[Elecraft] Variable bandwidth crystal filter

[Ron D'Eau Claire]

It's always a good idea to have the selectivity as early as possible in a
receiver. At the antenna input would be ideal, but no one yet has invented a
truly wide-range tunable filter with the performance of a fixed-frequency
filter at a suitable intermediate frequency (I.F.). 

That's why Armstrong's superhet design "took off" in the 1920's even though
it was so much more expensive to implement than the TRF receivers in use
then, and it is still the standard format for virtually all high-performance
tunable receivers today. 

Decent filter performance is available with a tunable filter over a very
limited range, but I've yet to see one that matches a really good
fixed-frequency filter. Still, it's an acceptable compromise in all but the
most severe filtering requirements, it holds down the cost of the receiver
and the results are usually better than adding another level of filtering
nearer the phones or speaker such as DSP at a second I.F. (or an audio
filter in an all-analog receiver). 

Ron AC7AC

-----Original Message-----


It appears that the only way the K3 achieves its high test scores in dynamic
range is when the noise is outside the roofing filter's bandwidth.  Only the
roofing filter yields this high performance.

For example, if your roofing filter is 2.8Khz, but you hear an unwanted
nearby signal, and you use DSP to narrow it to 2.0khz, the reduction of the
unwanted signal will not be as great as if you switched to your 2.0khz
roofing filter (which you had not purchased yet).  This is because the
offending signal is within the roofing filter's 2.8khz bandwidth.

...Thus the interest in the "variable bandwidth crystal" roofing filter as a
major money saver - if it ever comes to fuition.

Howard