[Elecraft] Up-conversion.

[Geoffrey Mackenzie-Kennedy]

> There are some many practical problems with the holder capacitance,
> stray capacitance and the like that would make such a filter challenging, 
> even if someone were to deliver a box of 45 MHz crystals with measured Qs 
> of 2 million to my doorstep. And if  the box of crystals arrive, to obtain 
> frequency stability might require stabilizing the filter assembly in a 
> temperature controlled oven.
>
> The typical roofing filters at 45 MHz have a bandwidth of 20 KHz or so. 
> Thus the fractional bandwidth is 50 times larger and the Qx is down into 
> the 100K range, making these filters relatively easy to realize.

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An interesting 42.5 MHz filter design appeared several years ago using just 
four crystals in ladder configuration. The designer was not looking for 
narrow bandwidth nor good shape factor, but illustrating a "method". The 
bandwidth was 5 kHz, the shape factor was 4, stopband better than 80db and 
the response was symmetrical. In addition to the usual inductors across each 
crystal to take care of holder capacitance, as inspired by Zobel, two other 
other modifications were incorporated. (1) Parallel tuned circuits were 
placed at each end to prevent degeneration into a poor Cauer lowpass filter. 
(2) Parallel tuned circuits were used as coupling elements, presumably to 
tweak mesh frequencies. Insertion loss unknown, but a fair number of 
elements to compensate for temperature change.

73,
Geoff
GM4ESD