请教个滤波信号衰减测试的问题

RS422总线，老外客户在需求书上说要1MHz的截止频率。我们这边找了好久，没有找到，就叫他们推荐一款。

结果他们推荐了一款共模滤波器（TDK公司的ZJYS51R5-2PL-01），然后样品做出来了，他们来测试了。说共模滤波性能不测了，要测差模的，当时听了就很晕。

最让我想不通的是他们的测试方法。下面是他们邮件说的测试方法


Test method we were applying in Suzhou is quite simple:
1.
Apply white noise (1Vrms) from signal generator to the inputs of the signal line, setting as differential mode, connect to scope channel 1;
2.
Check outputs of signal line filter with channel 2 of scope, you don’t need to power up the system to test the performance of the filter;
3.
Set Math as FFT on channel 2 at scope, adjust resolution and scales, record 3db bandwidth referring channel 1, if it is less than 1MHz, test pass, otherwise, fail.
This test need to repeat on both common mode and differential mode noise performance, however, since RS485 buffer/transceiver on board has the capability to handle the common mode noise, we just only tested the differential mode.
Hope it is helpful, and let me know if you have any questions regarding the signal line filter testing.

他们的测试大概就是，把共模电感的一边线圈接地，另一线圈的一端用信号发生器产生白噪声，另一端用示波器去测量信号并看FFT频率，来判断噪声是否有衰减。

我的理解FFT不过看的是滤波后信号的频谱图，如何可以根据3DB频率来看信号的衰减呢？