain and GC2 setting. Use this graph to determine what GC2 should be set to in order to achieve the desired gain and required noise figure.


Figure 4. Noise Figure vs. Gain and GC2 Setting (DL=1)

Figure 4 shows the relationship between noise figure and gain and GC2 setting. Use this graph to determine what GC2 should be set to in order to achieve the desired gain and required noise figure.


Figure 5. Gain vs. Gain Control Voltage (GC1), DL=0Figure


6. Gain vs. Gain Control Voltage (GC1), DL=1

Summarize

This application note provides expanded noise figure data for the MAX2118. This data supplements the MAX2118 data sheet, which provides more details on the device's noise figure and gain data under typical operating conditions.