[Hammarlund] Hammarlund Crystal Filter Patent

[Richard Knoppow]

     Hammarlund had a patented crystal filter which was 
introduced in the HQ-120-X and used in the Super-Pro, 
SP-600-JX, and other receivers. It is the same circuit used 
by Collins and I think TMC on the GPR-90. It is superior in 
many ways to the original Lamb Single Signal filter 
introduced by National and used by them and by 
Hallicrafters.
     The patent is _Selective Wave Transmission_, USP 
2.222,043 applied for in 1939 and issued in 1940. I had 
thought it was earlier. The inventor is Donald K. Oram.
     I had searched for this several times in the past but 
did not have the patent number. Google Patents, while a very 
powerful tool, has some definite shortcomings and one of 
them is that it often does not get text searches right. In 
this case, it fails on the inventor's name and also fails 
for Hammarlund Manufacturing missing this and many other 
patents held by the company.
     The drawings make the operation fairly clear (for a 
patent) but do not show one of the important features, 
namely that the notch filter function is independant of 
center frequency, which is not the case for the Lamb 
circuit. It also has constant gain with bandwidth variation 
another improvement over the Lamb filter.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com