[time-nuts] GPS referencing of broadcast stations...
lester at veenstras.com
Thu Oct 13 15:56:04 EDT 2011
I wonder if you happen to remember a company called Cambridge
Thermionics. Located in Cambridge MA, they made ceramic slug tuned coil s
but in one corner was an individual with the off air frequency measuring
service. As a duty engineer up the read at WCOP in Lexington, I frequently
would get calls from him to tick a dummy plug in the modulator input patch
(turning the board gain down was too much residual noise), so he could
measure us, and more often, to remove the carrier for a few seconds so you
could measure some one co-channel. He never wanted to talk about how it did
it, and absolute would not accept visitors who might learn his dark trade
I had assumed that these days a GPSDO would remove the need for the
monthly "freq service" but I guess not.
I stood my last midwatch at COP and reported to the Boston Army station for
induction into the USN the next morning.
Lester B Veenstra MØYCM K1YCM
lester at veenstras.com
m0ycm at veenstras.com
k1ycm at veenstras.com
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From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Burt I. Weiner
Sent: Thursday, October 13, 2011 7:33 PM
To: time-nuts at febo.com
Subject: [time-nuts] GPS referencing of broadcast stations...
Several years ago there was some discussion about using a FM carrier
as a calibrating signal by driving close to a tower supporting a FM
broadcast antenna then adjusting the counter's time base so the
display agrees with the station's assigned frequency. The fellow
suggesting this soon found out that this was not such a great idea.
One of the most annoying problems I have in measuring FM stations is
the time involved. I need to catch the carrier, as the FCC refers to
it, at rest. This can take quite a while and I have to stare at my
equipment and not blink. In the olden days I could call the station
at 2 or 3 o:clock on Monday mornings during transmitter maintenance
or get them to remove modulation for 2 or 3 seconds by having all
faders down at the end of a song in order to get a quiet
carrier. Now-a-days, if the PD thinks there's even 1 second of
silence at 2 AM, all the listeners will all tune out never to come
back, they will not show up in the ratings, the station will go
bankrupt and they'll all lost their jobs and of course it will be his
fault for allowing it. It's not the FM modulation that's so much the
problem as it is the exciter's AFC loop chasing modulation. A real
good demonstration of this is to beat the incoming FM carrier against
a stable signal generator. If you were to suddenly remove the
modulation, say pull the BNC off of the composite input, you would
hear the beat flop back and forth until it settles. How fast and how
much it settles depends on the particular exciter and its AFC loop
constants. Some settle almost instantly, other's may take 2 to 3
seconds regardless of the reference source.
As you know, with FM exciters there is a problem with truly locking
the carrier (FMO) to any reference. Definitely you can tie the
exciter's internal reference to a GPS reference and lock that part of
the exciter. The real problem comes when you try and "hard lock" the
FMO portion of the exciter to that reference. The FMO has to be able
to be "springy" enough to be modulated by the lowest modulating
frequency; that was the whole point of moving away from crystal
controlled FMO's or multiplying a bazillion times back in the 40's
and 50's. If you were to hard lock the FMO to the reference it would
almost be the same as simply replacing the FMO with a crystal
controlled oscillator. I know there are some folks who are field
modifying the time constants and going to a lower impedance to drive
the varactor modulator diodes to reduce modulation peak overshoot,
but you can only take that so far before you lock the modulation
right out of the beast.
As far as synchronizing the audio between the digital and analog
signals goes... Well, I have a Radio Shack Accurian, a Radioosphphy
something or other, and a Boston acoustics receiver. None of them
agree in the delay through them in spite of using iBiquitiy's
software - and the difference can be quite noticeable. I suppose the
best you can do is to go by the delay in your IBOC modulation monitor
and hope for the best. A buddy of mine goes by his PD's radio which
results in the best sleep at night.
I realize my two diatribes have been sort of off topic for the group,
but since the subject of using broadcast signals as references had
come up I though I'd squeak up from my normal lurking.
At 07:39 AM 10/13/2011, time-nuts-request at febo.com 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
Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
biwa at att.net
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