电缆均衡器视频广播设备可通过串行数字接口输入信号,而且需要支持的传送范围一般都较长:高清晰度电视信号可以传送到140米范围以外的接收端,而标准清晰度信号的传送范围更长达300米以上。但我们必须采用电缆均衡器,才可支持长距离的信号传送。较长的电缆都有低通的特性。换言之,输入信号的衰减程度不但与电缆长度成正比,而且与频率的平方根值也成正比。图2显示100米长的Belden1694A电缆的频率响应,而Belden1694A是视频设备最常用的电缆。若要适应电缆的不同频率响应,设计工程师要小心为滤波器加入各个零点,以确保最终的响应非常接近电缆的频率平方根响应。由于必须同时具备高增益及高带宽的特性,因此均衡器电路必须采用先进的高速工艺技术制造,例如美国国家半导体的0.25μm BiCMOS SiGe工艺。LMH0044电缆均衡器便是其中一款采用这种工艺技术制造的均衡器。即使采用长达200米的Belden1694A电缆,而传输速度甚至高达1.5Gbps,这款芯片都可将其中传送的信号恢复。信号衰减程度与频率的正比关系当然有它的用处,例如,我们可以更容易选定理想的互补滤波器设计(互补滤波器是我们赖以恢复信号的电路)。但话虽如此,我们真正需要的是眼图,眼图会显示究竟有没有足够的空间,可以让我们将信号恢复。图3是1.5Gbps信号(高清晰度/串行数字接口信号)的眼图,图中的信号分别在不同长度的电缆上传送。根据本文作者的观察,趋肤效应的主要影响是,每当频率越高,电缆用以传送信号的截面面积便越少,因此高频区出现的信号衰减情况比低频区严重。信号衰减的响应函数曲线与频率的平方根成正比,因此要采用标准型号的滤波器来提供补偿便更感困难。 Cable Equalizers Video broadcasting devices can input signals over the serial digital interface and generally require longer transmission ranges to support: High-definition television signals can be transmitted to receivers other than 140 meters while standard-definition signal transmission ranges As long as 300 meters above. But we have to use a cable equalizer to support long distance signal transmission. Longer cables have low-pass characteristics. In other words, the attenuation of the input signal is not only proportional to the length of the cable but also to the square root of the frequency. Figure 2 shows the frequency response of a 100-meter Belden1694A cable, which is the most commonly used cable for video equipment. To accommodate the different frequency response of the cable, the design engineer must be careful to add each zero for the filter to ensure that the final response is very close to the square root of the frequency response of the cable. Equalizer circuits must be manufactured using state-of-the-art high-speed process technology, such as National’s 0.25μm BiCMOS SiGe process due to the combination of high gain and high bandwidth. The LMH0044 cable equalizer is one of the equalizers built using this process technology. Even with up to 200 meters of Belden1694A cable, and even up to 1.5Gbps transmission speed, the chip can be transmitted from the signal recovery. The proportional relationship between the degree of signal attenuation and frequency, of course, has its use, for example, we can more easily select the ideal complementary filter design (complementary filter is the circuit we rely on to restore the signal). But even so, what we really need is the eye diagram, which shows whether there is enough room for us to recover the signal. Figure 3 is an eye diagram of a 1.5Gbps signal (high definition / serial digital interface signal) in which signals are transmitted over cables of different lengths. According to the authors of this article, the main effect of the skin effect is that the higher the frequency, the less the cross-sectional area of ​​the cable used to transmit the signal, so the signal attenuation occurring in the high frequency region is more severe than in the low frequency region. The response curve of the signal attenuation is proportional to the square root of the frequency, making it more difficult to use a standard model filter to provide compensation.