AM Radio, LSB or USB?

Discussion in 'General Electronics Chat' started by samski, Dec 1, 2011.

  1. samski

    Thread Starter New Member

    Nov 23, 2011
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    So I know that AM radio transmission are single-side band carrier suppressed. But which sideband is transmitted?

    the reason i ask is because i have just done an experiment with a superheterodyne receiver, the final stage of which used an envelope detector. The system used one IF and used High side injection. As far as i can tell, that would cause spectrum inversion right? I could hear the station, so if the LSB were transmitted, that would make sense.

    The real puzzler is at the end of the experiment, the radio was configured as a 'crystal set' (ie with no LO or mixer) and the channel was still demodulated properly.

    so is spectrum inversion something that does not apply with envelope-detector type demodulators? or is there a hidden step that inverts the spectrum again that I'm not seeing?
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    In the US, AM Radio uses both upper and lower sidebands.

    There was a time for AM stereo where they tried left channel on USB and right channel on LSB, but that didn't last long, implemented at one station.

    SSB (Single Sideband) transmission is only used in Amateur Radio / HAM Radio, Where the transmitter chooses Upper or lower sideband.
     
  3. PatM

    Active Member

    Dec 31, 2010
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    I don't know where you got that information, it is incorrect.

    AM radio transmissions are double sideband -with a carrier.

    What you are describing is SSB transmissions which can be either USB or LSB
     
  4. samski

    Thread Starter New Member

    Nov 23, 2011
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    ah right, thanks very much. im UK based but i would guess similar system hear.

    i did however read (and measure) that channels have 9 kHz of audio bandwidth and that also channels were spaced by around 10 kHz. if both USB and LSB are transmitted, should channels not be spaced 20 kHz apart?
     
  5. thatoneguy

    AAC Fanatic!

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    Transmitters are rarely put within 100kHz of each other in practice, though PLL tuners allow closely packed signals today, stations were kept wide apart in geographical areas, covering the dial at night time, as the low frequency allows the signal to carry quite a distance when the sun is down.

    Here is a diagram from wiki that shows it better, and the power savings if only USB or LSB were used. The problem is, Decoding cannot be done with a simple diode, encoding and decoding a sideband transmission is much more difficult (compared to a detector only).

    [​IMG]
     
  6. samski

    Thread Starter New Member

    Nov 23, 2011
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    okay. so does a pll tuner lock to the input frequency, then add a dc offset to the signal inside the pll loop to step up the frequency by the IF? (i know that it is not good to step down to baseband as you can transmit the LO out through the mixer)

    This from Wiki:
    Medium wave is 520 kHz–1,610 kHz. In the Americas (ITU region 2) 10 kHz spacing is used; elsewhere it is 9 kHz. ITU region 2 also authorizes the Extended AM broadcast band between 1610 kHz and 1710 kHz. Medium wave is by far the most heavily used band for commercial broadcasting. This is the "AM radio" that most people are familiar with.

    EDIT: ahhh also found this: In the rest of the world, the separation is 9 kHz, with sidebands of ± 4.5 kHz.
    how odd that i have a FFT printout from by oscilloscope showing audio information up to around 8/9 kHz
     
    Last edited: Dec 1, 2011
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    PLL Tuners lock onto the carrier, which I was referring to in channel spacing.

    Consumer equipment had a hard time staying tuned on the station in cars, or even with cheap radios, PLL and other improvements have eliminated that issue. On old TV shows you'll see what I mean, people gathered around the radio to listen to the news, and one guy is always tweaking the tuning knob. It was just something that was "part of radio" until the 80's.
     
  8. samski

    Thread Starter New Member

    Nov 23, 2011
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    yea i just wondered if the PLLs were being used to generate a 'LO' to give an Intermediate frequency...
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I'm sorry to have to say that the answer to that is that in the crowded broadcasting conditions of UK and Europe, the transmitted audio bandwidth has to be quite limited. Not all broadcasters will necessarily play it by the rules though, but I think most do.

    By the time the signal has passed through some radios with narrow IF filters optimised for selectivity and low crosstalk, the bandwidth may have fallen even further to just a bit more than 3kHz - like a telephone: adequate for reasonable speech intelligibility, but hardly Hi-Fi.

    One broadcasting application I do know of for vestigial sideband AM used in UK and elsewhere is for the soon-to-be-defunct analogue television systems, where the larger bandwidth needed for the video signal justified the extra complexity, even in the past when this was harder to achieve.
     
  10. samski

    Thread Starter New Member

    Nov 23, 2011
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    great, thank-you for the information. It's good to clarify the specs as i have a report to write and i can say "this observation agrees/doesnt agree with the specs". that, and personal interest of how it's actually done. Ive always believed that standard AM modulation was the basic example to show us how it basically worked, but carrier suppression and the like are the norm. i suppose if the carrier isnt suppressed either, it also allows easier phase locking onto a signal (like when noone is talking... :D).
     
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Non-suppression of the carrier also allows for the use of very simple radio circuits where there is no "locking-on" happening at all, and perhaps not even an internal oscillator of any kind in the receiver. This is how things started in the early days of broadcasting, when complex circuits were difficult to make, costly, bulky or even simply not invented! This remained the case for many years, and by the time more sophisticated arrangements were available broadcast systems were well established.

    There is another reason for transmitting at least a vestigial carrier with AM systems, and that is to ensure accurate demodulated frequencies, and especially to recover the DC component successfully. If there is any beating between the local carrier signal and the true carrier this will not happen. A bit of frequency error might be tolerated on speech, as witness radio hams copying SSB on drifty old radios, but it's not great for music. TV signals need a consistent DC level.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    Actually it is pretty easy anyhow, it is the only stable frequency in the mash, so eventually the PPL will lock onto it. What is eventually for electronics is PDF (Pretty Darn Fast) to humans.

    Basic long distance circuits, before the internet, used suppressed carrier USB signal every 4Khz on a base band. This is why telephony rolls off at 3.4Khz, to create that gap and prevent cross talk. I wonder if it is still used at all?
     
  13. Audioguru

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    Dec 20, 2007
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    If the central office rolls off frequencies above 3400Hz then you are lucky if you are closeby because the capacitance of twisted-pair telephone lines cuts high audio frequencies. The Bell spec limit is -7.5dB one-way at 3kHz.

    I tested the telephone lines at work and found a round trip from one line to another had 3khz at -10dB so I complained to Bell and found out that it was normal. The high frequencies began rolling off a little less than 2kHz. No consonant sounds of speech were passed (just like my local TV station news anchors) so communication was difficult.
     
  14. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    It was phased out during the dialup craze in the late 90's.

    52k was the max bandwidth having the limit in place, 56k is the max with the new 4kHz/channel, IIRC.

    Google L-Signal to find current specs, with everything going digital, I've no clue what standards area actually applied in various areas anymore.
     
  15. Wendy

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    Mar 24, 2008
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    The 3.4Khz is a hard core standard, but it doesn't really have to be anymore. You still find old systems in some of the weirdest places, and maybe not so weird. 3rd world countries will use old equipment to death. 56Kb has to fit in the 3.4Khz standard.

    For telephony, needed or not, 3.4Khz is here to stay. I was referring to the Base Band scheme, where the USB AM signals are stacked like cordwood.

    When I was working with long distance carrier boards for Collins Radio during the late 80s I noticed they had a notch at 2.6Khz for information purposes. I broke out the function generator and tried it while talking to my brother long distance. Sure enough the notch was there.
     
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