[time-nuts] What are these towers?

[Lamar Owen]

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?
>>
>> -Bob
>>
> 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  
depth.

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- 
topic contributions.....
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