88-108 Mhz modes - Page 1
Forum section: electronics / rf-microwave

author: silly sausage
author_rank: Regular Contributor
timestamp: January 27, 2025, 05:02:03 am
content: Is there any reason fm is used on the 88-108 Mhz broadcast band and not any am used ?.

author: pqass
author_rank: Super Contributor
timestamp: January 27, 2025, 06:05:43 am
content: "Edwin Howard Armstrong developed a competing transmission technology, "wide-band frequency modulation", which was promoted as being superior to AM transmissions, in particular due to its high-fidelity and near immunity to static interference. ... One area of concern was the effects of tropospheric and Sporadic E propagation, which at times reflected station signals over great distances, causing mutual interference. ... FM band needed to be shifted to higher frequencies [to 88-108MHz] in order to avoid this potential problem."FM stations may also be broadcasting AM signals in the form ofsubcarriersat 67 kHz and 92 kHz from the main carrier. Typically broadcasting low quality music, religious programming, and data.

author: CaptDon
author_rank: Super Contributor
timestamp: January 28, 2025, 03:42:25 am
content: Yes, Because FM provides a better quality recovered signal in low signal strength listening areas. The limiter section of FM receivers holds the carrier to a relatively steady level and the FM modulation is then recovered in one of several ways with much less noise. Because of the wide modulation bandwidth FM requires much more spectrum so we see AM commercial radio stations jammed into a portion of spectrum about 1MHz wide while we see FM commercial station grouped in a 20MHz portion of spectrum. As a side note about AM, it is in fact used in the aviation band from 108MHz to 136MHz. The reasons are a whole new discussion. The FM broadcast band being 20MHz wide also allows for a higher fidelity signal approaching 15KHz of audio bandwidth were as the AM band tends to brick wall the signal at 9KHZ.

author: TimFox
author_rank: Super Contributor
timestamp: January 31, 2025, 02:02:11 pm
content: Statute law and FCC regulations.That band is reserved for FM broadcast in the US by the FCC.

author: radiolistener
author_rank: Super Contributor
timestamp: January 31, 2025, 05:43:38 pm
content: Quote from: silly sausage on January 27, 2025, 05:02:03 amAM/SSB is good for long range distance communication, while SSB is good for stationary source and receiver (when the path distance is fixed) and AM is good for moving source/receiver and low non-linear distortions. In addition SSB uses most narrow bandwidth and is very energy efficient.But AM/SSB is vulnerable for pulse interferences like produced with digital electronics and SMPS. This is where FM modulation shine. FM requires much more wider bandwidth than AM/SSB, FM is bad for long distance communications when signal strength is too weak, FM is very energy inefficient and FM has non-linear distortions by its nature. But FM works very good with strong signals on short distance and is noticeably less sensitive to pulse interferences.Since broadcast stations are targeted for receivers on short distance, uses pretty high power and works at MHz frequency band (where wide bandwidth transmission is not an issue), it uses FM modulation. It allows to have good reception despite the pulse interference smog in the modern cities.At air VHF band AM modulation is used, because aircraft needs long range distance communication and moving too fast, AM is not sensitive to frequency shift due to the Doppler effect, so there is no need for dynamic frequency correction. And in addition AM modulation is tolerant for mixing several transmitter sources at close carrier frequencies, you will hear all sources with no issue with AM, while with FM you will have distorted noise in this case. This is very important for aircraft to avoid blocking communication with important dispatcher stations with some random carriers and illegal transmitters at air band.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 31, 2025, 08:32:23 pm
content: Reading between the lines, your general question seems to ask why some modulation modes are observed only on specific frequency bands.The non-technical explanation is that laws and regulations say so.The technical explanation is generally bandwidth (but other characteristics may also apply).

author: wasedadoc
author_rank: Super Contributor
timestamp: February 02, 2025, 09:55:21 pm
content: Quote from: CaptDon on January 28, 2025, 03:42:25 amThat 9kHz (or 10kHz) in some regions is the AM double sideband width. The audio bandwidth cannot exceed half of that.

author: radiolistener
author_rank: Super Contributor
timestamp: February 03, 2025, 05:50:15 am
content: Quote from: wasedadoc on February 02, 2025, 09:55:21 pmSome time ago, AM stations with a bandwidth of 20-40 kHz could be found on shortwave, they had very good quality, even better than on FM broadcast, with 4 times more narrow bandwidth than FM broadcast.I also hear that there are AM stations with stereo broadcasting, but I don't have a ready AM stereo demodulator to check this. I will be glad to see AM stereo broadcast stations on shortwave, so I can debug demodulator.

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2025, 01:56:13 pm
content: Quote from: silly sausage on January 27, 2025, 05:02:03 amThere are other advantages, however the main one is that an FM receiver rejects AM noise sources, and most noise is AM.

author: Retep
author_rank: Regular Contributor
timestamp: February 03, 2025, 02:36:21 pm
content: Quote from: radiolistener on February 03, 2025, 05:50:15 amApparently AM Stereo was/is a thing in the USA:The way the stereo signal is injected into the AM carrier is very different from how it is done with FM stereo.

author: CaptDon
author_rank: Super Contributor
timestamp: February 03, 2025, 04:00:45 pm
content: As far as the '5KHz' thing goes, consult the NRSC-1 standards which state for an Analog only (no stereo or other subcarriers) the transmitter frequency response shall be -15dB below reference at 10KHz. Which side steps the '10KHz bandwidth' rule. During proof of performance testing the transmitters audio response is verified as flat to 10KHz with a maximum distortion level also specified due to the pre-emphasis curve which rises to 10dB at 10KHz. Our audio processing chain included a brick wall filter at 9KHz to comply with the NRSC rolloff measurements at 10KHz, 10.2KHz and beyond. We did not brick wall at 5KHz as some have suggested. Transmitter output spectra has different regulations with the different modulation modes. We used Orban OptiMod-AM units and a separate audio brick wall filter device. The Gates MW-5 and the newer solid state Gates / Harris both could easily do 110% forward mod while not going beyond 99% negative.

author: TimFox
author_rank: Super Contributor
timestamp: February 03, 2025, 04:05:17 pm
content: Quote from: David Hess on February 03, 2025, 01:56:13 pmAnother way to think of noise in FM and AM is that the frequency-modulated signal (with parameters defined by the FCC for FM broadcast) takes up more bandwidth than an equivalent AM signal, which seems paradoxical, but results in better SNR. The "deviation" parameter (75 kHz for US FM broadcast) can be adjusted to get wide-band FM, but the bandwidth for AM is defined by the bandwidth of the baseband (audio modulating signal). The math for understanding FM is complicated, so one should consult the relevant textbooks.Politically, the history of the fight between E H Armstrong (advocate for FM) and D Sarnoff at RCA (who was more interested in pushing television) makes interesting reading. Some say that the FCC decided on FM for the audio transmission in TV to spite Sarnoff after Armstrong's suicide.

author: CaptDon
author_rank: Super Contributor
timestamp: February 03, 2025, 04:10:56 pm
content: For the U.S. AM broadcast see NRSC-1-C section 7.2 (fig 4). When I was working in the field we were still at the 'new' NRSC-1-A. We did have too make changes to our audio processing chain to meet the 'new back then' standard.

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2025, 04:32:03 pm
content: Quote from: Retep on February 03, 2025, 02:36:21 pmThere were 4 methods of generating the stereo AM signal, which I remember had to do with a patent thicket, however only 2 different decoders were necessary. It worked, but the result was too noisy and low fidelity compared to stereo FM.

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2025, 11:33:38 pm
content: Quote from: TimFox on February 03, 2025, 04:05:17 pmJust to be clear, the FM deviation is unrelated to FM audio bandwidth, which is determined by the audio cutoff frequency. The RF bandwidth is a function of the peak deviation and audio bandwidth, and goes to infinity but an RF filter limits it cutting off the further sidebands.I think what is going on here is that the phase noise is a smaller proportion of a greater peak deviation FM signal, increasing the ratio of signal to noise.

author: TimFox
author_rank: Super Contributor
timestamp: February 04, 2025, 12:08:21 am
content: Yes.The additional “deviation” parameter is the shift in carrier frequency for maximum audio signal level (peak voltage).The ratio between deviation and audio bandwidth is called the “modulation index”.For a single audio tone, the magnitude of the sidebands is expressed in terms of Bessel functions that extend to infinity but fall off with frequency.This gives the system designer additional freedom over what is available with AM.

author: BrianHG
author_rank: Super Contributor
timestamp: February 04, 2025, 12:40:57 am
content: Quote from: CaptDon on January 28, 2025, 03:42:25 amIt's much wider than 15khz for mono FM broadcasts.

author: David Hess
author_rank: Super Contributor
timestamp: February 04, 2025, 01:59:09 am
content: Quote from: BrianHG on February 04, 2025, 12:40:57 amI had to look it up and it says 200 kHz.Stereo broadcast FM uses an AM subcarrier to encode the stereo, neatly explaining the increase in hiss when in stereo mode.

author: pqass
author_rank: Super Contributor
timestamp: February 04, 2025, 03:19:43 am
content: "Normally each channel is 200 kHz (0.2 MHz) wide, and can pass audio and subcarrier frequencies up to 100 kHz. Deviation is typically limited to 150 kHz total (±75 kHz) in order to prevent adjacent-channel interference on the band."Yes, the main channel mono signal is 15KHz as expected as any higher bandwidth is pointless for us humans. But there is more than just that. See attached (fromhere).

author: radiolistener
author_rank: Super Contributor
timestamp: February 04, 2025, 12:34:31 pm
content: Usually some FM stations working with more than 200 kHz bandwidth - up to 300-350 kHz and if you demodulate it with sharp brickwall filter at ±100 kHz cut-off, it can lead to distortions. Here is example - highlighted area is 200 kHz.Regarding to bandwidth, FM modulation bandwidth is a complex function which can be estimated by Carson's equation, as follows: FM Bandwidth = 2(Δf + fm), where Δf is the peak frequency deviation and fm is the highest modulating frequency

author: CaptDon
author_rank: Super Contributor
timestamp: February 04, 2025, 02:13:51 pm
content: On our FM outlet we ran the 38KHz pilot, our 67KHz subcarrier returned transmitter site data produced by the 'Hallikanen & Friends' remote data logging system which was streaming continuously and we ran a 91KHz subcarrier used for 'Studio Talk Back' during our local ball game broadcasts. I had a McMartin 67KHz decoder at home and could monitor the transmitter site from my home also. The data stream could easily be parsed.