2
\$\begingroup\$

What I am asking is if it is possible to transmit a frequency modulated wave in AM range(550-1650 Khz) and similarly to transmit an Amplitude modulated wave in FM band (88-108 Mhz). I would like to know about the technical aspects of the problem like what problems we may face if try to do so.

\$\endgroup\$
  • \$\begingroup\$ I don't see why not. But the AM band is likely packed much closer than the FM band. (You need some BW space in order to frequency modulate the signal.) Other problems: 1. The FCC will probably hate you. 2. There may be anomalies / problems you will encounter trying to frequency modulate lower frequencies carriers. \$\endgroup\$ – st2000 Dec 17 '16 at 18:10
  • \$\begingroup\$ You can transmit any kind of modulation you'd like to in either of those bands... There's nothing specific about the band that limits the modulation schemes that you can use (aside from regulation). \$\endgroup\$ – Captainj2001 Dec 17 '16 at 18:10
  • 3
    \$\begingroup\$ Yes, to both questions. No technical difficulties but you will be breaking the law in most countries as you will be unable to get a licence. \$\endgroup\$ – Leon Heller Dec 17 '16 at 18:11
  • 2
    \$\begingroup\$ FM uses 75kHz deviation with spread on 200kHz spacing , AM uses 10Khz bandwidth or so NO WAY. This is why the frequency response and SNR is higher. There are many technical reasons why FM cannot be used in the AM band, CNR to SNR improvement factor of FM requires more bandwidth not available and thus using a lower deviation ratio to match the BW of AM would result in worse performance than AM. Also narrow band AM in the FM band would be clobbered by AM noise from adjacent carriers and would need extra narrow filters or triple HET conversion. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 17 '16 at 18:18
  • \$\begingroup\$ I was thinking if we transmit AM signal in FM band it will be facing more attenuation due to smaller wavelengths so will not be able to travel longer distances lie normal AM wave travels. Similarly for transmitting FM using AM band : as the bandwidth is limited to few 100's of Khz only in AM band so we will not be able to have more frequency deviations and thus it will be more noisy. \$\endgroup\$ – Nikhil Pandya Dec 17 '16 at 18:19
3
\$\begingroup\$

AM now supports Quadrature Stereo AM but still fits within 10Khz channel spacing with some guardband.

FM cannot fit into the AM band , wwhile AM is even used in the ISM band at 928MHz with 6kHz channel spacing, so there is no reason why AM cannot be used at any frequency.

FM has higher SNR and signal bandwidth that prevents application in the lower AM bands.

However one can use AM or FM for cable communication at any frequency. enter image description here Can you see how to fit the above baseband FM signal in the 10KHz channel spacing of the AM band? If you do it's worth a ton of money.

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ For me your comment was more helpful and easy to understand,also solved my problem. So thanks. \$\endgroup\$ – Nikhil Pandya Dec 17 '16 at 18:42
  • \$\begingroup\$ the AM band spans a little over 1MHz (0.53 to 1.60) so there's room, but it would use the frequency space needed for 10 or so AM broadcasts. \$\endgroup\$ – Jasen Dec 18 '16 at 4:25
  • \$\begingroup\$ for conducted AM FM no problem use any BW you need for radiated, there are regulations. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 18 '16 at 14:56
  • \$\begingroup\$ In FM stereo broadcasting a variety of modes are used. FM for the mono audio, wide band amplitude modulation for the left -right audio with suppressed carrier if I remember right Phase modulation for the stereo pilot beacon just to name a few. \$\endgroup\$ – Old_Fossil Feb 6 '19 at 8:26
  • \$\begingroup\$ Thanks @Old_Fossil for the bit on AM-SC part. I know Nikhil’s logic is correct on BW compression as Shannon-Hartley’s Law proves this in the deviation ratio of FM BW to baseband BW. Or the conversion of CNR to SNR thru demodulation, Meaning the higher deviation ratio increases SNR and conversely as well. Yet carefully balanced so stereo (AMSC) drops to mono in Rx when the CNR gets too low. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 6 '19 at 13:55
3
\$\begingroup\$

You could do AM on the FM band .You will be able to squeeze in many more channels but you wont get the SNR advantages of FM .If you tried to do wideband FM on the standard MW AM band the number of channels would be very low .Also the type of Atmospheric fading on the MW band would make the stereo music sound terrible .

|improve this answer|||||
\$\endgroup\$
2
\$\begingroup\$

Technically any mode of transmission is feasible on any frequency if bandwidth is available. The reason that AM mode is used on the lower broadcast band and FM on the higher broadcast band is by reason of government regulation as well as signal propagation issues. The broadcast FM band in the '40's was originally around 40 Mhz the got moved to 88-108 Mhz band. Depending on the application a multitude of modes are used.

|improve this answer|||||
\$\endgroup\$
0
\$\begingroup\$

There is no technical aspect. The aspect is standard equipment and frequency licencing. Waves themselves know nothing about it, so you can construct your own equipment to use any band you want.

|improve this answer|||||
\$\endgroup\$
0
\$\begingroup\$

There is no technical reason you couldn't do what you describe. In fact, there is at least one system for AM stereo that is based on modulating the carrier simultaneously with AM and FM.

However, depending on what jurisdiction you live in, there may be regulatory issues.

|improve this answer|||||
\$\endgroup\$
0
\$\begingroup\$

AM and FM waveforms: -

enter image description here

In case 1 you move the amplitude up and down a bit. In case 2 you wobble the frequency a bit. This can occur at any point in the radio spectrum.

Both can occupy the same amount of spectral bandwidth. Narrow band FM is scattered all over the spectrum and, for low bandwidth speech signals is perfectly OK. Wideband FM used in broadcasts of stereo music occupies a large chunk of bandwidth hence it can't really be used in the traditional AM broadcast region of the spectrum.

|improve this answer|||||
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.