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KC9TNQ
10-24-2011, 10:12 PM
Hello all, I was observing the FM broadcast spectrum in the frequency domain and all stations looked like blobs as I assumed they would:
76706
But I came across a talk station which when silent ( in-between words ) would turn into three distinct signals, one at 0hz from center and two at +/- 19khz:
76709
I presumed the two outside signals are the stereo pilot, but that is modulated and should not appear on the spectrogram. I now believe they are the FM sidebands produced by modulating the pilot tone and they are at 19khz by chance. Can someone enlighten me as to what they are or just point me to the math that would explain them?

Thank you all!

K7JEM
10-24-2011, 10:29 PM
Hello all, I was observing the FM broadcast spectrum in the frequency domain and all stations looked like blobs as I assumed they would:
76706
But I came across a talk station which when silent ( in-between words ) would turn into three distinct signals, one at 0hz from center and two at +/- 19khz:
76709
I presumed the two outside signals are the stereo pilot, but that is modulated and should not appear on the spectrogram. I now believe they are the FM sidebands produced by modulating the pilot tone and they are at 19khz by chance. Can someone enlighten me as to what they are or just point me to the math that would explain them?

Thank you all!

An FM multiplex signal will indeed appear as 3 separate signals, as you have observed. But that 19KHz pilot is not modulated at all, it is simply inserted into the modulator at about 10% deviation. The stereo signal is actually modulated by impressing the L+R audio onto the main modulator, and the L-R audio onto a subcarrier centered at 38KHz. That modulator is a DSB suppressed carrier signal. The RX takes the pilot, doubles it, then reinserts the carrier at 38KHz, then it is demodulated, summed with the main channel, and subtracted from the main to develop audio for L and R channels.

Music stations are so heavily compressed that you rarely see the three signals, since there is almost always something modulating them.

Joe

KC9TNQ
10-24-2011, 10:47 PM
Right but the pilot is still frequency modulated with the rest of the signal and so should not be visible as a separate signal from the main FM signal, it should be one carrier moving back and forth at 19khz, right?

Edit: to clarify that picture is of the signal BEFORE FM demodulation and so the audio and pilot shouldn't be visible.

K7JEM
10-24-2011, 11:09 PM
Right but the pilot is still frequency modulated with the rest of the signal and so should not be visible as a separate signal from the main FM signal, it should be one carrier moving back and forth at 19khz, right?

Edit: to clarify that picture is of the signal BEFORE FM demodulation and so the audio and pilot shouldn't be visible.

The pilot is a pure 19KHz signal modulated on the main channel. If you look at it on a spectrum analyzer, you see the main channel and two sidebands spaced at 19KHz, where the only modulation is, when there is no program material being transmitted.

FM signals look very complex on a spectrum analyzer, when there is decent modulation. When there is no modulation, there is only a carrier. When there is a single tone modulating the TX (as is the case with a multiplex TX and no program material), you will see a carrier and sidebands spaced at integers of the modulating frequency, depending on specific deviation of that tone. Since the pilot is transmitted at 10% deviation, we see only one significant sideband on each side of carrier. If we were to increase the deviation of the pilot, we would see multiple sidebands at 19, 38, 57, 76, etc as the deviation increased. We would also see a Bessel null of the main carrier when the deviation of the pilot reached 45.6 KHz of peak deviation. Bessel nulls are interesting to look at if you have the equipment to produce them and look at them. That is the way FM transmitters can have an accurate deviation set on them, precise down to less than .1 db. But that is beyond your question.

Joe

KC9TNQ
10-24-2011, 11:37 PM
you will see a carrier and sidebands spaced at integers of the modulating frequencyWhy? The distance the carrier is from the center is determined by amplitude of the signal not the frequency, correct? Then why do the sidebands relate directly to frequency, wouldn't it depend on the deviation and final frequency? Also the sidbands are at ~18khz not 19khz, and I sort of see 5 signals now with more gain on my equipment -36khz, -18khz, 0, 18khz, -36khz. So if I change the pilot frequency how will this effect the final output, what if a change the pilot amplitude?

Thank you for your help!

K7JEM
10-25-2011, 12:33 AM
Here's a couple of pictures. The first one is a diagram showing significant sidebands on an FM TX. The second picture shows a spectrum analyzer photo of an FM TX at Bessel null. You will see that the carrier has disappeared entirely, and the sidebands remain.

You are probably seeing the 19KHz sidebands, your resolution just shows it at 18. And the second sideband at a much lower level, at 38 KHz. If you were to increase pilot level, the sidebands would increase in amplitude, and a third significant sideband would show up, then a fourth and fifth.

FM has a theoretical infinite bandwidth, if you do the math.

Click the photos for a larger image.

76714

76715

Joe

WB2WIK
10-25-2011, 01:16 AM
Yeah, it's 19 kHz not 18 kHz, guaranteed.

Many FM BC stations also have pilot carriers to carry other data as well. As soon as "Stereo FM multiplex!" became available several decades ago, this has been the case. If you watch on an analyzer, you'll see the carrier null completely when the modulation hits the right frequency. Doesn't matter, they're still really there.:o

KC9TNQ
10-25-2011, 01:32 AM
Hmm I wonder if I could use GNUradio to slowly move the deviation and plot the results frequency/time domain like my pictures, I would love to see the carrier and sidebands changing and interacting.

Also if the stations deviation or modulation index is not right could the pilot end up on 18khz? ( I'm sure it's not, just wondering if it's possible )

Edit: Also I see why it looked like 18khz, the center carrier is at -1khz, I must have some frequency drift going on... at 100Mhz its only 10ppm, not to bad.

KB3LIX
10-25-2011, 01:42 AM
If the pilot subcarrier was at 18kHz, the receivers would NOT be able to
decode the stereo program.
The receivers use the 19kc pilot and double
it to 38kc, then use that 38kc as an injection freq to demodulate the
L-R program.
No pilot...No workie.

It would only detect the main channel program L+R

WB2WIK
10-25-2011, 01:45 AM
Hmm I wonder if I could use GNUradio to slowly move the deviation and plot the results frequency/time domain like my pictures, I would love to see the carrier and sidebands changing and interacting.

Also if the stations deviation or modulation index is not right could the pilot end up on 18khz? ( I'm sure it's not, just wondering if it's possible )

Edit: Also I see why it looked like 18khz, the center carrier is at -1khz, I must have some frequency drift going on... at 100Mhz its only 10ppm, not to bad.

Why are you concerned about this?

Are you planning to set up an FM broadcast station? If not, ummmm...."who cares?":o

NO2A
10-26-2011, 09:42 PM
I don`t know if department stores still use multiplex for their "muzak" that you hear,as well as "elevator music". I`ve heard engineers use those frequencies to talk to each other on fm,maybe tv too.

W9GB
10-26-2011, 10:58 PM
I have not tracked how many comemrcial FM stations are still using the Subsidiary Communications Authority (SCA) subcarrier
67 kHz was used, as I remember.
http://en.wikipedia.org/wiki/Subsidiary_Communications_Authority

Being in West Lafayette, IN ... I would hope that Purdue has not let the engineering slip on their local FM broadcasters.

Maybe Andrew will pick up the baton from the late Bruce Elving (a strong advocate of SCA and FM experimentation)
http://en.wikipedia.org/wiki/Bruce_Elving

w9gb

KA0GKT
11-01-2011, 05:43 AM
Subcarriers broadcast on FM broadcast stations are used for many things. RDS (Radio Data Service) is on a 57 kHz subcarrier which provides call sign, artist name, album name and song name which will scroll across an LCD screen on FM receivers designed to receive the data. Additionally, the RDS can set the clock in the receiver, provide alternative frequencies (Like for Translators) for the receiver to auto tune to in the event of main channel signal loss, and digital traffic information, along with tons of other data related services.

It used to be that an FM ststion needed authorization from the FCC to broadcast a subcarrier, SCA stands for Subsidiary Communications Authorization. SCA was deregulated in the 1980s. Subcarriers are used for things like Radio Talking Book, Physicians Radio Network, IFB (Interruptible Feedback) where audio cues can be given to radio (or TV) tallent on remote broadcasts in the field, Telemetry for the transmitter remote control, and just about anything which can be sent in a voice-quality audio channel. SCA subcarriers are usually but not exclusively at 67 and 92 KHz. In analog NTSC TV broadcast, video subcarriers were sometimes put to a similar use, and data (Like Time Code and Black Reference) was sent in the vertical blanking interval.

NO2A
11-02-2011, 04:39 AM
Anyone know if broadcast stations ever have swr problems?

K8JD
11-02-2011, 05:25 PM
Don't forget HD digital signals now piled on top of the other signals from FMBC stations !

K8JD
11-02-2011, 05:26 PM
Anyone with an exposed antenna on top of a tower could develop Defects causing high SWR.
Detected reflected power usually caused those 10-20 kW transmitters to shut down and maybe switch to a backup antenna and restart..

KC9TNQ
11-02-2011, 10:50 PM
Hello everyone, I have been sick the past couple days but i'm better and back to messing around. I wanted to see exactly how sidebands form from a single moving signal, so I took a carrier and frequency modulated it with an increasing in frequency sine wave, here are the results:
77399
Speed up the frequency acceleration:
77400
I believe the dark spots in the carrier are the nulls you were mentioning.

Then I wanted to replicate the received signal, so I modulated the carrier at 19khz and got a mess of sidebands:
77401
Then I turned down the amplitude of the modulating sine to 10% and turned on some background noise:
77402
Looks pretty close!


I would hope that Purdue has not let the engineering slip on their local FM broadcasters.
This IS Purdue's own radio station WBAA-FM, I hope they didn't mess it up :)


HD digital I have seen these, I am attempting to decrypt them, they are supposedly proprietary, and no one knows the exact modulation and compression scheme, but that doesn't sit right with me, so I want to see what I can do.

W8JI
11-03-2011, 10:07 AM
Yep. 19 kHz is the stereo pilot that is doubled in the receiver to 38 kHz so it can be used to detect the L+R and L-R signals for stereo.

Sub carriers for SCA are at 67 and 97 kHz I believe. 67 kHz is easier to meet FCC bandwidth but has more problems with crosstalk, and the 97 is at the edge of allowable bandwidth and needs one sideband suppressed.

AC0OB
11-06-2011, 03:55 AM
I have seen these, I am attempting to decrypt them, they are supposedly proprietary, and no one knows the exact modulation and compression scheme, but that doesn't sit right with me, so I want to see what I can do.

The compresson is done via a PAC but the modulaton is done via COFDM.

If you go to the IEEE's Broadcast Engineering journal you can read all about it.

IMHO, HD radio is a bill of goods sold by Ibiquity, with the help of the NAB, to non-technical station managers of large radio conglomerates with the promise that they could get up to four more subchannels out of a single channel. Further, Hybrid Radio or IBOC takes twice the bandwith of a purely analog channel.

Ibiquity also miscalculated digital power levels so now many FM stations are scrambling to raise their power levels.

How many HD radios are out there? Not as many as promised.

Phil

WA7KKP
11-10-2011, 10:42 PM
Why? The distance the carrier is from the center is determined by amplitude of the signal not the frequency, correct? Then why do the sidebands relate directly to frequency, wouldn't it depend on the deviation and final frequency? Also the sidbands are at ~18khz not 19khz, and I sort of see 5 signals now with more gain on my equipment -36khz, -18khz, 0, 18khz, -36khz. So if I change the pilot frequency how will this effect the final output, what if a change the pilot amplitude?

Thank you for your help!

If the audio has pre-emphasis (FM stations run 75 usec) the amplitude of the audio at the FM modulator will be greater at higher audio frequencies.

I strongly suggest you get some good books on the subject. I remember one from years ago, "FM Simplified" by Kiver . . . that was a bit thick to read but I got it and read it over and over again.

Gary WA7KKP

KA0GKT
11-11-2011, 05:37 AM
78143
Anyone know if broadcast stations ever have swr problems?

Why Yes, Yes they do.

Ice build-up on the antenna elements or the Radomes around the elements will cause the VSWR to rise on any antenna and FM broadcast antennas are no exception.

Some FM and TV broadcast antennas have deicers built in. They are usually calrod heating elements housed within the antenna elements. Broadcast antennas are usually tuned slightly high; when the ice builds up on the elements, the resonant point drops in frequency and the VSWR initiallly begins to lower until it passes through the operating frequency and then continues to rise until the transmitter VSWR trips off.

The photograph attached is from the winter of 2009. The iced up towers are atop Mt. Bigelow just north of Tucson, AZ. Some of the stations I take care of are on the tower which is the second from the left. Our FM antennas are in Radomes as are the panels for our channel 30 DTV station (54 KW TPO).

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