What type of frequency modulation is this  - Page 1
Forum section: electronics / rf-microwave

author: sgaudio
author_rank: Newbie
timestamp: February 17, 2025, 11:14:08 pm
content: Hello to all members,nice to meet you,i'm glad to be here !I'm newbie on electronics,Here is the question,a friend given to me a wireless microphone to check if it is transmitting signalI have tinysa ultra spectrum analyzer,i tested it but i can't understand well what type of modulation,i think it is FSK but not sureI will post the photo of modulation which showed in tinysa ultraYour help will be appreciated

author: TimFox
author_rank: Super Contributor
timestamp: February 17, 2025, 11:49:07 pm
content: The spectrum of an FM signal is interesting: look it up online or in a textbook.At 20 kHz/div (if I read the screen correctly), that looks like an FM spectrum from a single 30 kHz tone.The sidebands occur at integer multiples of the modulating frequency spacing from the carrier (center) frequency.The amplitudes of the sidebands fall off asJn, whereJnis the Bessel function of the first kind andn-th order (as I said, it is interesting), but go out to infinite order as you go away from the carrier.

author: LM21
author_rank: Regular Contributor
timestamp: February 18, 2025, 09:19:54 pm
content: Some distortion too. There are several other peaks too, that are not from 30kHz signal. Text books give very nice and clean spectrum/Spectre (or whatever), this is not that. Can you feed some steady tone to your mic to see if something useful appears.

author: CaptDon
author_rank: Super Contributor
timestamp: February 19, 2025, 03:33:30 am
content: It is highly doubtful the O.P. would be modulating a wireless mic with a 30KHz signal? They typically only have good response out to about 15KHz for the good ones. A 30KHz sampling rate for a 15KHz response would seem about right and some of the wireless systems are in fact doing ADC DAC transmit / receive. I think most of my Audio Technica wireless units spec 15KHz deviation. They are compandered through the transmit / receive chain. I would have to look at the spec but I think they are 50Hz to 10KHz audio response rated. Mine are VHF analog running in the television channels 10-13 spectrum. Most of the new UHF band stuff is digital and much of the old UHF stuff is deemed illegal since the U.S. UHF television repacking fiasco.

author: ftg
author_rank: Frequent Contributor
timestamp: February 19, 2025, 08:30:54 am
content: Not having measured the output spectrum of any analog wireless microphone, could it be a pilot tone?One well above the <15kHz pass band would make sense.Having a pilot tone would make for a better working squelch circuit that would be less affected by noise.It would also allow measuring the SNR even when there is nothing being spoken and thus make the life of a possible diversity receiver easier.

author: radiolistener
author_rank: Super Contributor
timestamp: February 19, 2025, 11:28:19 am
content: It maybe AM or FM, or some other type, its hard to say from the static picture. 30 kHz FM bandwidth is too small for high quality audio signal, so it may be high quality AM, or low quality FM. It also may be some digital stream.If you can record some short WAV file with IF, it will be more easy to decide what is it.

author: eb4fbz
author_rank: Regular Contributor
timestamp: February 23, 2025, 10:18:01 am
content: They are usually wide-FM (200-500KHz) with a subtone or pilot tone to open the receiver squelch.

author: Sassy Taste
author_rank: Contributor
timestamp: February 27, 2025, 10:59:37 am
content: I agree. 20 hz to 20k hz. Broader bandwidth is better. FM or AM. Just broader is better on the frequency to the microphone.