Frequency deviation in FM broadcast - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: March 29, 2025, 10:20:22 am
content: Asking about analog radio and analog TV sound. Let's say I'm looking at the output of one of the IF stages.What is the frequency deviation around the 10.7MHz in an FM radio? +/- how many kHz?Same question for the 4.5MHz and 6.5MHz IF of analog TV sound. Living in Europe/Romania.Searched online for a few minutes and it is not clear to me what is the frequency deviation for FM radio, and for TV sound. I've looked at the datasheets of a few ceramic 4.5/6.5/10.7 MHz filters, and for each central frequency they are available in many different bandwidths (for examplehttps://www.transko.com/img/product/filterCeramic_pdf/03%2010.7M.pdf), so couldn't deduce the expected frequency deviation from the filter's datasheet.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 29, 2025, 11:19:31 am
content: As far as I remember FM broadcast is 75 KHz and the PAL sound channel is 50 KHz.

author: TimFox
author_rank: Super Contributor
timestamp: March 29, 2025, 02:27:40 pm
content: US standard for FM broadcast is +/- 75 kHz, but the bandwidth occupied by an FM signal with normal and stereo multiplex audio is considerably wider (technically infinite, but it falls off as a Bessel function).The spectrum of a modulated FM signal is an interesting matter: you should consult textbooks for the messy details.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 29, 2025, 02:50:50 pm
content: Thank you both. I am interested only in the expected deviation, because I think I have an idea for an FM demodulator.My problem was: "What would be the required slope of a demodulator (say, in mV/kHz) such that to get a decent audio-out amplitude from an FM broadcast?"Was asking for both TV-audio and FM-radio F deviations because, as possible FM signal sources at hand, I could try to revive either an old (all analog) FM radio, or a TV tuner remaining from a scrapped analog TV. And then to use the signal from their IF to test my FM demodulator.

author: bdunham7
author_rank: Super Contributor
timestamp: March 29, 2025, 03:07:34 pm
content: The answer here is 75kHz, IDK if it is the same where you are but IIRC from years ago, at least pre-1990 western Europe was the same.Here's an application note on FM broadcast specifications and testing. Also, a screenshot of the spectrum of a standard broadcast multiplexed FM signal at 101.5MHz with 1kHz input on both channels. The deviation is set for 67.5kHz for the L+R (main) channel and 6.75kHz for the 19kHz pilot tone. Those get reduced slightly if there's an SCA signal, IDK if RDS matters at all.https://bh.hallikainen.org/uploads/harold/TekFmBroadcastMeasurementsUsingTheSpectrumAnalyzer.pdf(note: if this link goes dead in the future, contact me as I have a copy but it is too large to post)

author: TimFox
author_rank: Super Contributor
timestamp: March 29, 2025, 04:38:01 pm
content: For a FM transmitter, deviation is defined this way, with 1 V peak (2 V pk-pk) used as an example.The exciter contains a voltage controlled oscillator that shifts the frequency by 75 kHz for 1 V (pk) input.The audio processor that drives the exciter takes the audio (including audio frequency limit, pre-emphasis, multiplexed channels, and pilot tone used to demultiplex in the receiver) and controls the output to limit it to 2 V pk-pk (+/- 1 V) before applying it to the exciter.The spectrum shown above is for 1 kHz audio, but the results for more complex audio are even more cluttered.For the demodulator, if you design the detector to give 1 V output for 75 kHz deviation, then the maximum output signal will be 2 V pk-pk.Thereafter, you need to extract the pilot tone, demultiplex the stereo channels, and apply de-emphasis. US uses a 75 us pre/de-emphasis, while the European standard uses 50 us. If the receiver network does not match the transmitter, there will be a 3 dB error at about 10 kHz in the overall response between the two standards.The pre-emphasis is just a high-pass filter, and the de-emphasis is a low-pass filter with the same constant. Since the audio is band-limited before pre-emphasis, the high-pass filter does not “blow up” at high frequency.

author: CaptDon
author_rank: Super Contributor
timestamp: March 29, 2025, 11:58:21 pm
content: When you speak of FM slope detection it sounds like Foster Seeley to me which can also slope detect AM.

author: TimFox
author_rank: Super Contributor
timestamp: March 30, 2025, 01:03:31 am
content: Foster-Seeley discriminators get a better dV/dfover the relatively narrow frequency region than a simple slope detector, with better linearity.

author: radiolistener
author_rank: Super Contributor
timestamp: March 30, 2025, 02:04:33 am
content: Usually FM broadcast signal has bandwidth about 190-200 kHz, but some stations can have up to 250 kHz. So, for the most FM stations deviation is about 90-100 kHz. If I remember correctly, according to the standard the bandwidth should be 200 kHz.Usually I'm use 192 kHz filter:

author: TimFox
author_rank: Super Contributor
timestamp: March 30, 2025, 03:41:53 am
content: Deviation is defined as in my prior post, and is always less than the bandwidth occupied by the modulated signal.US FM broadcast uses a deviation of 75 kHz.This is a matter of regulation, since the hardware would be capable of much more frequency deviation, if allowed.AM is limited to 100% negative modulation (which reduces the output magnitude to zero, below which is impossible), but can go more than 100% positive (with resulting distortion of the demodulated signal).

author: radiolistener
author_rank: Super Contributor
timestamp: March 30, 2025, 07:05:30 am
content: yes, the total FM bandwidth will be a sum: 2*(message bandwidth + dF).I just tried to generate FM modulated signal with deviation 75 kHz at 300 kHz carrier and use digital white noise FIR filtered with cut-off 20 kHz and 100 dB attenuation as a message signal. Here is result:Spectrum of message signal:

author: jonpaul
author_rank: Super Contributor
timestamp: March 30, 2025, 11:59:50 am
content: See agency regs for your area, eg EBU and FCC on max dev.Most FM stations push to the max for loudness and coverage.Best FM demods are decades old, see the many books on modulation.We used the very fine Sid Smith FM det in the 1971 Sequerra tuner.Modern sets used PLL ICs.j

author: watchmaker
author_rank: Unknown
timestamp: March 30, 2025, 12:47:48 pm
content: Quote from: RoGeorge on March 29, 2025, 10:20:22 amRoGeorge,As you know I am a neophyte developing a STEM course around the Elenco FM/AM discrete and IF based kit (and identified most of the errors in the instructions). (Middle schoolers, main goals are to have them appreciate the role of RF in the things they use everyday, importance of paying attention to physics, algebra and trig, and to build a working FM radio model they can be proud of).First, I have downloaded a number of resources on FM and demodulation. PM me if you want the links or copies.I even purchased vintage HK and Sencore FM stereo generators. (Less than $75USD on eBay). Guess they are fairly useless today.Secondly, from what very little I know, in the US I would try the LT10.7MS2 or LT10.7MS3. The BW you want (I think) is 10.7 MHz +- at least .075MHz, or 150kHz total. MS2 would seem to give a nice flat curve in that range although there is likely a reason to consider MS3.But, as noted by others, the broadcast standards are country specific. Some of the vintage transistor radios included a switch to deal with this.For the grownups in the room, please improve my understanding (teach). As you have noted, this is not an easy area to understand for a neophyte and I am intentionally testing my limits of understanding. I would expect a lot of what I "know" does not apply to modern PLL receivers.

author: watchmaker
author_rank: Unknown
timestamp: March 30, 2025, 12:51:14 pm
content: Quote from: bdunham7 on March 29, 2025, 03:07:34 pmTHANKS and WOW. This is very comprehensive and is quite useful for what I am doing.

author: TimFox
author_rank: Super Contributor
timestamp: March 30, 2025, 02:42:01 pm
content: Quote from: watchmaker on March 30, 2025, 12:47:48 pmRadiolistener’s formula for bandwidth (above) is a reasonable approximation: your filter bandwidth must be more than twice the deviation, due to the audio frequencies. The actual spectrum falls off as a Bessel function.For a full stereo signal, the baseband, including multiplex, is at least 53 kHz. The “L-R” component is DSB about a suppressed carrier of 38 kHz (demodulated with the second harmonic of the pilot tone at 19 kHz).From that formula, the required filter bandwidth is 2 x (75 + 53) = 256 kHz (US standard deviation of 75 kHz).For a simple mono baseband (US standard 15 kHz max audio), that would be 180 kHz minimum.US FM broadcast channels are 200 kHz apart, and the FCC tries to avoid “adjacent channels” in the same location.PLL demodulation can be considered as an audio filter “swinging” with the deviation, and gives a better SNR due to the narrower filter bandwidth, compared to discriminator detection.

author: watchmaker
author_rank: Unknown
timestamp: March 30, 2025, 02:58:13 pm
content: THANKS!I thought there was something about MPX that required a higher BW, but I was not sure. The radio model we are using is mono and that is where I have concentrated my learning.But, in for a penny, in for a pound. So I am ready to travel down the rabbit hole and learn this stuff.Again, thank you.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 31, 2025, 10:30:48 am
content: Quote from: watchmaker on March 30, 2025, 12:47:48 pmYou are too kind, thank you. For now I have all the info, just that I didn't find the time to continue the experiments.I've give up the idea of using a TV tuner (has digital controls and will be too big of a fuss to reverse engineer that). Meanwhile looked through the scrap boxes, and the FM radio I thought it was fully analog, it's in fact a digital FM radio, with antenna-in and audio out (Si4831), so no IF available to hijack.There are a few other scrap boards, 2 of them seem fully analog, with transistors and can IF filters. Didn't find the time to make them work. Will post any results as soon as I'll have any.Thank you all for your answers, it is much clear now.

author: ftg
author_rank: Frequent Contributor
timestamp: March 31, 2025, 03:09:40 pm
content: The tuner controls are likely just one I2C bus and one analog AGC line for the frontend gain.Depending on the age the I2C can terminate into a single tuner+pll chip like the Taifuns from Infineon, or there can be a pile of chips.Just look inside the can to determine what pll/tuner it has and see if you can find datasheets for it.The fully analog TV tuners usually require a +28 to +33v tuning voltage, +12V and sometimes also +5V rails.It depends on the unit.

author: jwet
author_rank: Frequent Contributor
timestamp: April 04, 2025, 02:13:07 pm
content: Watchmaker, I applaud what you're doing.One of the very best books that I've ever seen is "Building Transistor Radios" by Ronald Quan. Its mostly AM radios from the ground up with a good level of technical content for a STEM student. I don't know if going into both FM and AM would be as effective pedagogically to use a big word.AM modulation is very simple to understand even with just some HS trigonometry knowledge. The receivers are much more forgiving to build and will likely have a high success rate. Many FM kits use a prebuilt FM section where isn't much to be learned in the building. The other issue is as Tim Fox and others have said, to truly understand FM mod and demod, at more than handwaving leve, some pretty sophisticated math like Bessel Functions is required. I might recommend sticking with a good AM, discrete transistor superhet with a few IF stages. The Quan book is nice background and might be useful for further study but decent AM kits would be my vote.

author: shabaz
author_rank: Frequent Contributor
timestamp: April 04, 2025, 03:15:52 pm
content: Quote from: RoGeorge on March 31, 2025, 10:30:48 amUnfortunately a lot of modern low-cost radios have that type of design. Photo shows a Tecsun R-909 however, which I tore apart for a slightly similar purpose; to get at the IF signal for some experimentation. The radio itself isn't very good, but at least it has a superhet design with that IF output. I took the insides of the radio and mounted to a piece of plastic to access both sides easily. Quite an ugly job but it served its purpose.Loaded with a 10X probe and scope set to AC, I saw about 30 mV p-p at the 10.7 MHz IF. The photo unfortunately doesn't show where the IF was tapped from (it is the green wire), but it's from the ceramic filter of course, the wire was soldered on the board underside and brought out to the top side.

author: watchmaker
author_rank: Unknown
timestamp: April 05, 2025, 01:49:25 pm
content: Quote from: jwet on April 04, 2025, 02:13:07 pmThank you for your support and ideas. We have identified the errors in the Elenco AM/FM discrete radio kit and have both made good radios. I am in contact with Elenco about this. We are preparing lab procedures to use in place of Elenco's botched alignment descriptions and even some diagrams. Elenco apparently assigned document updates to the unwilling who cut and pasted but no one ever reviewed the final product line by line.This can be followed in my thread about preparing a STEM course for middle schoolers around this kit.https://www.eevblog.com/forum/beginners/after-school-stem-based-on-fm-radio-kit/msg5834173/#msg5834173The main thrust is to have them appreciate the use of RF in the devices they use everyday, as well as their coursework in algebra, trig and physics. We will ensure they complete the radios successfully by replacing some parts (LM 386, switch, etc) and by teaching them how to correct their mistakes. (desoldering braid, replacing incorrect components).We figure to take 6 students and to supply kits, tools, silicone mats, eyepro, etc for about $125@.For those who cannot afford the lab fee, we will pay. Met with the school superintendent and since they cannot find a pre-engineering teacher, we may fit a need. Even include a demo on the dangers of electrolytic capacitors. And give them each a TA7642 IC (3 lead AM radio). Plus I am collecting related You tubes as resources for them.Of course, the tariffs casue some problms. We will see.We will have them use our DMMs at the benches, but we will directly supervise the use of our scopes and AWGs while having them do the setups and adjustments.

author: radiolistener
author_rank: Super Contributor
timestamp: April 06, 2025, 10:19:03 am
content: Here is interesting details of FM modulation spectrum.Message signal is white noise with LPF cutoff 20 kHz.FM deviation is 75 kHz.Top window shows message signal spectrum.Bottom windows shows FM modulation signal spectrum.