如果没有可配置的测试装置，我可以降低VNA的频率范围吗？

我有一个8720D（50 MHz到20 GHz），并且想知道是否有任何方法可以通过使用外部耦合器，混频器等将频率范围降低到1 MHz或更低。我对此并不感到困扰
动态范围或准确度下降。
我主要对天线的反射测量感兴趣，所以测量S11在最坏的情况下是可以接受的，但是当然能够测量所有4个S参数会很好。
我知道通过使用外部设备可以将VNA的频率范围扩展到mm波，但我所看到的只需要一个可配置的测试装置。
问题是，除了测试端口之外，我对此VNA内部信号路径的唯一访问是后* R通道上的* 10 MHz参考输入，它被买到前面板。
后部的10 MHz参考输入意味着信号发生器，VNA可以具有相同的10 MHz参考。
我想通过使用200 MHz本地振荡器和混频器将VNA用作200-250 MHz范围内的可调谐IF。
在该频率下，图像响应应比VNA工作在GHz频率时更容易滤波。
8720D的动态范围在50 MHz时比在较高频率下低，因此我认为在200-250 MHz左右工作可能是明智的。
考虑到对内部信号路径的访问相当有限，我对这是多么实用有任何想法？
有没有人得到一个已知有效的方块图？
戴夫



     以下为原文

  I've got an 8720D (50 MHz to 20 GHz), and are wondering if there is any way I can drop the frequency range down to 1 MHz or less by using external couplers, mixers etc.  

I'm not too bothered about a drop in dynamic range or accuracy. I'm mainly interested in reflection measurements on antennas, so just measuring S11 would at a worst case be acceptable, though being able to measure all 4 S parameters would be nice of course.  

I know the frequency range of VNAs can be extended up to mm wave by the use of external devices, but all that I have seen require a configurable test set.  The problem is, apart from the test ports, the only access I have to this VNA internal signal paths are

* 10 MHz reference input on rear
* R channel which is bought out to the front panel.  

The 10 MHz reference input on the rear means a signal generator and VNA could have the same 10 MHz reference.  

I was thinking of perhaps using the VNA as a tunable IF in the range of say 200-250 MHz, by use of a 200 MHz local oscillator and mixer. At that frequency, the image responses should be easier to filter than if the VNA was working at GHz frequencies. The dynamic range of the 8720D is lower at 50 MHz than at higher frequencies, so I was thinking working around 200-250 MHz might be sensible.  

Any thoughts on how practical this is, given the rather limited access I have to the internal signal paths? Has anyone got a block diagram of something that is known to work?  

Dave