It was patented by TenTec in 1988 (US patent #5051711)
commonly known as the Jones filter after the designer. It
is used in many of the TenTec transceivers.
Also used in several of the Elecraft transceivers and
other manufactures as well as in many homebrew QRP
transceivers. There is quite a bit of information on
these types of filters if you search for variable
bandwidth crystal filters. The circuit in the patent
document is pretty close to what is commonly used, except
that most implementations use a fixed resistors instead of
the variable resistor that allows tailoring the bandpass
shape. The only problem I remember reading about is that
along with the bandwidth, the center frequency also
changes slightly. Somewhere I read about a slightly
different circuit that keeps a constant center frequency
when the bandwidth is changed, unfortunately I can not
find the article at this time.
On 9/14/2015 6:36 PM, Will zl1tao@...
Would it be practical to use varicap diodes to vary
bandwidth of xtal
filters? Instead of using fixed value caps it is
obviously possible to
replace or use in parallel with fixed capacitor
between xtals to ground.
However would the low Q of varicaps and or the
resistor used to feed the
DC to them upset the Q too much. Also would it effect
impedances so that the matching was not matched.