High-performance analog switch for set-top box design

introduction
As set-top boxes are designed to incorporate more emerging features, such as hard drives as video programs, compact flash interfaces, and video on demand (VOD) machines, analog switches continue to be widely adopted in such applications. With the right switches, designers can not only shorten time-to-market to improve designs, but also address important design challenges such as maintaining signal integrity. In some designs, proper switching also helps reduce the cost of bill of materials (BOM). This article explores the role of analog switches in different types of set-top box applications and explains how to use the right switches to reduce design complexity and component cost. Application example IP Digital Set-Top Box As end-user demand for video-on-demand and VoIP services continues to grow, high-throughput video streaming lines will operate over Internet networks such as DSL or cable modems. Figure 1 shows a simple functional block diagram showing how a wideband analog switch can be used in an IP set-top box design to easily route incoming 100Base-T Ethernet connections. At the same time, the switch provides a high degree of flexibility to return 10/100 Base-T Ethernet signals to another Ethernet RJ-45 interface, allowing end users to easily access Ethernet functions from the front end of the set-top box. Since each pair of twisted pairs of 100Base-T cable usually has a bandwidth of 100 Mbps, the frequency band of the switch is more than 100 MHz -3 db, which is suitable for such applications. Since 100Base-T operates in full-duplex mode, near-end and non-near-end crosstalk performance is important to reduce mutual interference between twisted pairs. The key function of the analog switch here is to remove the suspension traces generated when the RJ45 interface is not connected to the PC, and the parasitic capacitance of other connectors to improve the return loss.
Set-top boxes with personal video recording (PRV) set-top boxes with personal video recording (PRV) have been widely used by end users to record favorite TV shows while away from home or watching other TV shows. Some set-top box designs even have multiple RF inputs to receive different TV programs simultaneously, so that the user can more easily record another while watching a program. Such applications have traditionally used relatively expensive relays to switch between two RF-tuned demodulator outputs (CVBS) that share the same analog front-end video decoder. In addition to the higher cost of BOM, traditional relays are also larger in size. As today's and tomorrow's markets are increasingly demanding slim and mobile designs, better solutions such as analog switches in smaller packages are naturally welcome, and in theory have a long life that can be switched indefinitely. The single-pole, double-throw (SPDT) low-ohmic video switch shown in Figure 2 is ideal for this type of application. Since most CVBSs have a bandwidth of less than 7MHz, typical analog switches built on CMOS technology are very easy to meet. Important criteria for selecting the appropriate video switch for this type of application are differential gain and differential phase, which are directly related to color saturation and hue as a function of brightness. Fairchild's recently introduced FSA4157 switching device is ideal for this application, with on-resistance as low as 1 ohm and HBK ESD (human discharge model electrostatic damage protection) performance of 8KV. Since there is no need to add an ESD protection diode to the application, the cost of the BOM can be further reduced because the switch is placed on the edge of the video RCA connector.
Of course, recording large amounts of information requires embedding in a memory such as a hard disk drive (HDD). In general, ATA I/O is the standard interface between the HDD and its controller. In some designs that use dual hard drives in response to end user requirements, a 16-bit SPDT switch can be used to switch between the primary and secondary hard drives. The bus switch can be used here to isolate the hard drive from the ASIC, giving the designer a high degree of flexibility, with or without a hard drive. At the same time, the isolated hard drive protects the internal controller from external ESD statics. Of course, in this type of design, a 5V-driven bus switch must have enough bandwidth (-3db) to pass the high-speed ATA data stream. In fact, this does not pose a problem for general bus switches based on CMOS technology, and the internal switching channel uses only the NMOS structure. The NMOS-only architecture reduces the parasitic capacitance of the channel and helps improve the data channel bandwidth of -3db.
<br> set-top boxes with dual video output with the video output format to the development of high-definition set-top box (HD), 720p / 1080i even higher resolution such as 1080p format appears on the consumer electronics market is. Some video drivers can drive dual video outputs even in the case of DC coupling, such as dual SCART outputs. Usually, only one output connection is displayed at a time, and the other is used as a float. The floating output can seriously affect the HD video signal image effect, that is, because the transmission line is not terminated at the video driver interface end, resulting in edge distortion and/or overshoot distortion or undershoot distortion. Fortunately, the wideband video switch solves a design problem that cuts off the trace when the video output is not used (as shown in Figure 2). Since the high-definition analog video signal has a fast rise/fall time, the switch must have enough bandwidth signal to pass. Wide bandwidth is especially important for A/D converters at the back end of the data channel; there is enough bandwidth to reliably sample valid analog video signals because the high-definition pixel clocks are much shorter than standard-definition video clocks. At the same time, when considering the 75 ohm remote termination resistor on the side of the TV display, the insertion loss must be small to reduce the loss of color saturation. In general, a video switch with a 5 ohm on-resistance is sufficient. But bandwidth needs to be greater than 500MHz to reduce nonlinear distortion in high-definition video signals such as 720p and 1080p. The higher the bandwidth of the switch, the less distortion it produces.
<br> with external set-top box in addition to embedded hard disk storage, removable storage devices such as compact flash or other small-size memory cards have been widely used in the design of portable products, including mobile phones and digital cameras, to meet Increasing market demand. Future set-top boxes will also have enough intelligence to read these storage devices and drive the TV to display digital camera photos. As shown in Figure 3, in this type of application, the core CPU USB output of a typical set-top box shares the same interface as the embedded or removable storage controller to communicate with the PC. There must be a wide bandwidth high-speed USB switch to overcome the design challenges for two reasons: First, the Hi-Speed ​​USB 2.0 signal has a fast rise/fall time (500 ps), which requires the switch to cut off all useless traces. , thereby minimizing the reflected signal while improving the eye opening indicators required for compliance testing. Second, both the CPU and flash controller outputs have 1.5K boost resistors, which must be isolated from each other to reduce potential fluctuations.
Choosing the right switch to reduce BOM cost <br> analog switches in addition to solve these design challenges, but in some applications the use of a suitable switch can also help designers to significantly reduce the number of BOM and save board space. As shown in Figure 4, some set-top boxes themselves can drive a composite video output such as CVBS. When the set-top box is in standby, other external video sources such as VCR or DVD players can also directly drive the TV. In these applications, the dual 74VHC4053 device will be used to switch between the set-top box audio/video encoder output and the VCR/DVD player A/V input to share a single display. One of the main disadvantages of the 74VHC4053 is its high on-resistance (up to 100 ohms), which can cause video brightness loss if the switch output is directly connected to the TV. A simple buffer (such as a discrete NPN component) is usually used to isolate the switch from the 75 ohm termination resistor on the TV side.
After the addition of NPN, the base of the NPN needs to apply a DC bias voltage, which means adding additional component costs, such as AC coupling capacitors, resistors, and ferrite rings for stabilizing the DC bias voltage, preventing stringing into the power supply. VCC whispers. All of these additional costs are due to the high on-resistance of the 74VHC4053. If you can choose a switch with low on-resistance, such as Fairchild's FSAV330 or FSAL200 (usually only 6 ohms), you can save all these extra components. Because of the large resistors and capacitors, the board area can be greatly reduced. . The low bandwidth of the 74VHC4053 results in higher differential gain and differential phase than typical CMOS video switches, which have excellent color saturation stability and hue. Figure 4 shows a structural quick comparison of the front and rear of the FSAV330 or FSAL200, showing a significant reduction in the use of discrete components at a ratio of 9:1.
Conclusion <br> addition to traditional digital video decoding capabilities, set-top boxes in the home appliances increasingly worried played an important role, with more intelligent features, such as VoIP and video on demand. Future designs will use high-performance analog switches extensively to speed time-to-market. Dedicated broadband video switches improve signal integrity and help designers solve high-definition design challenges. In addition to reducing traditional BOM costs by sharing connectors between video outputs, higher performance video and Ethernet switches will help reduce the number of external discrete components, further reduce BOM cost, and save PCB space. The set-top box market is another area with huge growth potential, especially in China. This market will be more attractive as China’s State Administration of Radio, Film and Television has set a goal of fully implementing the digitalization of television systems in 2015. The development of high-definition video formats and low-cost communications such as VoIP will also bring tremendous boost to the set-top box and dedicated switch markets in the region.


:
0 times
Window._bd_share_config = { "common": { "bdSnsKey": {}, "bdText": "", "bdMini": "2", "bdMiniList": false, "bdPic": "", "bdStyle": " 0", "bdSize": "24" }, "share": {}, "image": { "viewList": ["qzone", "tsina", "tqq", "renren", "weixin"], "viewText": "Share to:", "viewSize": "16" }, "selectShare": { "bdContainerClass": null, "bdSelectMiniList": ["qzone", "tsina", "tqq", "renren" , "weixin"] } }; with (document) 0[(getElementsByTagName('head')[0] || body).appendChild(createElement('script')).src = 'http://bdimg.share. Baidu.com/static/api/js/share.js?v=89860593.js?cdnversion=' + ~(-new Date() / 36e5)];