|
AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
On Jun 30, 12:55 pm, John Smith I wrote:
Use simple logic, you can modulate a dc (0 Hz) with higher freq (voice), (hint, your telephone line is an example) right? The telephone does not use either AM or FM. It is simply the electrical equivalent of the sound that gets into the microphone. You input a 1 KHz tone into the microphone, telephone lines will carry a 1 KHz AC current to the destination. The louder the sound into the microphone, the stronger the amperage in the telephone lines. |
AM electromagnetic waves: 20 KHz modulation frequency on anastronomically-low carrier frequency
On 6/30/07 8:50 PM, in article , "John Smith I"
wrote: Don Bowey wrote: ... You are hearing the effects of the sidebands, not the Carrier. DUH! And, you only have the sidebands as a result of the carrier/modulation ... So what? You implied or inferred that what was heard from the FM radio was caused by the AM Carrier. Leave out the Carrier and you will hear the same thing. In a properly designed transmitter the Carrier amplitude does not change with modulation. I have better tools than FM receivers to prove that fact and theory agree for AM. And the time to argue the insignificant ... sharpen that razor blade, you can then successfully split much narrower hairs ... It is important that we not confuse a person new to electronics by the type of inane points you make. It doesn't matter a whit if someone's AM transmitter Carrier shifts on power peaks due to poor regulation. It has nothing to do with "AM" and everything to do with poor design. Side issues don't help the new folks. JS |
AM electromagnetic waves: 20 KHz modulation frequency on anastronomically-low carrier frequency
|
AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-lowcarrier frequency
Mike Kaliski wrote:
ELF communications are carried out at very slow data rates, only a few characters per hour at best. Actually its on the order of several characters per minute using a 64 character "alphabet". It is possible to communicate at a base band frequency of 0Hz. This is what happens when you talk down a hard wired telephone or intercom. At a telephone exchange (switching centre), the signals from each line are modulated onto a higher frequency for onward transmission down a trunk wire cable or fibre optic cable. The multiplexed high frequency modulated signals are down converted back to audio frequencies once they reach the intended destination. In the old T carrier (before 24 channel digital T1) carrier, each telephone conversation was modulated onto a low frequency radio frequency AM signal ranging from (and don't quote me as its been over thirty years since I worked T spans) 50 KC to 200 KC. Very similar in principle to the 5 kc wide AM radio station signals on the 530 kHz to 1700 kHz AM broadcast band. |
AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-lowcarrier frequency
Radium wrote:
You input a 1 KHz tone into the microphone, telephone lines will carry a 1 KHz AC current to the destination. The louder the sound into the microphone, the stronger the amperage in the telephone lines. On a side note, its actually voltage modulation towards the subscribe and current modulation back to the central office. The earpiece is a high impedance (2,000 ohm) device that responds to voltage variations. The carbon microphone element 220 to 200 ohms modulates the talk battery current. |
AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
DTC hath wroth:
Mike Kaliski wrote: ELF communications are carried out at very slow data rates, only a few characters per hour at best. Actually its on the order of several characters per minute using a 64 character "alphabet". It is possible to communicate at a base band frequency of 0Hz. This is what happens when you talk down a hard wired telephone or intercom. At a telephone exchange (switching centre), the signals from each line are modulated onto a higher frequency for onward transmission down a trunk wire cable or fibre optic cable. The multiplexed high frequency modulated signals are down converted back to audio frequencies once they reach the intended destination. In the old T carrier (before 24 channel digital T1) carrier, each telephone conversation was modulated onto a low frequency radio frequency AM signal ranging from (and don't quote me as its been over thirty years since I worked T spans) 50 KC to 200 KC. Very similar in principle to the 5 kc wide AM radio station signals on the 530 kHz to 1700 kHz AM broadcast band. Argh, that brings back fond nightmares of Ma Bell. 4Hz per voice channel with FDM (frequency division mux). Most were FM systems, but there were some AM implimentations (to avoid patent infringement). Later, there were SSB systems that doubled the number of channels. No voice Spectrum BW channels KHz kHz AT&T ITU-T 12 60-108 48 Group Group 60 312-552 240 Supergroup Supergroup 300 812-2044 1232 Mastergroup 600 564-3084 2520 Mastergroup 3600 564-17548 16984 Jumbogroup -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-lowcarrier frequency
Radium wrote:
.... You miss the simple point, the dc is the carrier ... instead of dc, you could put a 1 hz signal on the line and modulate it with your voice, indeed, you can put a 30 hz signal on the line and modulate it with your voice--if you can tollerate a bad 30 hz hum! But, who knows, perhaps you are tone deaf to the 30 hz hum and would like it ... JS |
AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-lowcarrier frequency
Don Bowey wrote:
... You are an idiot ... bother some one who has the time to take you to task ... JS |
AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-lowcarrier frequency
Jeff Liebermann wrote:
DTC hath wroth: Mike Kaliski wrote: ELF communications are carried out at very slow data rates, only a few characters per hour at best. Actually its on the order of several characters per minute using a 64 character "alphabet". It is possible to communicate at a base band frequency of 0Hz. This is what happens when you talk down a hard wired telephone or intercom. At a telephone exchange (switching centre), the signals from each line are modulated onto a higher frequency for onward transmission down a trunk wire cable or fibre optic cable. The multiplexed high frequency modulated signals are down converted back to audio frequencies once they reach the intended destination. In the old T carrier (before 24 channel digital T1) carrier, each telephone conversation was modulated onto a low frequency radio frequency AM signal ranging from (and don't quote me as its been over thirty years since I worked T spans) 50 KC to 200 KC. Very similar in principle to the 5 kc wide AM radio station signals on the 530 kHz to 1700 kHz AM broadcast band. Argh, that brings back fond nightmares of Ma Bell. 4Hz per voice channel with FDM (frequency division mux). Most were FM systems, but there were some AM implimentations (to avoid patent infringement). Later, there were SSB systems that doubled the number of channels. No voice Spectrum BW channels KHz kHz AT&T ITU-T 12 60-108 48 Group Group 60 312-552 240 Supergroup Supergroup 300 812-2044 1232 Mastergroup 600 564-3084 2520 Mastergroup 3600 564-17548 16984 Jumbogroup That does bring back memories. I worked on STC built systems that used AM modulation using Double-Balanced Modulators and depending on "Group" classification used either the lower or upper sideband. Up to the 60 "voice" Ch's, we had some low Baud rate Modems on as well with the signalling frequency disabled, the spectrum usage was the the same however the next step up was 16 Supergroups using 60-4028KHz with Supergroup 2 not being translated. We also had a 30 Ch PCM link which worked very well apart from the "Regenerators" being susceptible to lightning. |
AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
In message , cledus
writes Radium wrote: Hi: Please don't be annoyed/offended by my question as I decreased the modulation frequency to where it would actually be realistic. I have a very weird question about electromagnetic radiation, carriers, and modulators. No offense but please respond with reasonable answers & keep out the jokes, off-topic nonsense, taunts, insults, and trivializations. I am really interested in this. Thanks, Radium The fundamental answer is no, it is not possible to generate AM where the baseband signal is a pure 20 kHz sinewave and Fc20kHz. The reason is that the modulated waveform consists of the sum of a sinewave at Fc, a sinewave at Fc+20kHz, and a sinewave at Fc-20kHz. If Fc20kHz then one of the components becomes a "negative" frequency. So the carrier must be greater than the baseband signal to prevent this. I'm afraid that this is not correct. The 'laws of physics' don't suddenly stop working if the carrier is lower than the modulating frequency. However, there's no need to get into complicated mathematics to illustrate this. Here is a simple example: (a) If you modulate a 10MHz carrier with a 1MHz signal, you will produce two new signals (the sidebands) at the difference frequency of 10 minus 1 = 9MHz, and the sum frequency of 10 plus 1 = 11MHz. So you have the original carrier at 10MHz, and sideband signals at 9 and 11MHz (with a balanced modulator - no carrier - only 9 and 11MHz). (b) If you modulate a 1MHz carrier with a 10MHz signal, you will produce two new signals (the sidebands) at the difference frequency of 1 minus 10 = minus 9MHz, and the sum frequency of 1 plus 10 = 11MHz. The implication of the negative 'minus 9' MHz signal is that the phase of the 9MHz signal is inverted, ie 180 degrees out-of-phase from 9MHz produced in (a). So you have the original carrier at 1MHz, and sidebands at 9 and 11MHz (again, with a balanced modulator - no carrier - only 9 and 11MHz). The waveforms of the full composite AM signals of (a) and (b) will look quite different. The carriers are at different frequencies, and the phase of the 9MHz signal is inverted. However, with a double-balanced modulator, you will only have the 9 and 11MHz signal so, surprisingly, the resulting signals of (a) and (b) will look the same. [Note that, in practice, many double-balanced modulators/mixers put loads of unwanted signals - mainly due the effects of harmonic mixing. However, the basic 'laws of physics' still apply.] Finally, although I have spoken with great authority, when I get a chance I WILL be doing at test with a tobacco-tin double-balanced mixer, a couple of signal generators and a spectrum analyser - just to make sure that I'm not talking rubbish. In the meantime, I'm sure that some will correct me if I'm wrong. Ian. -- |
| All times are GMT +1. The time now is 04:28 PM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com