[Milsurplus] Re: BC 604 FM modulation Milsurplus digest, Vol 1 #578 - 4 msgs

[Marty R's GI-stuff haunt]

>  So why didn't Galvin just use the Armstrong phase modulator? 

There's at least 3 theories

  - Technical.  See below.

  - Perhaps NIH on Dan Noble's or Galvin's part?

  - Then the patent idea.  There wasn't a war in 1940 & god only knows
    the difference between 'temprora et mores' in courts then vs. later

   Marty

 = = = = = = BELOW = = = = = =

  The Armstrong phase modulator only gave about 22 degrees useful
  phase shift.  (third of a radian)  So what?  Here's what, look 
  at the formula

          delta-phase = delta-freq./modulation freq. (radians)*

  Consider 10kc deviation and 1kc modulation.  Then above quotient
  is 10.  And 30x multiplication needed from phase modulator.  So
  to get to 20mcs you'd start at 600kcs - right in the BC-604 regime 
  (all those FT171 Channel XX xtals are down there)

  Now the saturable reactor "FM modulator" wasn't, apparrently, good for
  much more phase shift but it it's output was more harmonic-rich - there
  was measurable 9th harmonic output & it looked like a way to reduce
  parts count.

  But it seems unexploited in the production 604s since they were
  all 32x multiplication.  So after all the fuss, it was "uh, oh - back 
  in old Lodi again"



* I know there's a 'flat world theory' on phase modulation being
  fundamentally different from FM.  Well PM does give a 'pre-emphasis'
  effect that must be treated after detection.  But they both
  detect the same.

  All the FM ham jobs & GI FMs since the BC-1000 use PM.  Probably.