[time-nuts] What are these towers?
lowen at pari.edu
Sat May 21 19:43:54 EDT 2011
On May 21, 2011, at 2:51 AM, cook michael wrote:
Le 21/05/2011 08:30, Robert Darlington a écrit :
>> Guys, I gotta ask, what does this have to do with time keeping?
>> Am I
>> missing something?
> I know what you mean. I was desperately fighting down the urge to
> reply to Lamar's post to query the significance of cows and horses.
My apologies for drifting even more off-topic than the OP.
However, there are some serious timing issues present in both AM
broadcast phased arrays. Much of the same techniques are used.
Obviously the first one is basic oscillator stability. Most of the AM
transmitters I have seen have had ovenized oscillators, with assorted
stabilization circuits. The FCC's requirements aren't quite as
stringent to require more than oven-stabilized quartz, but newer
digital stuff does require much more stability.
The second is the need for accurate phase monitoring of a phased
array. This gets us into phase-coherent transmission line issues,
dielectric variance (with accompanying change in propagation
velocity), as well as being able to accurately monitor the phase of
the RF (at up to 1700kHz) to the FCC's precision requirements. Can
you imagine the precision timing/ frequency issues an 11-tower (ten
phase measurements) phase monitor could have?
The third is historical, but this group of all groups should grasp
some of the fundamental issues with the old CONELRAD system. The
basic idea was to throw off incoming missile timing and aiming by
taking all radio stations on the AM band away from their normal
frequency and to either 640kHz or 1240kHz, whichever was the farthest
away from the station's ordinary frequency, and 'timeslice' the
stations all with the same audio program, on the same frequency, but
at different synchronized times. The wikipedia article goes into more
And the fourth area is that of synchronous AM repeaters, to extend an
AM station's coverage using a phase-synced transmitter located at some
distance away from the main transmitter but on the same frequency.
I'll leave as an exercise the explanation of selective fading in AM,
due to ionospheric scatter.
There are other disciplines that benefit greatly from techniques that
'time-nuts' take for granted; high-end analog to digital and digital
to analog converters, for instance, benefit from non-PLL stable clocks
to reduce jitter (at 24 bit samples clock jitter is a significant
noise/distortion issue, at the converters).
So it is tangental, but just barely so, and I apologize for my off-
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