[Hammarlund] Noise Filter

[Richard Knoppow]

-----Original Message-----
>From: Charles Ochs 
>Sent: Aug 26, 2013 9:20 AM
>To: "hammarlund at mailman.qth.net" 
>Subject: [Hammarlund] Noise Filter
>
>I believe this was known as the "Lamb" noise filter, and was the best of all the older designs, for the reasons that you mentioned. Definitely better than the circuit used by National in the HRO and others. I have a Hammarlund-Roberts "Hi-Q" set in my collection, and I always assumed it to have been the beginning of the use of the "Hi-Q" model designation-but I could be wrong. You all know about that word "assume". I have never read anything to substantiate this, but it sounds good to me.  ChuckN1LNH 		 	   		  

     Not sure if you mistyped in the Subject.  James Lamb had two important inventions, a practical and simple crystal filter and a noise blanker.  The filter was used first by National and then by Hallicrafters and probably many others.   The blanker, like the filter was described in QST before being applied to any commercial equipment and then  adopted by Hallicrafters for the SX-28.  All of the noise blankers used since are based on Lamb's idea although the embodiment is quite different in many.  The SX-28 filter was evidently not too successful because Hallicrafters later dropped it in favor of a plain "series-valve" type noise clipper.  The blanker has the advantage that it can be placed before any highly selective circuits which allows it to work on short pulses.  It also works whether or not a BFO is used.  Blankers are very good for auto ignition noise but often don't work on typical power line noise.