Questions regarding FM audio transmission - Page 1
Forum section: electronics / projects

author: Entropia
author_rank: Regular Contributor
timestamp: January 12, 2014, 11:55:58 am
content: I'm a beginner with radio frequency modulation stuff. If I'd like to receive or transmit FM audio, what would be the best book to get info from? I have a basic understanding of frequency modulation. I think it goes something like this; an amplitude modulated audio signal changes the frequency of the radio wave at baseband. If the amplitude goes negative, it decreases baseband frequency and vice versa. Correct?But then come the bits that I see in block diagrams but don't really know what they do. Pre-emphasis, de-emphasis, discriminator, limiter? I know pre- and de-emphasis are to increase the transmitted audio quality (emphasize higher frequencies in pre-emphasis and vice versa in de-emphasis?). I think limiter is a frequency deviation limiter? I.e. it would cut off incoming AM signal if it was to deviate the carrier too much from the baseband?If we were to talk about transmitting audio frequency FM at RF, am I on the right track if I would use a PLL to frequency modulate a carrier at, say 10 MHz, and then mix it with a local oscillator of 450 MHz to get 460 MHz FM?Also, I see that 10.7 MHz seems to be the "de facto" intermediate frequency for FM receivers. Is there any magic behind this number, or can I just as well use a 10 MHz IF, if my PLL's VCO goes there? I'm think this 10.7 MHz might be a left-over from decades ago or something like that, that just stuck...Anything else I should know on the topic?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: January 12, 2014, 12:33:55 pm
content: ARRL handbook is a good place to start.http://www.amazon.com/Best-books-for-Ham-Radio/lm/RF1MZRUPQU1WQ]

author: qno
author_rank: Frequent Contributor
timestamp: January 12, 2014, 02:08:36 pm
content: It is from the older days where analog flters where not so steep.If the intermediate frequencies where not so close to eachother you could get good filtering with cheap LC filters.The industrie standard was 455 kHz and 10.7 Mhz so ready made filters are cheap and available.You are free to choose your own frequency but then you have to make your own filters.The modern way is Software Defined Radio where you can do direct conversion and demodulate the audio in software.

author: Entropia
author_rank: Regular Contributor
timestamp: January 12, 2014, 04:38:37 pm
content: For a direct-conversion receiver I would need seriously more horsepower than a simple 8-bit microcontroller... So it's out of the question.The 2013 edition of ARRL Handbook for Radio Communications seems to be available at my local library. Sweet! Better go grab that tomorrow.

author: Entropia
author_rank: Regular Contributor
timestamp: January 24, 2014, 10:18:27 pm
content: So, I've been binging on information regarding FM modulation and demodulation. I even ordered a few 74HC4046A PLLs to experiment with. My first intention was to frequency modulate speech using the VCO. I would set up the VCO so that my carrier frequency (say 100 kHz) would be at 2.5 volts VCOin. Then my speech signal would be scaled so that it would deviate the carrier by 5 kHz either way. This could be very temperamental in regards to tuning the VCO, but in theory it should work, right?I talked to some experienced RF guys on the IRC and was told that the "conventional" way of doing FM is by having the PLL lock on to a carrier frequency and then AC-couple the modulating signal to VCO input. I tried to think this over but I just can't seem to get a grip on how this would turn into a schematic. Where do I feed the carrier? AC-coupling the modulating signal is easy, but surely the single-ended supplied VCO can't handle negative control voltages, so it would have to be DC-biased first.I've tried Googling with atleast 50 different search terms but I just can't find a simple tutorial on this thing. Everything is either pure mathematical theory regarding PLLs, or full-blown FM transmitters (don't want the transmitter part, just the modulation, and even the modulation is usually done some other way). So, again, I'm a bit lost. Anyone care to enlighten me?

author: w2aew
author_rank: Super Contributor
timestamp: January 24, 2014, 11:45:13 pm
content: Have you watched my video on the basics of a PLL? It features the 4046. It doesn't go into frequency modulation per se, but I do show 'pulling' the VCO a little.

author: dannyf
author_rank: Super Contributor
timestamp: January 25, 2014, 12:04:14 am
content: QuoteDirect conversion receivers - the most widely used receiver on earth.Quoteamplitude.QuoteFrequency +/- 10.7Mhz * n vs. [88Mhz -- 108Mhz]

author: Co6aka
author_rank: Unknown
timestamp: January 25, 2014, 02:45:37 am
content: Right up your alley... (Literally!)http://www.sral.fi/

author: vk6zgo
author_rank: Super Contributor
timestamp: January 25, 2014, 07:13:40 am
content: Quote from: dannyf on January 25, 2014, 12:04:14 amWhere are all these direct conversion receivers?Almost all receivers are still some version of a Superheterodyne.

author: Entropia
author_rank: Regular Contributor
timestamp: January 25, 2014, 10:34:45 am
content: Quote from: w2aew on January 24, 2014, 11:45:13 pmI've been your subscriber for a good while but somehow I missed that video. At the end, when you "yank" on the VCO control voltage, the control voltage change must be so small that it seems like just the phase is changing, when in reality the frequency changes by a tiny amount, right? So I could modulate the output like this. Have the PLL lock on to my carrier frequency and AC-couple audio signal via a series resistor to VCO input?

author: dannyf
author_rank: Super Contributor
timestamp: January 25, 2014, 11:40:30 am
content: -Where are all these direct conversion receivers?-your cell phones, blue tooth headsets, GPS, to name a tiny portion.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 26, 2014, 02:41:48 am
content: I would think they would be regarded as Sofware Defined Radios,or perhaps DirectDigitalConversion.Direct Conversion receivers are more generally regarded as those which produce an analog baseband signal directly from the mixing process.Apart from such things,the vast majority of receiversarestill various versions of Superhet.Overcoming Image interference is a reaspnably easily solvable problem in Superhets,but requires fairly complex analog circuitry with Direct Conversion.If you are looking for a known type of signal,with a specific format,you can solve this problem with software,but it isn't nearly as easy for general purpose receivers.

author: dannyf
author_rank: Super Contributor
timestamp: January 26, 2014, 02:52:37 am
content: QuoteWhen reality contradicts with our perception, we can always refine reality.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 26, 2014, 03:29:06 am
content: Quote from: dannyf on January 26, 2014, 02:52:37 amWell,it looks like it's your perceptions versus mine,& a lot of other people who make the same distinction.Anyway,your original comment had nothing much to do with the OP's rather muddled perception of FM.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 26, 2014, 03:33:04 am
content: Quote from: Entropia on January 25, 2014, 10:34:45 amIf the phase changes,so does the frequency,& vice versa--both modes are called "Angle Modulation".

author: lgbeno
author_rank: Frequent Contributor
timestamp: January 26, 2014, 03:59:31 am
content: Here's an example of the simplest FM modulator you can build and it works. Will drift all over the place by works...http://www.buildcircuit.com/simple-steps-for-making-fm-transmitter/Certainly software defined radio is extremely common, not the same as analog direct conversion just a band pass filter and then a under sampling ADC with sufficient bandwidth for the input signal band width. Some dsp and it's all digital circuits.Pick up a RTL-SDR dongle off amazon for $16 and you can learn quite a bit!

author: G0HZU
author_rank: Super Contributor
timestamp: January 26, 2014, 03:25:22 pm
content: My advice to Entropia would be to buy an old 1980s FM CB radio and study how it works.Everything is there for the classic methods of FM transmission and reception (apart from decent pre emphasis and de emphasis)In the receive path you will see a typical FM IF strip with a hard limiter and a quadrature (FM) detector for you to probe and test.In the transmit section you should see a simple single loop PLL with a low loop bandwidth with the external modulation fed in from the microphone buffer amplifier etc.The idea here is that the PLL has a very lazy response time so it can only correct frequency errors that occur at about 30Hz or less. So if you feed in modulation from 300Hz to 2500Hz (ten times faster) then the loop can't 'remove' this modulation as an error because it is happening too fast. So the net result is the loop stays locked but it also has FM on top of it.If you reduced the modulation frequency down to, say 10Hz, then the PLL would be able to correct it and the FM deviation would collapse to a very small amount because the PLL will track and cancel the modulation.So with a cheapo CB you would have a very common/typical platform to play with and you can buy an FM CB for peanuts

author: Entropia
author_rank: Regular Contributor
timestamp: January 26, 2014, 07:45:03 pm
content: lgbeno: I do have an RTL-SDR dongle, it's mighty fun to play around with! It's also my idea to use it to test if my modulation setup works. (No antennas involved, so no regulations authority to get pissed off either.) I got one with an SMA to BNC pigtail included.G0HZU: Very good tip. I'll see if such radios are available locally so that I could pick one up. I wonder if I have to get an amateur radio operator certification to use such equipment in transmit mode though. Or I could just terminate the antenna output into sufficiently large dummy load.