What I find interesting is uBitx V3 though 6 the terminations for the filter that is
the crystal filter plus matching network is between a Mixer (DBM) and TIA amp.
Both of which are NOT 500 ohms and are optimized for 50 ohms.? So the matching
network has that as its?input and output? terminations.
The filter is characterized as 1500ohms 2pf That is not what it presents,
its what it expects as a load to produce the desired and expected filter shape.
However... I have parts tha texpect 470ohms to as high as 2000ohms resistive
for the termination. So you do need the data sheet for the filter in use.
The above is critical to the filter used as when done tight the loss though the
filter will match the data sheet and also the passband frequency and shape will
be as specified on the datasheet.? Crystal filters are not an exact match to LCR
filters, most LCR fitlers are designed for restive termination where most crystal
filters are designed to expect some reactance as part of the termination.? Altering
that alters the filters behavior (pass band and shape).
Also I have the experience with V3 and a lot of testing.? I found with the supplied
filter and the matching network it performed only marginally well.? Testing using VNA
(Agilent) the matching network values used both skewed the filter response and
created a large amount of ripple in the pass band. causing RX degradation and
also degrading RX and TX audio (for mine).? The values I used were close to
calculated once I had filter data.? I did have to adjust for the trace to ground
capacitance.? The results when properly matched improved.? The filter
pass band was properly centered at 45.000mhz and the loss was lower.?
FYI the filter used wanted a termination of 1500ohms and parallel of 3pf
as a match.
And like Gerry said its not a pure resistive match.
--
Allison
------------------
Please use the forum, offline and private will go to bit bucket.
