Last week's announcement of Microtune's MT2131 tri-mode tuner illustrates increasing manufacturer interest in introducing products for off-air DTV reception. It is especially encouraging that tuner and VSB decoder manufacturers are touting compliance with ATSC Recommend Practice A/74 for ATSC receiver performance.
Microtune's MT2131 is a double conversion design -- typical image rejection is specified at 75 dBc, which helps it avoid the image problems often associated with traditional signal conversion tuners. The tuner features integrated RF power detectors that sample RF output from the first and second mixers and a closed-loop AGC around the input low-noise amplifier and the variable gain amplifier following the filter after the first mixer. The RF AGC system has a range of 55 dB. Another variable gain amplifier driving the IF output to the demodulator accepts an AGC voltage from the demodulator and has an AGC range of 39 dB. The MT2131 product brief shows the typical noise figure in the "off-air" mode as 6 dB, which is 1 dB better than the FCC planning factors require at UHF.
The MT2131 exceeds ATSC A/74 requirements and performed better than the tuners listed in the FCC report in ET Docket 05-182. (See Appendix C of the FCC Report to Congress - Study of Digital Television Field Strength Standards and Testing Procedures.)
"The MT2131 outperformed tuner cans and far outpaced all other silicon tuners that were evaluated in our internal tests against the A/74 and DCR technical specifications and in tests conducted by leading TV manufacturers," said James A. Fontaine, Microtune president and CEO. "For us, the MT2131 is another milestone achievement in our history of innovating the industry's most advanced RF integrated circuits. It reflects our continuing success in raising the tuner performance bar for our customers across their TV, cable, computer and mobile products, while significantly reducing their RF solution costs."
Microtune has posted benchmarks comparing the MT2131 with the DTV receivers tested by the FCC and with ATSC A/74 requirements. In FCC tests, the average receiver had a sensitivity of -81 dBm. ATSC A/74 specifies a sensitivity of -83 dBm. The worst-case sensitivity measured with the MT2131 was -84 dBm and the average sensitivity was better than -85 dBm.
Microtune's benchmarks show D/U performance at only one signal level, -68 dBm, corresponding to the "weak desired" signal level in A/74. The recommended adjacent channel D/U ratio is 13 to 14 dB higher at weak signal levels than at the "strong desired" signal level of -28 dBm. Given the 55 dB range of its RF AGC system, the MT2131 should maintain adjacent channel performance in the presence of a stronger desired signal. At -68 dBm, the adjacent channel D/U ratio is approximately -48 dBm. This means an adjacent channel has to be 48 dB stronger than the desired channel before it will cause interference. At this level, out-of-band emissions from the adjacent channel transmitter may have a greater impact than tuner performance. The indicated MT2131 performance is seven to eight dB better than required by A/74. For undesired signals two to thirteen channels away from the desired channel, the MT2131 exceeds A/74 requirements by three to five dB. The D/U ratio required by A/74 drops to -50 dB for signals 14 or 15 channels from the desired channel, reflecting the image response of single conversion tuners. Since the MT2131 uses a dual conversion tuner, these signals pose no problem for it and D/U ratios better than -60 dB reflect this.
Power consumption is especially important for manufacturers of portable or handheld DTV sets. The MT2131 can operate at 3.3 volts with a typical power consumption of 80 mA (264 mW). Most discrete ATSC tuners require at least 5 volts and some require multiple voltages, so this should give the MT2131 an advantage in portable devices. Size is another specification important to portable device manufacturers. The MT2131 excels in this area -- look at the MT2131 photo gallery to see how its 7mm by 7mm size compares with a popular candy.
According to the Microtune announcement the chip is priced at less than $3.00/unit in volume quantities. Microtune said samples of the chip have been sent to select customers. While my focus has been on the over-the-air DTV performance of the tuner, it is also able to receive NTSC broadcasts and meets CableLabs standards for "Digital Cable Ready" (DCR) tuners.
