Bert: 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 secrets.
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. 73 Les Lester B Veenstra MØYCM K1YCM les...@veenstras.com m0...@veenstras.com k1...@veenstras.com US Postal Address: PSC 45 Box 781 APO AE 09468 USA UK Postal Address: Dawn Cottage Norwood, Harrogate HG3 1SD, UK Telephones: Office: +44-(0)1423-846-385 Home: +44-(0)1943-880-963 Guam Cell: +1-671-788-5654 UK Cell: +44-(0)7716-298-224 Jamaica: +1-876-352-7504 This e-mail and any documents attached hereto contain confidential or privileged information. The information is intended to be for use only by the individual or entity to whom they are addressed. If you are not the intended recipient or the person responsible for delivering the e-mail to the intended recipient, be aware that any disclosure, copying, distribution or use of the contents of this e-mail or any documents attached hereto is prohibited. -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of Burt I. Weiner Sent: Thursday, October 13, 2011 7:33 PM To: time-nuts@febo.com Subject: [time-nuts] GPS referencing of broadcast stations... Hi Mike, 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. Burt, K6OQK www.biwa.cc At 07:39 AM 10/13/2011, time-nuts-requ...@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. b...@att.net www.biwa.cc K6OQK _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.