感谢分享。
结合ba1io和bd2bh的帖子，总算弄明白了一些问题。
找了ft-857d的英文说明书，把相关的内容又翻译了一遍。实际上ft-857d的S表指示的就是AGC电压。随着RF gain旋钮逆时针调节，AGC电压上升，指示条也上升，RF / IF增益减小。这与是否有信号无关。弄明白了以上内容，所以以后大家再将RF gain旋钮调到最小，却发现S表指示最大，电台也没声的时候，就不会感到奇怪了。
The SQL/RF Gain control is configured differently,depending on the country to which the FT-857D has been exported. In the U.S. version, the default function of this control is “RF Gain.” The configuration of the SQL/RF Gain control is set via Menu Mode No080 [SQL/RF GAIN]; see page 110 for details.
依据FT-857D出口国的不同，“SQL/RF Gain”控制旋钮有着不同的配置。在美国版本中，此旋钮的默认功能是“RF Gain”。可以通过080号菜单[SQL/RF GAIN]对“SQL/RF Gain”控制旋钮功能进行设置。详情见110页。
If your transceiver is configured for “RF Gain” use, rotating this control fully clockwise in the SSB/CW/Digital modes will provide best sensitivity. To reduce the receiver’s RF Gain somewhat, rotate this control counter-clockwise slightly. You will observe an increasing
number of bars on the S-meter as you rotate the RF Gain control counter-clockwise; this indicates increasing AGC voltage, which is causing the front-end gain to be reduced. In the FM and Packet modes, with “RF Gain” selected, the FT-857D goes into an “Auto Squelch” mode, with the Squelch level being preset at the factory.
如果你的收发信机设置为“RF Gain”功能，在SSB/CW/数字模式时，将此旋钮顺时针旋转到底，可以提供最佳的灵敏度。有时为了减少收信机的射频增益，可稍微逆时针旋转该旋钮。当你逆时针旋转RF增益旋钮时，你会发现S表指示条在增加。这表示AGC电压增加，相应的前端增益降低。在FM和分组模式下，当选中“RF Gain”功能时，FT-857D会进入“自动静噪”模式，静噪等级已在工厂预设。
If this control is configured for “SQL” operation, the FT-857D’s RF Gain will be set for maximum sensitivity in all modes, and the SQL/RF Gain control will function solely as a Squelch control. In this case, rotate the SQL/RF Gain control to the point where the background noise is just silenced; this will provide the best sensitivity to weak signals, while keeping the receiver quiet when no signal is received. The LED just above the Main DIAL will glow Green when the squelch is opened by an incoming signal or noise.
如果这个旋钮被配置为“SQL”操作时，FT-857D的射频增益在所有模式下都会设为取得最大灵敏度状态，“SQL/RF Gain”旋钮将仅作为静噪旋钮功能。在此情况下，旋转“SQL/RF Gain”旋钮到背景噪音刚好消失的位置；这一操作将为弱信号提供最佳的灵敏度，同时在没有收到信号时可保持收信机安静。主拨轮上方的LED在输入信号或噪声打开静噪时会变绿。
If you disable the AGC by pressing the
(AGC) key (to make the parentheses disappear), the S-meter (which monitors AGC voltage) will cease to function.Depending on the setting of the RF Gain control, incoming signals will probably be distorted if the AGC is turned off.
如果你通过按
（AGC）键（使括号消失）来禁用AGC功能，S表（监控AGC电压）将停止运行。取决于“RF Gain”旋钮设定位置，输入信号在AGC关闭时可能会失真。
If noise is present at a level which causes indication on the S-meter, the performance of the Noise Reduction filter may be enhanced by rotating the SQL/RF (RF Gain) control in a counter-clockwise direction so as to set the (fixed) S-meter reading at the same level as the noise peaks. This adjustment raises the AGC threshold of the receiver.
如果S表显示存在一定强度的噪声，可以逆时针旋转SQL/RF (RF Gain)旋钮，设置（固定）S表读数与噪声峰值强度相同，以增强降噪滤波器的性能。这一调整会提高收信机的AGC门限。
[ 此帖被zly在2015-04-10 14:32重新编辑 ]
