救急:100Hz低频纹波噪声及高频噪声电路分析(有图)

本人有个电路百思不得其解:
请看:电源原Y1电容接法在冷热地之间,次级输出接负载电话,但在电话段可以听到明显的100HZ脉冲噪声,可是此时次级的纹波噪声仅有60~100mV,如明显视频干扰．如图1500) {this.resized=true; this.width=500; this.alt='这是一张缩略图，点击可放大。\n按住CTRL，滚动鼠标滚轮可自由缩放';this.style.cursor='hand'}" onclick="if(!this.resized) {return true;} else {window.open('http://u.dianyuan.com/bbs/u/29/1116215172.jpg?x-oss-process=image/watermark,g_center,image_YXJ0aWNsZS9wdWJsaWMvd2F0ZXJtYXJrLnBuZz94LW9zcy1wcm9jZXNzPWltYWdlL3Jlc2l6ZSxQXzQwCg,t_20');}" onmousewheel="return imgzoom(this);">
当完全取消冷热地之间的Y1电容时,在电话段听不到到明显的100HZ脉冲噪声．但此时电源的纹波噪声高达2V,在视频输出的显示端可见明显的视频干扰噪声．
当我们降接地的方法改了一下后,在电话段也听不到到明显的100HZ脉冲噪声,但实际的纹波噪声居然有400~600mV,无视频干扰图象的噪声,如图2:500) {this.resized=true; this.width=500; this.alt='这是一张缩略图，点击可放大。\n按住CTRL，滚动鼠标滚轮可自由缩放';this.style.cursor='hand'}" onclick="if(!this.resized) {return true;} else {window.open('http://u.dianyuan.com/bbs/u/29/1116215191.jpg?x-oss-process=image/watermark,g_center,image_YXJ0aWNsZS9wdWJsaWMvd2F0ZXJtYXJrLnBuZz94LW9zcy1wcm9jZXNzPWltYWdlL3Jlc2l6ZSxQXzQwCg,t_20');}" onmousewheel="return imgzoom(this);">
请大虾帮我们分析一下,为什么会有如此的结果?