[time-nuts] AM Broadcast stations as a frequency reference...

Mike Seguin n1jez at burlingtontelecom.net
Thu Oct 13 14:38:52 UTC 2011

We don't have any AM stations, but the big reason for GPS locking of 
our FM stations for IBOC is to stabilize diversity delay.

The Analog component of the audio for HD-1 is delayed to synchronize 
with the digital components so that if the digital carriers are lost, an 
IBOC receiver will "blend" back to Analog. If it's not synchronized with 
in a few samples, you hear a jump in the audio.

Mike - Vermont Public Raido Engineering

Mike, N1JEZ
"A closed mouth gathers no feet"

On Thu, 13 Oct 2011 07:27:02 -0700, Burt I. Weiner wrote:
> There are some AM broadcast stations in the U.S. that are GPS
> referenced.  This has primarily come about as a result of Digital
> (IBOC) Broadcasting.  I do off-air frequency measurements for many
> stations, primarily in the Southern California area, so I have a
> fairly good idea of who in this area is doing what and their 
> reference
> reliability.  In my local area there are three AM stations that are
> reliably referenced to GPS most of the time.  They are: KFWB 5,000
> Watts non-directional on 980 kHz, KNX 50,000 Watts non-directional on
> 1070 kHz, and KDIS 50,000 Watts directional on 1110 kHz.    These
> three stations seem committed to the IBOC system, but for how long,
> who knows. There are other's, but with them it is sort of an on 
> again,
> off again situation.  A couple of caveats are that these stations may
> not always be operating on their GPS referenced exciter or
> transmitter.  These days, individual transmitter maintenance is 
> mostly
> done during the daytime and only antenna repairs are generally done
> during the nighttime hours.
> Not all stations broadcasting digital IBOC are GPS locked.
> Supposedly being GPS locked would allow receivers that are GPS locked
> to lock up faster and stay locked better.  So far I don't know of a
> single IBOC receiver that has the capability of being GPS referenced,
> so the GPS referenced transmitter has never been of great importance.
> I know of several AM stations that have the capability but have never
> bothered to hook up the GPS antenna to their GPS capable exciter and
> really have no interest in doing so.  In many cases, AM stations have
> shut down their IBOC systems due to lack of interest on the part of
> the listening radio audience and in some cases due to adjacent 
> channel
> interference caused by the digital sidebands.  In some cases they've
> actually removed the equipment from their racks and are using it,
> appropriately enough, as something to sit on.
> So, the bottom line is that while there are some GPS referenced AM
> broadcast stations out there, they are not a reliable day to day
> reference.
> Another aspect of this whole thing is that while it would be simple
> enough to GPS lock/reference an AM broadcast transmitter, this may 
> not
> always be desirable.  Having two co-channeled stations precisely on 
> or
> very near the same frequency, even though they may be thousands of
> miles apart, can have serious fade issues.  If they are exactly on 
> the
> same frequency they can cause deep nulls in their coverage depending
> on the relative signal strengths.  Paging transmitters have in the
> past gone to precision offsets of a only a few Hz in order to get 
> what
> they called "rolling fades.  While this may be desirable for paging
> systems where data can be repeated several times, it can be very
> annoying in the broadcast radio world.  A listener, depending on
> location, could hear consistent deep cyclic fades.  As most of you
> know, AM broadcast stations are allowed a carrier tolerance of +/- 20
> Hz.   Co-channel fades are much more tolerable to the listening
> audience if they are in the range of 15 to 20 Hz.  While this 15 to 
> 20
> Hz offset may cause some low frequency intermodulation (tone) in the
> audio, it is much less objectionable than a deep, fraction of a Hertz
> difference causing a station to slowly come and go.  Because of this 
> I
> have several clients that have deliberately opted to go with an 
> offset
> of 10 to 15 Hz, depending on the absolute frequency of an interfering
> co-channel station.  While broadcast engineers are a scientific 
> bunch,
> they are never the less in "Show Business", and the need to present
> their programming (such as it is) in the most listenable manner comes
> first.
> Burt, K6OQK
>>Well its a funny thing actually. Looking at an amateur effort its not 
>> all
>>that hard at 1.6-7.0 MHz to generate reasonable power of say 100-500 
>> watts.
>>Certainly its not hard to create an exciter at those frequencies that 
>> are
>>derived from a quality reference. Heck many time nuts have CS 
>> references
>>etc. the concept is quite flexible if you consider adding some pahse
>>modulation perhaps. All in all pretty do-able. A few things tend to 
>> get in
>>the way like rules and regs etc.
>>But there is an alternate that would be very reasonable. The 
>> broadcast band
>>AM transmitters run all the time and if their exciters were 
>> controlled by
>>the reference you would have major portions of large areas covered. 
>> The
>>Broadcasters are already paying for the power, transmitter, and 
>> antenna upkeep.
>>Of course nothing like that would happen and am modulation can have 
>> effects
>>on the carrier.
> Burt I. Weiner Associates
> Broadcast Technical Services
> Glendale, California  U.S.A.
> biwa at att.net
> www.biwa.cc
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