[time-nuts] What are these towers?

[Bob Camp]

Hi

At least back when dirt was new and I got my first phone license the accuracy required for AM broadcast was very loose. What drove them to OCXO's was the need to use X cut crystals rather than AT's. The age of digital dividers had not yet dawned, so if you wanted 500 Kcps you used a crystal at that frequency or some sub-multiple of that frequency.

Bob


On May 21, 2011, at 7:43 PM, Lamar Owen wrote:

> 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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