Thursday, 19 November 2015

Using Time Domain Reflectometry to align filters

Time Domain

Frequency Domain
Here is my first attempt at using TDR to align a 437 Mhz BPF filter.
The technique is described in Agilent AN 1287-8
The dips in the Time domain represent the resonators in the filter, S11 from one
end of the filter and S22 from the other. TDR allows you to see
the results of adjusting each resonator fairly independently of the others.
The aim is to increase the dips as much as you can. The peaks are related to
the inter resonator coupling. For a full explanation you need to read the Appnote.

As you can see from the frequency domain plot my efforts were not perfect
but I suspect practice will make perfect. The tune elements were 6 mm brass
studding and these were not the easiest thing to adjust. After I had done the
measurement I got a warning saying the calibration had been aborted. I had been
at it for about 30 mins and was getting cold, there is no heating in my workshop.
I will however try again. The filter is adequate for my 70 cms DATV station.


  1. This is very interesting, and quite hard to get one's head around!

    Colleagues of mine who are specialists in the use of network analysers (I am not!) often remark that the "best" way to tune complex filters (eg multiple bandpass filters as used in starpoint combiners, etc, is to use the "polar" function (which I think is an S11 function, but I could be wrong!).

    What makes my brain hurt is the fact that you have a swept oscillator and an x axis reading time ... I think you need to get "hands on" to work out what is going on!

  2. The Agilent Appnote does a better job at describing it. Basically the instrument is measuring in the frequency domain (swept oscillator) doing an FFT and then displaying in the time domain. Yes it is measuring S11. I believe the guy that wrote the Appnote did the research as part of his PhD

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