E4426B AM频谱差

嗨，我使用的是E4426B，我一般都非常满意。
信号质量很好。
我现在要用AM调制测量一些放大器。
我有20MHz并调制1kHz和90％（值实际上并不重要，它的行为与不同的值相同）为了找到AM失真（THD + N）我正在使用FAM调制分析仪或者我给解调器提供信号然后分析
音频。
无论如何，在这两种情况下我都会得到大约1％的失真，这似乎没问题。
如果我将发生器连接到频谱分析仪，我在载波+ 1kHz和载波+ 2kHz之间的距离仅为19dB。
这将导致约10％的失真。
我不明白坏谱来自哪里。
看一下不同的信号发生器，我在这两个频率之间的差异超过30dB，失真几乎相同。
信号上是否存在相位调制或相位噪声？
这可以在频谱分析仪中看到，但不能在解调信号中看到。
有没有人用这些发生器观察到相同的不良AM光谱？
感谢您的提示和建议。



     以下为原文

  Hi,
I am using a E4426B and I am in general very happy with it. Signal has good quality. I now was going to measure some amplifiers with AM modulation.  
I have 20MHz and modulate with 1kHz and 90% (values actually dont really matter, its same behaviour with different values)

To find the AM distortion (THD+N) I am using either FAM Modulation analyzer or i give signal to demodulator and then analyze the audio. Anyway, in both cases i get a distortion of around 1%, which seems ok. If i connect the generator to a spectrum analyzer, i have only 19dB distance between carrier+1kHz and carrier+2kHz. This would lead to a distortion of around 10%. I dont understand where the bad spectrum comes from.

Looking at a different signal generator, i have more than 30dB difference between those two frequencies and the distortion is almost the same.  

Is there a phase modulation or phase noise on the signal? This could be seen in spectrum analyzer but not at the demodulated signal. Did anyone observe the same bad AM spectrum with those generators?

Thanks for hints and suggestions.