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Can I decrease frequency range of a VNA without a configurable test set?

Question asked by drkirkby on Sep 25, 2014
Latest reply on Oct 2, 2014 by drkirkby
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  

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