[Hallicrafters] s-76

[Grant Youngman]

On Mar 12, 2010, at 11:31 AM, Carl wrote:

> 
> A 50 kc IF was simply the easiest way to get decent skirt selectivity in the 
> days before multi pole crystal filters became affordable. 

 A low IF frequency also impact "nose" selectivity that can be achieved using LC (really RLC) filters.  The best you can do in terms of nominal bandwidth is about 1-2% of the filter center frequency.  So a 455Khz IF -- say 4.5-10Khz.  If you want  500Khz selectivity, then 50Khz is a better choice.  Quite a few radios did pretty well at voice bandwidths using multiple stagger tuned 455Khz circuits in the IF, but still included a crystal filter for CW bandwidths.   The 50Khz (or similar) IF's eliminated the need for the crystal filter, and have the advantage of better skirt selectivity overall, with the added benefit that the circuits can be loaded to give wider bandwidths for voice, etc.  Hammarlund did the same thing in the HQ-170/180 radios with excellent results.

Grant/NQ5T