Andy:
That;s all right. I misread your 2nd instance as 500 ohms, not 50 ohms,
If it is indeed 50 ohms, then a simple 1:1 transformer or LC match will suffice. That may not even be needed
as there are capacitors to isolate the circuit...I suspect it is not really 50 ohms though.
Note that the "QER" topology is not the same as the ladder filter topology. It is a variation of
the Cohn filter, but the effect of the parallel crystals at the end has the effect of greatly reducing
the passband ripple. It also changes the termination impedance of the filter somewhat. A Cohn
filter has a terminal impedance somewhere around 200 ohms. (That varies a great deal depending
on the terminations of course, but simple typical values are around 200 ohms. The QER topology changes that.
So, an ordinary Chebyshev or Butterworth filter, which is the basic design of the ladder filter, is NOT
reflected in the Cohn filter, which in turn is complicated by being a QER topology. Theoretically, Dishal
takes that into account...but only theoretically.
The net result is what you are seeing. You are assuming the filter is a Butterworth or Chebyshev, but it is not.
Perhaps the impedance match is closer to 9:1 rather than 4:1. Perhaps not. This is not exactly a simple
subject, and Dishal does not exactly account for the QER design (even though it was modified to do so;
I don't think we yet exactly understand the QER topology). What we do know is that the basic filter is
the inside ladder (or Cohn), but the parallel crystals on the end change things rather dramatically.
First, make sure that you are measuring INTO a 50 ohm impedance both at the input and output of the
filter. Second, make sure that your measuring instruments are really telling you the truth. Then
go by what you measure. You know what the outside world is putting into and taking out of the filter
(50 ohms). Then your measurements will tell you what is needed to match. You will probably have to experiment
a bit to find the "right" combination. However, even a not so good looking filter in theory, may be perfectly
acceptable to our ears. The true test is always how it really performs in our rig.
There was a fairly recent discussion on e-ham.net about directly measuring the impedance of the filter.
Google it. Also look at the various papers and files dedicated to filter design. There are lots...and they to date
still have not figured it out completely. Know that 99% or more of how the filter works is related to how closely matched
the crystals are; the other parameters are important, but the ear hardly notices the difference.
john
AD5YE
