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

paul swed paulswedb at gmail.com
Thu Oct 13 11:50:01 EDT 2011

Talk about a great write up on station stability.
I thought that there might be some AMs that were GPS locked.
So in reality if the ole US Government wanted a cheap backup
freq dissemination they might easily have it after all with a bit of
thought. (Sorry. I used a silly concept)

That certainly would be an approach thats easily implemented. As for the
offsets. If they are known I do not feel they are that much of an issue
today. Seems like we have a lot of tricks in the bag such as DDS etc.

Time might be transmitted on a subcarrier... Or could it be sent on the main
carrier slowly?
Alas all silly talk accept for actually looking at and monitoring one of the
suggested stations.
Time to warm up a HP 3586 locked to GPS and spectrumlab for some fun

On Thu, Oct 13, 2011 at 10:38 AM, Mike Seguin
<n1jez at burlingtontelecom.net>wrote:

> 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
> ---
> 73,
> 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.
>>> Regards
>>> Paul
>>> WB8TSL
>> Burt I. Weiner Associates
>> Broadcast Technical Services
>> Glendale, California  U.S.A.
>> biwa at att.net
>> www.biwa.cc
>> K6OQK
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