[Hallicrafters] s-76

[Carl]

>
> 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


Getting the nose selectivity that is wanted was assumed in my prior post.
The low IF concept goes back into the 20's with early superhets but the 
double conversion ability to get rid of the pesky images was still in the 
future until better tubes and higher frequencies were occupied by commercial 
interests.

Philco had RLC variable selectivity in 35 and Hammarlund introduced their 
variable coupling IF's at the same time.

Zenith developed the mechanical filter in 1939 and tried to sell it to 
Collins who turned it down and went on to pretty much steal the idea in the 
early 50's. Zenith had a few consumer models with the filters on the market 
in 40-41 with good reports but it was uneconomical in their main price range 
market.

Collins had no problem getting down to 1 kc selectivity with LC filters at 
455 kc in the R-390. All it took was money.

Carl
KM1H