Re: WLLH-AM Lawrence transmitter off-air

[John Mullaney <john@minutemancomm.com>]

I realize this is a tad different but might interest some people in the area of
frequency synchronization ... The Mass State Police's new 800 Mhz trunked radio
system has multiple synched transmitters working together all over the State.
Now these are are FM transmitters connected by a large statewide microwave
system so the charistitics are a little different but the transmitters frequency
stabilization is controlled with some sort of GPS system. When you are between
two of them you hear kind of a mobile flutter kind of sound but with more of a
regular pattern.
Some sites that explain the Trunk are at:

http://lynx.dac.neu.edu/s/stjohnso/ematrunk/msp/msp.html
http://24.128.24.108/massstate.ram



Dan Strassberg wrote:

> Bingo! Absolutely correct. Engineers have been fiddling with synchronization
> schemes for AM TXs since the 20s, when oscillator frequency stability was
> dreadful by the standards of even 20 years ago. What a thread could get
> started on this one--if anyone had enough details; unfortunately, even I am
> not old enough to remember any of the sync deals first-hand. Back when the
> whole sync idea started, there was apparently no appreciation of phase noise
> in frequency multipliers. The scheme that was apparently most often used was
> to divide the "master" station's carrier down to audio, send the audio over
> ordinary phone lines, and multiply the frequency back up to the original at
> the "slave" station. Unless the divider and multiplier have zero phase
> noise, the nulls between the two signals walk around within the hash zones,
> giving the listener the same effect as a beat note between carriers at
> slightly different frequencies. It's best if the hash zones can be confined
> to areas where hardly anyone lives. In such cases, the frequency stability
> of today's crystal oscillators is quite good enough. Best example is KIPA in
> Hilo HI on 620. KIPA runs 5 kW-U ND and has two "synchronous" TXs, one with
> 5 kW-U ND and the other with 10 kW-U ND (and a Paran antenna--but that's yet
> another thread). They are all on the same island--at different points along
> the coast, but it's a large, triangular-shaped island and is very sparsely
> populated between the major communities on the coast and pretty much
> everywhere inland. The hash zones mostly exist in the areas of low
> population density.
>
> --
>
> Dan Strassberg, dan.strassberg@worldnet.att.net
> Phone: 1-617-558-4205, eFax: 1-707-215-6367
>
> -----Original Message-----
> From: SteveOrdinetz <steveord@wavewizard.com>
> To: dan.strassberg@att.net <dan.strassberg@att.net>
> Cc: BostonRadio <boston-radio-interest@bostonradio.org>
> Date: Monday, January 10, 2000 11:32 PM
> Subject: Re: WLLH-AM Lawrence transmitter off-air
>
> >Does there need to be phase synchronization as well as frequency sync for
> >this to work?  It would seem to me that if the signals were in phase with
> >each other the hash zone would be smaller.
> >