FM and IBOC TX freq

[kcbooboo]

In a configuration where the analog signal comes from one transmitter/antenna and the IBOC signal comes from a second transmitter/antenna, I can easily measure the frequency of the analog transmitter by removing modulation and looking at the mod meter sample jack.

As the IBOC transmitter is only outputting the digital carriers between (roughly) +/- 100 kHz and +/- 200 kHz, how does one check and/or measure the frequency of that signal? The analog and digital transmitters are not connected together so the frequency of one won't affect the other. In a combined system where the IBOC exciter provides both the analog and digital signals and just a single PA/antenna is used, this is not a problem, but when they're two separate transmitters and antennas, it is. No GPS antenna is installed.

I'm guessing that I would have to reconfigure the IBOC transmitter so it outputs just an analog carrier and no digital carriers, then measure that. Has anyone had to check the frequency of an IBOC transmitter? Are there other ways?

Bob M.

[BroadcastDoc]

WIth IBOC, you're concerned with occupied bandwidth and analog:digital ratio as opposed to frequency. You'd take a look at it in a spectrum analyzer - with and without the analog carrier on.

[kcbooboo]

However, if the digital carrier master oscillator is 2 kHz high, will anyone see or even notice that the entire spectrum has shifted 2 kHz, when the span is at least +/- 200 kHz? That's less than 1% of the screen width and I bet most people would never even think about zooming in to check that. Hence the original question about checking and possibly adjusting the center frequency of the digital exciter.

It may never be possible for that oscillator to be so far off frequency, but stuff happens. If I'm already at the site measuring the analog carrier frequency, might as well do the digital side of things, especially when it's a completely separate TX, feed line, and antenna.

Bob M.

[RFWarrior]

However, if the digital carrier master oscillator is 2 kHz high, will anyone see or even notice that the entire spectrum has shifted 2 kHz, when the span is at least +/- 200 kHz?

I think that was Doc's point (hopefully he'll clarify if I'm mistaken ) - since we're only concerned about OBW on the digital carriers, whether or not they are centered on the carrier frequency is irrelevant, since the receivers are only looking for the analog carrier as a starting point to locating and decoding the digital carriers - the only significant effect that might result is increased time for the HD receiver to lock on to the digital carriers. Since, like AM, the digital signals are GPS clocked, for the digital transmitter to be off frequency by any significant amount would be a rarity.

Best,

Jeff

[BroadcastDoc]

Yep, that was my point. As long as you're in the mask, the frequency of the IBOC carriers is largely irrelevant.

[kcbooboo]

I think my point is that the digital carriers could be within the mask, as far as the spectrum analyzer is concerned, yet the center frequency of the digital exciter could be off by more than 2 kHz. The SA isn't a high-precision instrument when it comes to absolute frequency and unless you happen to notice that the upper or lower edge of the waveform is right on the mask line, you may not even give the overall frequency accuracy another thought. In today's deregulated environment, I don't even think the FCC specifies the frequency tolerance of the IBOC system.

Thanks.

Bob M.

[w9wi]

Strictly curious, how far off-frequency do receivers look for the carriers?

[RFWarrior]

I think my point is that the digital carriers could be within the mask, as far as the spectrum analyzer is concerned, yet the center frequency of the digital exciter could be off by more than 2 kHz. The SA isn't a high-precision instrument when it comes to absolute frequency and unless you happen to notice that the upper or lower edge of the waveform is right on the mask line, you may not even give the overall frequency accuracy another thought. In today's deregulated environment, I don't even think the FCC specifies the frequency tolerance of the IBOC system.

Well, no, I'm sure they don't - but again, if the digital carriers are in the mask (i.e., within their occupied bandwidth) and since the digital transmitter operates in a DSB suppressed carrier mode, I can't think of any reason why the nominal center frequency would be of any significance whatsoever, except insofar as it could affect the ability of receivers to lock on to the digital carriers. There's technically no signal there to be off frequency. Since receivers are "theoretically" designed to see the digital carriers anywhere within the range of allowable occupied bandwidth, the only issue would be longer acquisition/lock times if they have to work to detect and decode digital carriers that are off center.

Strictly curious, how far off-frequency do receivers look for the carriers?

Good question, Doug - I don't know exactly. Theory says that as long as the carriers are within the spec'd occupied bandwidth, the receiver should be able to detect them. I've never purposely tried to test this - but offsetting a digital transmitter in 1kHz increments of frequency relative to the analog carrier wouldn't be an impossible task, might be fun to try it someday! Any volunteers?

Best,

Jeff

[eadler]

I've never purposely tried to test this - but offsetting a digital transmitter in 1kHz increments of frequency relative to the analog carrier wouldn't be an impossible task, might be fun to try it someday! Any volunteers?

If only someone had a place where they test transmitters and could tweak parameters like that...

[RFWarrior]

If only someone had a place where they test transmitters and could tweak parameters like that...

Wait! I know somebody like that!!! I was just thinking it would be more fun into an antenna... with somebody else's equipment.... Besides, into a combiner and a dummy load, you don't get to create nearly as much havoc on the air!!!

Best,

Jeff