The Old versus the New -- Why bother with the new one?
I recently measured the most recent bandpass filter (built in 2010) and
the original filter used in the 1998 QST Spectrum Analyzer. The
measurement was done with a spectrum analyzer and tracking generator that
tune a bit beyond 500 MHz, an instrument that was not available for the design
of the 1998 QST SA. The two filters have nearly identical response
at 110 MHz, but the new filter is much better in the UHF stopband.
These measurements scan from 60 to 560 MHz.
This is the result obtained with the old
filter. The junk at the right results from a filter construction
without shields between resonators. An auxiliary low pass filter
cascaded with the bandpass should eliminate these spurious responses.
Here is the response measured with the new filter.
None of the UHF feedthrough is evident, even though no lid was
included with this filter. The extra low pass filter should not be needed
with the new bandpass circuit.
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