Xtal Filters & Noise Limiters

[pwdrive]

Here is a link to the basics of single xtal filters as used in the early comms receivers explaning the phasing/selectivity arrangements etc also some info on amplified noise limiters.

http://www.jumpjet.info/Pioneering-Wirel...936-04.pdf

Lawrence.

[Skywave]

Yes, two well-written and informative articles.
I see that amongst the directors of that organisation, the name of E.H. Armstrong appears. I assume that that is THE Mr. Armstrong?

Al.

[pwdrive]

Hi Al, yes I would take it to be thee Armstrong the great man himself, the basis of my post was to try and find a non to technical explanation of how that type of single xtal filter works as it was a very common type employed, there were a few slight variants ie: center tapped feed winding, center tapped capacitor feed etc. but all basicaly the same principle.
So far as I understand it, it is a bridge circuit, the phase control capacitor being used to balance out stray capacitance in the xtal arm (xtal holder etc) it also acts to tune the parallel resonant notch frequency of the xtal, this feature being useful for nulling out hetrodyne whistles ie: Xtal at series resonance then that arm of the bridge is in effect a very sharp high Q series acceptor circuit balanced by the phasing capacitor, Xtal at parallel resonance then that arm of the bridge is a very sharp rejector circuit the rejection notch being tunable by the phasing capacitor, have I got this right? Any input welcome.

Lawrence.

[pwdrive]

Here is another good read on Xtals from one of the manufactures, a description of the single xtal filter as used in early receivers is also to be found in it, it's in the Xtals At Resonance section. This manufacturer supplied many of the Xtals used in the early American receivers.

http://www.bliley.net/XTAL/docs/eng_bull...lletin.pdf

Lawrence.