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MAX2108 Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX2108 Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 12 page Direct-Conversion Tuner IC 6 _______________________________________________________________________________________ _______________Detailed Description The MAX2108 downconverts signals in the 950MHz to 2150MHz range directly to baseband in-phase/ quadrature-phase (I/Q) signals. It is designed for digital DBS tuner applications where a direct downconversion provides a cost savings over multiple-conversion approaches. However, the MAX2108 is applicable to any system requiring a broadband I/Q downconversion. Internally, the MAX2108 consists of a broadband front- end variable gain stage, a quadrature downconverter, a 90° quadrature generator, a divide-by 32/33 prescaler, and high-linearity I and Q baseband buffers. The front-end gain-control range is over 50dB. Specifically, when the MAX2108 operates in an auto- matic gain control (AGC) loop, VGC is adjusted by the loop so that a sine wave at RFIN ranging in power from -70dBm to -20dBm produces a sine wave across IOUT, IOUT and QOUT, QOUT at 10mVP-P differential. The noise figure is at its minimum when GC is at its maxi- mum gain setting. The quadrature downconverter follows the front-end variable-gain amplifier. The mixer LO ports are fed with the two LO signals, which are 90° apart in phase. These quadrature LO signals are generated internally using the signal from the LO and LO pins. The resulting I/Q baseband signals are fed through separate I-channel and Q-channel baseband buffers. The outputs are capable of driving lowpass filters with 100 Ω characteristic impedance (that is, the equivalent of an AC-coupled 100 Ω load). The baseband -3dB out- put bandwidth is approximately 150MHz. Applications Information Front-End Tuner Circuitry for DBS Tuners In a typical application, the signal path ahead of the tuner includes a discrete low-noise amplifier/buffer and a PIN-diode attenuator. Since the MAX2108 satisfies the noise and linearity requirements for DBS, this front- end circuitry is not required. In some very high linearity applications, such as single channel-per-carrier (SCPC), a varactor-tuned pre- selection bandpass filter is added between a discrete LNA and the MAX2108. The filter provides a means of broadly filtering adjacent interference signals, thus improving the intermodulation performance of the tuner. Additionally, the filter removes RF interference at twice the LO frequency, which otherwise adds to the co- channel interference. The MAX2108 rejects this carrier to approximately 25dBc. LO Port The MAX2108 accepts either a single-ended or differ- ential LO signal. For single-ended drive, AC-couple the LO signal into LO with a 47pF capacitor, and bypass LO to ground with a 47pF capacitor in series with a 25 Ω resistor. Drive LO with a 50Ω source at -5dBm. Prescaler The prescaler requires a stable logic level at PS_SEL 4ns before the falling edge of PSOUT, PSOUT to assert the desired modulus. The logic level at PS_SEL must remain static until 2ns after this falling edge. Baseband Buffers The MAX2108 baseband buffers provide at least 10mVp-p differential swing across IOUT, IOUT and QOUT, QOUT, and are capable of driving an AC-cou- pled 100 Ω differential load. In a typical application, IOUT, IOUT, QOUT, and QOUT drive a 5th- or 7th-order lowpass filter for ADC anti-aliasing purposes (see the Filters in Direct-Conversion Tuners section ). In general, additional gain is required, after the filters. This is accomplished with a pair of video-speed op amps, such as the MAX4216 dual video op amp, or a simple transistor circuit. Contact Maxim for more information about the MAX4216. Layout Considerations Observe standard RF layout rules. A ground plane is essential; when connecting areas of ground plane between layers, use vias liberally. If a ground plane is used under the lowpass filters, note that the filter response may be slightly offset due to parasitic capaci- tance. In a direct-conversion receiver, LO leakage to the RF input connector is a major issue, since filtering of the LO is impossible (the LO operates at the same frequen- cy as the RF input). Observe the power-supply bypass capacitor connections in the Pin Description table, notably pins 3, 5, 12, and 21. Traces from these IC pins to the bypass capacitors must be kept on the top side of the board and as short as possible. |
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