[Elecraft] OT mechanical v crystal filters OT

[Brett gazdzinski]

John, 
Please describe the quadrature phasing part of your receiver.

On weak signals, I often find narrower filter settings of the K2
make the signal harder to copy.
Part of that might be my bad hearing and the higher pitch
sidetone I like...
The noise and the signal seem to blend into one pitch...

I suppose a receiver with a 50 KHz or 100 KHz IF
frequency would work quite well in regards to ringing.......

 
> Mechanical filters, which operate in an acoustical domain, 
> may have somewhat
> higher insertion loss and lower dynamic range than properly 
> designed crystal
> filters.

They operate at 455 KHz, I don't think even dogs can hear that high!



> 
> However, I've heard some glowing on-air comments regarding 
> the performance of
> the old Collins mechanical filters, especially with respect 
> to CW reception.
> 
> Well-designed direct conversion receivers are often noted for 
> their superior
> audio quality compared to highly filtered superhets.
> 
> Recently, I've designed a highly filtered (100 Hz bandwidth) 
> quadrature phasing
> direct conversion receiver, and was somewhat amazed by the 
> clarity of CW
> reception.


Was not the old heathkit hw7 a direct conversion receiver?
I had a lot of fun with that rig...


> 
> >From these observations I have concluded the following:
> 
> Narrow-band reception requires sharp selectivity (obviously). 
>  The higher the
> frequency at which the selectivity takes place, the higher 
> the required 'Q'. 
> The higher the 'Q', the greater the amount of ringing and group delay
> distortion that will occur.  Ringing and group delay 
> distortion cause listener
> fatigue.
> 
> In a direct conversion receiver, a 100 Hz bandwidth can be 
> obtained at 1000 Hz
> with a 'Q' of only 10. (Very low ringing)
> 
> A 100 Hz bandwidth at an IF of 455 kHz, however, requires a 
> 'Q' of 4550. (More
> ringing)
> 
> A 100 Hz bandwidth at an IF of 5 MHz requires a 'Q' of 
> 50,000.  (An RF "echo
> chamber"!)

Brett
N2DTS