[Elecraft] OT mechanical v crystal filters OT

John Magliacane kd2bd at yahoo.com
Tue May 8 12:26:41 EDT 2007


> Off Topic but with so much expertise flying around I thought I would ask how
> mechanical filters compare with crystal filters.  
> 
> I think they were fairly low in frequency (455kHz?), so, not suitable in the
> present discussion,  but it would be good for our education at least.
> 
> David
> G3UNA

Hi David.

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

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.

>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"!)

My K2/100 experiences have lead me to believe that these effects occur with HF
SSB crystal filters as well, but more so at the passband edges rather than in
the center, provided it is essentially flat.  The K2's filter shape is slightly
asymmetric, so the filter edge with the sharpest cut-off is the one where the
greatest ringing occurs.

In order to minimize the effects filter ringing can have on SSB audio quality,
I have found it advantageous to attenuate the audio frequencies that strike the
corners of the sideband filter (prior to the balanced modulator), especially
when the filter operates at HF (rather than lower) frequencies because the 'Q'
is so high at these points.

(This isn't so much of an issue in reception because SSB signals have already
been bandwidth limited in the other person's transmitter.)

Just some greatly over-simplified and otherwise totally random thoughts on the
subject...


73, de John, KD2BD


Visit John on the Web at:

	http://kd2bd.ham.org/
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