Re: Xtal filter bandwidth

DuWayne Schmidlkofer

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.
DuWayne KV4QB

On 9/14/2015 6:36 PM, Will zl1tao@... [PHSNA] wrote:

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 the termination
impedances so that the matching was not matched.


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